A controller mimics the function of an actual physical
controller in a somewhat rudimentary way. It can sense one
node variable, compare it with its setpoint, and determine the
desired value for another node variable. It should be noted
that a controller cannot span a loop manager boundary (but a
Setpoint Manager can). Thus, in an air loop (AirLoopHVAC), the
sensed node and the controlled device must be in the air loop
simulation. This means that a single zone system cannot be
modeled with a simple controller sensing zone temperature and
controlling coil water flow. Instead this must be modeled as a
Setpoint Manager sensing a zone temperature and resetting the
supply air temperature setpoint each timestep. This is
artificial but should simplify the control modeling task. It
should also be noted that there are various types of
controllers and that each controller might hook into loops in
a slightly different way. As a result, each controller type is
described separately below.
This controller is really a solution inverter. For a water
coil the simulation cannot be inverted where the mass flow
rate of the water through the coil can be solved directly
given an air temperature. Thus, this controller will
numerically step through all of the water flow possibilities
by an interval-halving technique until the mass flow rate is
determined to meet the specified outlet air temperature within
a specified user tolerance.
As the reader probably noted when reading the descriptions
of the coil syntax shown earlier in this section, there were
no controls attached directly to a particular component. This
is because the input can be simplified somewhat by entering
node names to be controlled. This avoids having to search
through multiple lists of component types for the sake of
simply controlling components. The Controller:WaterCoil
shown below is a way of controlling variables at one node
based on conditions at another node. After the identifying
name for the controller, the user must define which control
variable the controller is managing. These options include
Temperature, Humidity Ratio, TemperatureAndHumidityRatio, or
Flow.
The next parameter in the input syntax is the Action of the
control, which determines how the controlled variable (e.g.,
mass flow rate through a water coil) is changed based on the
control signal. The following input parameter is the actuator
variable, which is currently limited to mass flow through a
water coil.
The next two parameters in the input syntax are node names
corresponding to the node that is being sensed and the
actuated node which controls the water mass flow through the
coil. For example, in the case of a cooling coil the control
variable might be the outlet air temperature of the coil while
the actuated variable might be the water flow rate through the
coil. These two parameters are followed by the controller
convergence tolerance. Finally, the last two input parameters
represent the maximum and minimum values allowed for the
actuated variable at the actuated node.
This was setup to be generic but to date has only been used
for temperature control, or temperature and humidity ratio
control, of a water coil in the air loop simulation. The
keyword Temperature is used for air temperature control and is
normally specified for the coils outlet air node. The keyword
TemperatureAndHumidityRatio is used for controlling both air
temperature and high humidity levels, and is normally
specified for a cooling coils outlet air node. The keyword
HumidityRatio is used for humidity control and would normally
be specified for a dehumidifier outlet node. These two
keywords require a ZoneControl:Humidistat
object and a maximum humidity setpoint manager object
(SetPointManager:SingleZone:Humidity:Maximum,
SetPointManager:MultiZone:MaximumHumidity:Average or
SetPointManager:MultiZone:Humidity:Maximum). If the coil is
located in the outdoor air stream, it may also be necessary to
use SetpointManager:OutdoorAirPretreat.
The next input refers to the action of the control. This
can be best described by an example. In a coil where water
mass flow rate is to be controlled, a coil will increase the
mass flow rate through the coil when more heating or cooling
is requested. In a heating coil, this increases the value of
heat transfer from the water to the air stream. As a result,
this is considered a Normal action controller. In a cooling
coil, an increase in water mass flow rate through the coil
decreases the value of heat transfer from the water to the air
stream (absolute value increases, but since cooling is
traditionally described as a negative number, an increase in
absolute value results in a decrease in the actual heat
transfer value). Thus, the cooling coil controller has Reverse
action since an increase in flow rate results in a decrease in
heat transfer.
This was again meant to be more generic but currently has
only been used to control the water mass flow rate of a
heating or cooling coil. This actuator variable must be set to
the keyword Flow to control the water mass flow rate.
The coil is controlled by knowing the outlet temperature
and/or humidity ratio specified by the setpoint managers, and
setting the outlet conditions from the coil to meet these
setpoints. The hot and chilled water coils use complex models
that cannot be inverted directly. Therefore, to determine the
correct mass flow rate for the hot or cold water the models
are inverted numerically using an iterative procedure. The
iterative solution uses an interval-halving routine and needs
a termination criteria that is set with the Controller
Convergence Tolerance parameter. The convergence tolerance is
the maximum difference between the actual temperature at the
setpoint node and the setpoint temperature. This control
offset is set to a small temperature difference, such as 0.01
to denote 1/100 degree C. The default is 0.1 degree C.
Set to the minimum design water flow (m\(^{3}\)/sec) for the water coil,
normally a shut off valve that is set to zero.
An example of this object in an IDF, along with appropriate
setpoint managers, is shown below:
Controller:WaterCoil,
Central Cooling Coil Contoller 1, !- Name
TemperatureAndHumidityRatio, !- Control Variable
Reverse, !- Action
Flow, !- Actuator Variable
VAV Sys 1 Outlet Node, !- Sensor Node Name
Main Cooling Coil 1 Water Inlet Node, !- Actuator Node Name
0.002, !- Controller Convergence Tolerance {deltaC}
0.025, !- Maximum Actuated Flow {m3/s}
0.0; !- Minimum Actuated Flow {m3/s}
SetpointManager:SingleZone:Humidity:Maximum,
Zone Max Set Point Manager, !- Name
VAV Sys 1 Outlet Node, !- Setpoint Node or NodeList Name
Zone 2 Node; !- Control Zone Air Node Name
SetpointManager:Scheduled,
Scheduled Set Point Manager 1, !- Name
Temperature, !- Control Variable
Seasonal Reset Supply Air Temp Sch, !- Schedule Name
VAV Sys 1 Outlet Node; !- Setpoint Node or NodeList Name
A mixed air box has its own controller type called Controller:OutdoorAir.
The purpose of the outdoor air controller is to provide
outdoor air for ventilation and also provide free cooling
(through additional outdoor air and/or bypassing an air-to-air
heat exchanger) whenever possible. The outdoor air controller
includes a number of user-selectable limit controls. If any of
the selected limits are exceeded, the outdoor airflow rate is
set to the minimum.
If all the limits are satisfied, the outdoor air controller
does the following for continuous air flow systems: if the
outdoor air temperature is greater than or equal to the mixed
air temperature setpoint, the outdoor air flow rate is set to
the maximum; if the outdoor air temperature is less than the
mixed air temperature setpoint, the outdoor air controller
will modulate the outdoor air flow so that the mixed air
temperature will match the mixed air setpoint temperature.
A time-of-day schedule may also be used to simulate an
increase in outdoor air flow rate for push-button type
economizer applications. When the schedule permits (i.e.,
schedule values are greater than 0), the outdoor air flow rate
is increased to the user-specified maximum outdoor air flow
rate.
The outdoor air controller can also account for changes in
the outdoor air flow rate during times when indoor humidity
levels are high. A zone humidistat must be used with this
control option. During high indoor humidity, the outdoor air
flow rate is modified in response to a high indoor humidity
condition. If high humidity control is based on the outdoor
air humidity ratio and the outdoor humidity ratio is greater
than the indoor humidity ratio, high humidity control is
terminated. When the economizer is used in conjunction with
the high humidity control option, high humidity control has
priority and controls the change in air flow rates. The Night
Ventilation Availability Manager has priority over high
humidity control and will use the controllers maximum outdoor
air flow rate when this Availability Manager cycles the fan on
(Ref. System Availability Managers
AvailabilityManager:NightVentilation)
The mixed air box connections are defined separately in the
OutdoorAir:Mixer
object.
Although the mixer will commonly be connected directly to
the outdoor air, other components may be placed on the outdoor
air path upstream of the mixer. When this is the case, any
modulation will be determined by the conditions at the inlet
node of the mixer rather than the outdoor air. This means that
the controller will account for any heat recovery or other
preheating/precooling components that may modify the condition
of outdoor air before it reaches the mixer.
If all the limits are satisfied, the outdoor air controller
does the following for cycling fan systems: the outdoor air
flow rate is set to the maximum when the fan cycles on. If the
limits are not satisfied, the outdoor air flow rate is at the
minimum when the fan cycles on.
There are various minimum and maximum limits on the outdoor
air flow rate which are applied in the following order. Limits
that fall later in the list can override earlier limits.
OA flow rate = Minimum Outdoor Air Flow Rate * Minimum
Outdoor Air Schedule
(OA flow rate)/(Current mixed air flow rate) \(\ge\) Minimum Fraction of Outdoor
Air Schedule
(OA flow rate)/(Current mixed air flow rate) \(\le\) Maximum Fraction of Outdoor
Air Schedule (even if this reduces the OA flow rate, can
override all minimums and economizer)
OA flow rate \(\le\)
Maximum Outdoor Air Flow Rate
Apply OA flow rate specified by demand
limiting
Apply OA flow rate specified by EMS
OA flow rate \(\le\)
Current mixed air flow rate (system flow rate)
The unique user-assigned name for an instance of an outdoor
air controller. Any other object referencing this outdoor air
controller will use this name.
Name of the node where the mixed air setpoint is set. The
outdoor air controller senses the temperature at this node and
attempts to control that temperature to the node setpoint.
The name of the node that is associated with the outdoor
air damper. This should be the outermost air node on the
outdoor air path connected to the outdoor air stream for the
mixer associated with this outdoor air controller.
Input for this field is the minimum outdoor air flow rate
for the system in cubic meters per second. This field may be
autosized. If a Controller Mechanical Ventilation Name is
specified, note that this value times the Minimum Outdoor Air
Schedule is a hard minimum that may override DCV or other
advanced outdoor air controls.
Choosing NoEconomizer means the economizer
will not operate and the outdoor airflow rate will be at the
minimum for the entire simulation.
Choosing FixedDryBulb means the economizer
will set the outdoor airflow rate at minimum if the outdoor
air temperature is higher than a specified dry-bulb
temperature limit.
Choosing DifferentialDryBulb will trigger
the outdoor airflow to minimum when the dry-bulb temperature
of outdoor air is higher than the dry-bulb temperature of the
return air.
FixedEnthalpy checks the upper limit of
the enthalpy given as a field input against the enthalpy
content of outdoor air and will set the outdoor airflow rate
to minimum if the latter is greater than the former.
DifferentialEnthalpy does the same thing
but compares the return air enthalpy with the enthalpy of
outdoor air. When the enthalpy of outdoor air is greater than
the enthalpy of the return air, the outdoor air flow rate is
set to minimum.
Choosing ElectronicEnthalpy enables the
simulation to calculate the humidity ratio limit of outdoor
air based on the dry-bulb temperature of outdoor air and a
quadratic/cubic curve, and compare it to the actual outdoor
air humidity ratio. If the actual outdoor humidity ratio is
greater than the calculated humidity ratio limit, then the
outdoor airflow rate is set to minimum.
Choosing FixedDewPointAndDryBulb compares
both the outdoor dewpoint temperature and the outdoor dry-bulb
temperature to their specified high limit values. If either
outdoor temperature exceeds the high limit value, the outdoor
airflow rate is set to minimum.
Another option
DifferentialDryBulbAndEnthalpy enables the
control strategy to be based on both the DifferentialDryBulb
and DifferentialEnthalpy economizer control strategies.
In addition to all economizer control types listed above,
each control type checks for user-entered values for the upper
limit of dry-bulb temperature, enthalpy limit, humidity ratio
limit and dewpoint limit. The outdoor air flow rate is set to
minimum if any of these entered limits are exceeded.
There are two choices for this Field:
MinimumFlowWithBypass and
ModulateFlow, with the default being
ModulateFlow if this input field is left
blank.
ModulateFlow means the outdoor air flow
rate will be increased to meet the mixed air setpoint
temperature, subject to the limits imposed via other inputs
for this object (e.g., Economizer Maximum Limit Dry-Bulb
Temperature, Maximum Outdoor Air Flow Rate, etc.).
MinimumFlowWithBypass is used exclusively
in conjunction with air-to-air heat exchanger:objects (Ref.
HeatExchanger:*) for providing free cooling operation in the
absence of a conventional air-side economizer (i.e., when
outdoor air flow rate is not increased during economizer
mode). The MinimumFlowWithBypass choice forces the outdoor air
flow rate to always remain at the minimum. However, when high
humidity control is used, the outdoor air flow rate is set to
the product of the maximum outdoor air flow rate multiplied by
the high humidity outdoor air flow ratio. The heat exchanger
uses the limit checking in the outdoor air controller to
decide whether or not to bypass the outdoor air around the
heat exchanger or turn off the wheel motor in the case of a
rotary heat exchanger. Heat exchange is also suspended when
high humidity control is active.
Field:
Economizer Maximum Limit Dry-Bulb Temperature[LINK]
Input for this field is the outdoor air temperature high
limit ( °C) for economizer operation. If the outdoor air
temperature is above this limit, the outdoor airflow rate will
be set to the minimum. This field is required if Economizer
Control Type FixedDryBulb or FixedDewPointAndDryBulb has been
specified.
No input (blank) in this field means that there is no
outdoor air temperature high limit control. This limit applies
to the conditions at the Actuator Node regardless of whether
or not there are any other components in the outdoor air path
upstream of the mixer. If non-blank, this limit is applied
regardless of the specified Economizer Control Type.
Input for this field is the outdoor air enthalpy limit (in
J/kg) for economizer operation. If the outdoor air enthalpy is
above this value, the outdoor airflow rate will be set to the
minimum. This field is required if Economizer Control Type
FixedEnthalpy has been specified.
No input (blank) in this field means that there is no
outdoor air enthalpy limit control. This limit applies to the
conditions at the Actuator Node regardless of whether or not
there are any other components in the outdoor air path
upstream of the mixer. If non-blank, this limit is applied
regardless of the specified Economizer Control Type.
Field:
Economizer Maximum Limit Dewpoint Temperature[LINK]
Input for this field is the outdoor air dewpoint limit (°C)
for economizer operation. If the outdoor air dewpoint
temperature is above this value, the outdoor airflow rate will
be set to the minimum. This field is required if the
Economizer Control Type FixedDewPointAndDryBulb has been
specified.
No input (blank) in this field means that there is no
outdoor air dewpoint limit control. This limit applies to the
conditions at the Actuator Node regardless of whether or not
there are any other components in the outdoor air path
upstream of the mixer. If non-blank, this limit is applied
regardless of the specified Economizer Control Type.
Input for this field is the name of a quadratic or cubic
curve which provides the maximum outdoor air humidity ratio
(function of outdoor air dry-bulb temperature) for economizer
operation. If the outdoor air humidity ratio is greater than
the curve’s maximum humidity ratio (evaluated at the outdoor
air dry-bulb temperature), the outdoor air flow rate will be
set to the minimum. This limit applies to the conditions at
the Actuator Node regardless of whether or not there are any
other components in the outdoor air path upstream of the
mixer. No input (blank) in this field means that there is no
electronic enthalpy limit control. If non-blank, this limit is
applied regardless of the specified Economizer Control
Type.
Input for this field is the outdoor air temperature low
limit ( °C) for economizer operation. If the outdoor air
temperature is below this limit, the outdoor airflow rate will
be set to the minimum.
No input (blank) in this field means that there is no
outdoor air temperature low limit control. This limit applies
to the conditions at the Actuator Node regardless of whether
or not there are any other components in the outdoor air path
upstream of the mixer. If non-blank, this limit is applied
regardless of the specified Economizer Control Type.
Choices for this field are NoLockout, LockoutWithHeating,
and LockoutWithCompressor. This field is used for packaged
systems with DX coils. LockoutWithHeating means that if the
packaged unit is in heating mode, the economizer is locked out
i.e., the economizer dampers are closed and there is minimum
outdoor air flow. LockoutWithCompressor means that in addition
to locking out the economizer when the unit is in heating mode
the economizer is locked out when the DX unit compressor is
operating to provide cooling. In other words, the economizer
must meet the entire cooling load; it is not allowed to
operate in conjunction with the DX cooling coil. This option
(LockoutWithCompressor) is sometimes called a nonintegrated
economizer.
When LockoutWithHeating or LockoutWithCompressor is
selected, the lockout may also be applied to non-packaged
systems for heating. If any air loop heating coil is
operating, the lockout control compares the mixed air
temperature at minimum outdoor air flow without heat recovery
(if any) to the mixed air temperature set point. If the mixed
air temperature at minimum outdoor air flow is less than the
mixed air temperature set point, then the economizer is locked
out and the outdoor air flow rate is set to the minimum. When
the economizer is locked out, the heat recovery bypass control
will be set to activate heat recovery (no bypass), if present.
This action is meant to minimize heating energy (this action
may also disable the heating coil on subsequent iterations,
see output variable Air System Outdoor Air Heat Recovery
Bypass Heating Coil Activity Status).
Choices for this field are FixedMinimum or
ProportionalMinimum. FixedMinimum means that the minimum
outdoor airflow rate is fixed no matter what the actual system
flow rate is. ProportionalMinimum means the minimum outdoor
airflow rate varies in proportion to the total system air flow
rate. The default is ProportionalMinimum.
The name of a schedule which uses decimal values (e.g., 0.0
or 1.0). These values are multiplied by the minimum outdoor
air flow rate. This schedule is useful for reducing the
outdoor air flow rate to zero during unoccupied or start up
hours. If this field is not entered, the minimum outdoor air
flow rate either remains constant during the simulation period
(Minimum Outdoor Air Control Type = FixedMinimum) or varies in
proportion to the supply air flow rate (Minimum Outdoor Air
Control Type = ProportionalMinimum).
Field:
Minimum Fraction of Outdoor Air Schedule Name[LINK]
The name of a schedule with decimal values which are
limited between 0.0 and 1.0. The current schedule value is
multiplied by the current system mixed air flow rate to set
the minimum outdoor air flow rate. If this schedule name is
blank, then it is not applied.
If this schedule is 1.0 (and there is no Maximum Fraction
of Outdoor Air Schedule Name), then the system works at 100%
outdoor air regardless of any other condition as long as the
Maximum Outdoor Air Flow Rate is \(\ge\) the system supply air flow
rate.
Field:
Maximum Fraction of Outdoor Air Schedule Name[LINK]
The name of a schedule with decimal values which are
limited between 0.0 and 1.0. The current schedule value is
multiplied by the current system mixed air flow rate to set
the maximum outdoor air flow rate. This schedule is applied
after all other limits (except EMS and demand-limiting
overrides). This schedule can override all minimums and
economizer opearation. For example, if this schedule is zero,
then the outdoor air flow rate will be zero, regardless of any
other settings in Controller:OutdoorAir
or Controller:MechanicalVentilation.
If this schedule name is blank, then it is not applied.
This optional field is the name of the mechanical
ventilation controller object to be used in conjunction with
this outdoor air controller. The Controller:MechanicalVentilation
object allows the user to define the minimum outdoor air flow
rate based on air flow per unit floor area and air flow per
person (occupant) for the zones being served by the air loop
that utilizes this controller.
This feature allows the user to perform a first-order
evaluation of carbon dioxide(CO\(_{2}\))-based demand controlled
ventilation (outdoor ventilation varied according to occupancy
levels).
If a valid name for a Controller:MechanicalVentilation
object is entered in this field, the minimum outdoor air flow
rate delivered will be the greater of:
the minimum outdoor air flow rate calculated by the
fields Minimum Outdoor Air Flow Rate, Minimum Limit Type, and
Minimum Outdoor Air Schedule Name as defined for this outdoor
air controller, or
Leaving this input field blank will bypass the Controller:MechanicalVentilation
object calculations and the minimum outdoor air flow rate will
be based on the other inputs associated with this outdoor air
controller object. Actual outdoor air flow rates may be higher
than the minimum if free cooling is available and the object
inputs are properly selected. Regardless, the maximum outdoor
air flow rate is limited by the field Maximum Outdoor Air Flow
Rate.
Note that the Minimum Outdoor Air Flow Rate times the
Minimum Outdoor Air Schedule is a hard minimum that may
override DCV or other advanced outdoor air controls specified
in the Controller:MechanicalVentilation
object. The Maximum Fraction of Outdoor Air Schedule (if
specified) times the current system supply air flow rate may
limit the outdoor air flow rate set by the Controller:MechanicalVentilation.
Field:
Time of Day Economizer Control Schedule Name[LINK]
This alpha field is the name of a schedule which controls
the outdoor air flow rate based on a time-of-day economizer.
Schedule values equal to 0 disable this feature. Schedule
values greater than 0 cause the outdoor air flow rate to
increase to the maximum. When an economizer is used in
conjunction with the high humidity control option, high
humidity control has priority.
This choice field establishes whether or not the outdoor
air flow rate is modified in response to high indoor relative
humidity. Valid choices are Yes and No. If Yes is selected,
the outdoor air flow rate may be modified when the indoor
relative humidity is above the humidstat setpoint. If No is
selected, this option is disabled and the following three
fields are not used.
This input defines the zone name where the humidistat is
located. This is the same name used in the ZoneControl:Humidistat
object. This field is required when the input field High
Humidity Control is specified as Yes.
This input is the ratio of the modified outdoor air flow
rate to the maximum outdoor air flow rate. When the high
humidity control algorithm determines that the outdoor air
flow rate will be changed (i.e., increased or decreased), the
operating outdoor air flow rate is equal to the maximum
outdoor air flow rate multiplied by this ratio. The minimum
value for this field is 0. If this field is blank, the default
value is 1. This field is used only when the input field High
Humidity Control is specified as Yes. When an economizer is
used in conjunction with the high humidity control option,
high humidity control has priority.
Field:
Control High Indoor Humidity Based on Outdoor Humidity
Ratio[LINK]
This choice field determines if high humidity control is
activated based on high indoor relative humidity alone or is
activated only when the indoor relative humidity is above the
humidstat setpoint and the indoor humidity ratio is
greater than the outdoor humidity ratio. Valid choices are
Yes and No. If No is
selected, high humidity control is active any time the zone
humidistat senses a moisture load. If Yes is selected, the
model also verifies that the outdoor humidity ratio is less
than the humidistat s zone air humidity ratio. This field is
used only when the input field High Humidity Control is
specified as Yes. The default value is
Yes.
This choice field determines if specialized control is used
to optimize the use of heat recovery. Valid choices are
BypassWhenWithinEconomizerLimits and
BypassWhenOAFlowGreaterThanMinimum. If
BypassWhenWithinEconomizerLimits is selected, heat recovery is
disabled any time the controller determines that the
economizer is active (i.e., all controls are within limits).
If BypassWhenOAFlowGreaterThanMinimum is selected, the model
first verifies that the economizer is active and then checks
to see if the outdoor air flow rate is greater than the
minimum. If it is greater than minimum, then heat recovery (if
any) is set to bypass. When this option is used with Time of
Day Economizer Control or High Humidity Control, this option
has priority. The default value is
BypassWhenWithinEconomizerLimits.
An Example IDF specification:
Controller:OutdoorAir,
OA Controller 1, !- Name
Relief Air Outlet Node, !- Relief Air Outlet Node Name
Outdoor Air Mixer Inlet Node, !- Return Air Node Name
Mixed Air Node, !- Mixed Air Node Name
Outdoor Air Inlet Node, !- Actuator Node Name
0.25, !- Minimum Outdoor Air Flow Rate {m3/s}
1.6, !- Maximum Outdoor Air Flow Rate {m3/s}
ElectronicEnthalpy, !- Economizer Control Type
ModulateFlow, !- Economizer Control Action Type
23., !- Economizer Maximum Limit Dry-Bulb Temperature {C}
, !- Economizer Maximum Limit Enthalpy {J/kg}
13.5, !- Economizer Maximum Limit Dewpoint Temperature {C}
ElectronicEnthalpyCurveA,!- Electronic Enthalpy Limit Curve Name
14., !- Economizer Minimum Limit Dry-Bulb Temperature {C}
NoLockout, !- Lockout Type
FixedMinimum, !- Minimum Limit Type
OAFractionSched, !- Minimum Outdoor Air Schedule Name
, !- Minimum Fraction of Outdoor Air Schedule Name
, !- Maximum Fraction of Outdoor Air Schedule Name
, !- Mechanical Ventilation Controller Name
TimeOfDayEconomizerSch, !- Time of Day Economizer Control Schedule Name
Yes, !- High Humidity Control
EAST ZONE, !- Humidistat Control Zone Name
0.9, !- High Humidity Outdoor Air Flow Ratio
Yes; !- Control High Indoor Humidity Based on Outdoor Humidity Ratio
Curve:Cubic,
ElectronicEnthalpyCurveA,!- Name
0.01342704, !- Coefficient1 Constant
-0.00047892, !- Coefficient2 x
0.000053352, !- Coefficient3 x**2
-0.0000018103, !- Coefficient4 x**3
16.6, !- Minimum Value of x
29.13; !- Maximum Value of x
Reports the average operating status of an air economizer
over the reporting interval. The economizer status is set to 1
when the conditions are favorable for the economizer to
operate (i.e., none of the control limits have been exceeded).
While conditions may be favorable for economizer operation, it
does not guarantee that the air-side economizer has increased
outdoor air flow above the minimum level since the actual
outdoor air flow rate is also governed by other controls
(e.g., mixed air set point tempeature, time of day economizer
control, maximum humidity setpoint, etc.). This variable is
set to 0 if conditions disable economizer operation or
NoEconomizer (Economizer Control Type) is specified.
Air
System Outdoor Air Heat Recovery Bypass Status [][LINK]
This indicates if the controls have determined if the
bypass mode for heat recovery is in effect or not.
Air
System Outdoor Air Heat Recovery Bypass Heating Coil Activity
Status [][LINK]
Reports the operating status of any heating coil in the air
loop. If the heating coil is active, the heat exchanger will
be activated (no air bypassed) and the heating energy will be
reduced or eliminated. While conditions may be favorable for
economizer operation, it does not guarantee that the air-side
economizer has increased outdoor air flow above the minimum
level since the actual outdoor air flow rate is also governed
by other controls (e.g., mixed air set point temperature, time
of day economizer control, maximum humidity setpoint, etc.).
This variable is set to 0 if conditions disable economizer
operation. This output variable is only available when using
Heat Recovery Bypass Control Type =
BypassWhenOAFlowGreaterThanMinimum.
Air
System Outdoor Air Heat Recovery Bypass Minimum Outdoor Air
Mixed Air Temperature [C][LINK]
Reports the outdoor air mixer s mixed air node temperature
at minimum outdoor air flow rate when the heat exchanger is
disabled (off). This temperature is calculated as the return
air temperature multiplied by the return air mass flow rate
plus the mixer s inlet node temperature multipled by the
minimum outdoor air flow rate. This quantity is then divided
by the mixed air mass flow rate. If this temperature is less
than the outdoor air mixer s mixed air node set point
temperature, and the Heat Recovery Bypass Control Type =
BypassWhenOAFlowGreaterThanMinimum, the outdoor air flow rate
is set to the minimum. This output variable is only available
when using Heat Recvoery Bypass Control Type =
BypassWhenOAFlowGreaterThanMinimum.
Air
System Outdoor Air High Humidity Control Status [][LINK]
Reports the average operating status of the controller s
high humidity control over the reporting interval. The high
humidity control status is set to 1 when the controller
determines that a zone high humidity condition exists
according to the settings specified in the
controller. This variable is set to 0 if conditions
disable high humidity control operation or High Humidity
Control is specified as No.
Reports the average mixed air mass flow rate of the HVAC
air loop associated with this outdoor air controller over the
reporting interval.
Air
System Outdoor Air Maximum Flow Fraction [][LINK]
Reports the average maximum limit of the outdoor air
fraction for the outdoor air controller over the reporting
interval. The maximum flow fraction is used to prevent DX
cooling coils from freezing, specified by ASHRAE Stadard 90.1.
This output variable is available when a corresponding SetpointManager:MixedAir
object specifies optional inputs of Cooling Coil Inlet Node
Name, Cooling coil Outlet Node Name, and Minimum Temperature
at Cooling Coil Outlet Node.
Air System
Outdoor Air Mechanical Ventilation Requested Mass Flow Rate
[kg/s][LINK]
Reports the average outdoor air mass flow rate requested by
the Mechanical Ventilation Controller
(Controller:MechanicalVentilation, if specified) over the
reporting interval.
This object is used in conjunction with an outdoor air
controller (Ref. Controller:OutdoorAir,
Field: Mechanical Ventilation Controller Name) to establish
the minimum outdoor air flow rate provided by a mixed air
box.
Ventilation standards provide guidance on appropriate
levels of outdoor ventilation air required for acceptable
indoor air quality. The Ventilation Rate Procedure (VRP) of
ASHRAE Standard 62.1-2007/2010 (www.ashrae.org) requires
outdoor ventilation rates to be determined based on the floor
area of each occupied zone plus the number of people in each
zone and considers the zone air distribution effectiveness and
system ventilation efficiency. The outdoor air ventilation
rate can be reset dynamically as operating conditions change
(e.g., variations in occupancy). The Controller:MechanicalVentilation
object implements the VRP for calculating these outdoor air
ventilation requirements and resetting them based on varying
occupancy levels and zone diversification. This is
particularly useful for large air distribution systems that
serve a number of different zone types with varying occupancy
levels. This object can also be used to model the Indoor Air
Quality Procedure (IAQP) as defined in Standard 62.1
The first five inputs for this object are the name, the
availability schedule, the zone outdoor air method, the system
outdoor air method, and the zone maximum outdoor air fraction.
The next three input fields define the zone name (or zone list
name), the design specification outdoor air object name, and
the design specification zone air distribution object name to
be applied to this zone (or zone list). The last three fields
are extensible
The unique user assigned name for an instance of mechanical
ventilation. Any other object referencing this mechanical
ventilation object will use this name.
The name of a schedule whose values are greater than 0 when
mechanical ventilation, as calculated by this object, is
desired. If the schedule s value is 0.0, then mechanical
ventilation is not available and flow will not be requested.
If the schedule s value is > 0.0 (usually 1 is used),
mechanical ventilation is available. If this field is blank,
the schedule has values of 1 for all time periods. This
schedule is useful for purging the building of contaminants
prior to occupancy (i.e., ventilation rate per unit floor area
will be provided even if the occupancy is zero).
This field indicates whether the air loop is capable of
doing demand controlled ventilation (DCV) to vary the amount
of outdoor air based on actual number of occupants in spaces.
Two choices: Yes and No. Default is No.
The method used to calculate the system minimum outdoor air
flow. Several choices are allowed: ZoneSum,
VentilationRateProcedure,IndoorAirQualityProcedure,
ProportionalControlBasedonOccupancySchedule,ProportionalControlBasedonDesignOccupancy,ProportionalControlBasedOnDesignOARate, and
IndoorAirQualityProcedureGenericContaminant.
ZoneSum sums the outdoor air flows across all zones served by
the system. VentilationRateProcedure (VRP) uses the multi-zone
equations defined in 62.1-2007 to calculate the system outdoor
air flow. VRP considers zone air distribution effectiveness
and zone diversification of outdoor air fractions.
IndoorAirQualityProcedure (IAQP) is the other procedure
defined in ASHRAE Standard 62.1-2007 for calculate the amount
of outdoor air necessary to maintain the levels of indoor air
carbon dioxide at or below the setpoint defined in the ZoneControl:ContaminantController
object. Appendix A of the ASHRAE 62.1-2010 user’s manual
discusses another method for implementing CO\(_{2}\)-based DCV in a single zone
system. This method (Proportional Control) calculates the
required outdoor air flow rate which varies in proportion to
the percentage of the CO\(_{2}\) signal range and has two
choices to calculate occupancy-based outdoor air rate. The
ProportionalControlBasedonOccupancySchedule choice uses the
real occupancy at the current time step to calculate outdoor
air rate, while the ProportionalControlBasedonDesignOccupancy
uses the design occupancy level to calculate outdoor air rate.
The former choice is a good approach to estimate outdoor air
rate. However, for practical applications, the zone controller
usually does not have the real time occupancy information, and
the design occupancy level is assumed. The latter choice is
used in the design stage. The
ProportionalControlBasedOnDesignOARate uses design outdoor air
flow rate to calculate demand outdoor air flow rate. The
IndoorAirQualityProcedure-GenericContaminant method calculates
the amount of outdoor air necessary to maintain the levels of
indoor air generic contaminant at or below the setpoint
defined in the ZoneControl:ContaminantController
object.
Note: When System Outdoor Air Method =
IndoorAirQualityProcedure or
IndoorAirQualityProcedureGenericContaminant is specified, only
the Zone
<x> Name fields are used. The other field inputs
described below are not used.
This positive numeric input is the zone maximum outdoor air
fraction. For VAV systems, when a zone requires outdoor air
higher than the user specified Zone
Maximum Outdoor Air Fraction, the zone supply air flow will be
increased (if damper not fully open yet) to cap the outdoor
air fraction at the maximum value. This allows the system
level outdoor air flow to be reduced while the total supply
air flow increases to meet zone outdoor air requirement. Valid
values are from 0 to 1.0. Default is 1.0 which indicates zones
can have 100% outdoor air maintaining backward compatibility.
This inputs work for single and dual duct VAV systems.
Field
Set (Zone Name, Design Specification Outdoor Air Object Name,
and Design Specification Zone Air Distribution Object
Name)[LINK]
The following three fields are needed to define the
parameters for the ventilation. This object is extensible by
duplicating these three fields.
If an Outdoor Air Schedule Name is specified in the DesignSpecification:OutdoorAir
object, the schedule will be applied to all types of outdoor
air calculations for the corresponding zone, regardless of the
System Outdoor Air Method selected. If the schedule value is
zero, then the zone will be completely removed from the system
outdoor air calcaulations.
Field:
Design Specification Zone Air Distribution Object Name
<x>[LINK]
As described previously, the Controller:MechanicalVentilation
object works in conjunction with Controller:OutdoorAir.
As such, the minimum quantity of outdoor air delivered via the
mixed air box will be the greater of:
the minimum outdoor air flow rate calculated by the
fields Minimum Outdoor Air Flow Rate, Minimum Limit and
Minimum Outdoor Air Schedule Name in the associated Controller:OutdoorAir
object, or
the outdoor air flow rate calculated by this
object.
The actual outdoor air flow rate may be higher than the
minimum if free cooling is available. Regardless, the outdoor
air flow rate will not exceed the Maximum Outdoor Air Flow
Rate specified in the associated Controller:OutdoorAir
object or the Maximum Fraction of Outdoor Air Schedule (if
specified) times the current system supply air flow rate.
An example input for this object is shown below:
Controller:MechanicalVentilation,
VentObject, !- Name
VentSchedule, !- Availability Schedule Name
VentilationRateProcedure, !- System Outdoor Air Method
1.0, !- Zone Maximum Outdoor Air Fraction
Resistive Zone, !- Zone 1 Name
DSOA1, !- Design Specification Outdoor Air Object Name 1
DSZADO1, !- Design Specification Zone Air Distribution Object Name 1
DCV Zone List, !- Zone 2 Name
DSO_ZList, !- Design Specification Outdoor Air Object Name 2
; !- Design Specification Zone Air Distribution Object Name 2
ZoneList,
DCV Zone List, !- Zone List Name
East Zone, !- Zone Name 1
North Zone; !- Zone Name 2
DesignSpecification:OutdoorAir,
DSOA1, !- Name
SUM, !- Outdoor Air Method
0.00236, !- Outdoor Air Flow per Person
0.000305, !- Outdoor Air Flow per Zone Floor Area
0.0, !- Outdoor Air Flow per Zone
0.0, !- Outdoor Air Flow Air Changes per Hour
; !- Outdoor Air Flow Rate Fraction Schedule Name
DesignSpecification:ZoneAirDistribution,
DSZADO1, !- Name
1.2, !- Zone Air Distribution Effectiveness in Cooling Mode
1.0, !- Zone Air Distribution Effectiveness in Heating Mode
, !- Zone Air Distribution Effectiveness Schedule Name
0.3; !- Zone Secondary Recirculation Fraction
This controller object is used exclusively by the stand
alone energy recovery ventilator
(ZoneHVAC:EnergyRecoveryVentilator, see Figure [fig:schematic-of-the-zonehvac]).
The purpose of this controller is to simulate economizer
operation for the stand alone ERV and provide free cooling
whenever possible or modify the outdoor air flow rate during
high indoor humidity conditions. During economizer operation,
if all of the limits are satisfied, the controller activates
economizer mode (fully bypassing the fixed-plate air-to-air
heat exchanger or stopping the rotation of a rotary heat
exchanger). If any of the selected limits are exceeded,
economizer operation is terminated. A time-of-day schedule may
also be used to simulate a push-button type economizer
controller.
In addition, the outdoor air flow rate may be modified and
heat exchange suspended in response to high indoor relative
humidities. When the indoor relative humidity exceeds the zone
humidistat s relative humidity set point, high humidity
control is activated. When activated, if high humidity control
is based on the outdoor air humidity ratio and the outdoor
humidity ratio is greater than the indoor humidity ratio, high
humidity control is terminated.
Schematic of the
ZoneHVAC:EnergyRecoveryVentilator compound object [fig:schematic-of-the-zonehvac]
The input for this field is the outdoor air temperature
high limit ( °C) for economizer operation. If the outdoor air
temperature is above this limit, economizer (free cooling)
operation is terminated. No input (blank) in this field means
that there is no outdoor air temperature high limit
control.
The input for this field is the outdoor air temperature low
limit ( °C) for economizer operation. If the outdoor air
temperature is below this limit, economizer (free cooling)
operation is terminated. No input (blank) in this field means
that there is no outdoor air temperature low limit
control.
The input for this field is the outdoor air enthalpy limit
(in J/kg) for economizer operation. If the outdoor air
enthalpy is above this value, economizer (free cooling)
operation is terminated. No input (blank) in this field means
that there is no outdoor air economizer limit control.
Input for this field is the outdoor air dewpoint limit (
°C) for economizer operation. If the outdoor air dewpoint
temperature is above this value, the outdoor airflow rate will
be set to the minimum. No input (blank) in this field means
that there is no outdoor air dewpoint limit control. This
limit applies to the conditions at the Actuated Node
regardless of whether or not there are any other components on
the outdoor air path upstream of the mixer.
Input for this field is the name of a quadratic or cubic
curve which provides the maximum outdoor air humidity ratio
(function of outdoor air dry-bulb temperature) for economizer
operation. If the outdoor air humidity ratio is greater than
the curve s maximum humidity ratio (evaluated at the outdoor
air dry-bulb temperature), the outdoor air flow rate will be
set to the minimum. This limit applies to the conditions at
the Actuated Node regardless of whether or not there are any
other components on the outdoor air path upstream of the
mixer. No input (blank) in this field means that there is no
electronic enthalpy limit control.
This input establishes whether or not there is a limit
control on the exhaust air temperature. The choices are
ExhaustAirTemperatureLimit or
NoExhaustAirTemperatureLimit. If
ExhaustAirTemperatureLimit is chosen, the
controller deactivates economizer mode whenever the outdoor
air temperature is greater than the exhaust air temperature.
If NoExhaustAirTemperatureLimit is chosen, no
limit check on the exhaust air temperature is performed.
This input establishes whether or not there is a limit
control on the exhaust air enthalpy. The choices are
ExhaustAirEnthalpyLimit or
NoExhaustAirEnthalpyLimit. If
ExhaustAirEnthalpyLimit is chosen, the
controller deactivates economizer mode whenever the outdoor
air enthalpy is greater than the exhaust air enthalpy. If
NoExhaustAirEnthalpyLimit is chosen, no limit
check on the exhaust air enthalpy is performed.
Field:
Time of Day Economizer Flow Control Schedule Name[LINK]
This alpha field is the name of a schedule which controls
the change in air flow rate based on time-of-day. Schedule
values equal to 0 disable this feature. Schedule values
greater than 0 activate the economizer. Note that heat
exchange between the air streams is suspended when the
economizer is active. This schedule can be used with or
without the high humidity control option. When an economizer
is used in conjunction with the high humidity control option,
high humidity control has priority.
This optional choice field establishes whether or not the
supply and exhaust air flow rates are modified in response to
high indoor relative humidity. Valid choices are Yes and
No. If Yes is selected, the supply and exhaust air flow rates
may be modified when the indoor relative humidity is above the
humidstat set point. If No is selected, this option is
disabled and the following three fields are not used. Note
that heat exchange between the air streams is suspended during
times when high humidity control is active. The default value
is No.
This optional input defines the zone name where the
humidistat is located. This is the same zone name used in the
Zone
Control:Humidistat object. This field is required when the
High Humidity Control Flag is specified as Yes.
This optional input is the ratio of the modified supply
(outdoor) air flow rate to the supply air flow rate specified
in the Energy Recovery Ventilator:Stand Alone ERV object. When
the high humidity control algorithm determines that the supply
air flow rate will be changed (i.e. increased or decreased),
the operating supply air flow rate is equal to the supply air
flow rate specified in the Energy Recovery Ventilator:Stand
Alone ERV object multiplied by this ratio. The minimum value
for this field is 0. This field is used to modify both the
supply and exhasut air flow rates when high humidity control
is active. The supply and exhasut air fan volumetric flow
rates must be able to account for the increase in air flow
when this input is greater than 1. The default value is 1.
Field:
Control High Indoor Humidity based on Outdoor Humidity
Ratio[LINK]
This optional choice field determines if high humidity
control is activated based on high indoor relative humidity
alone or is activated only when the indoor relative humidity
is above the humidstat set point and the outdoor
humidity ratio is less than the indoor humidity ratio. Valid
choices are Yes and No. If No is selected, high humidity
control is active any time the zone humidistat senses a
moisture load. If yes is selected, the model also verifies
that the outdoor humidity ratio is less than the humidistat s
zone air humidity ratio. This field is used only when the High
Humidity Control Flag is specified as Yes. The default value
is Yes.
Following is an example input for this stand alone ERV
controller object:
ZoneHVAC:EnergyRecoveryVentilator:Controller,
ERV OA Controller 1, !- ERV controller name
19., !- Temperature high limit {C}
14., !- Temperature low limit {C}
, !- Enthalpy high limit {J/kg}
15.55, !- dew point temperature limit (C)
ElectronicEnthalpyCurveA, !- electronic enthalpy limit curve name
NoExhaustAirTemperatureLimit, !- Exhaust air temperature limit
NoExhaustAirEnthalpyLimit, !- Exhaust air enthalpy limit
OutsideAirFlowSchedule, !- Time of Day Economizer Flow Control Schedule Name
Yes, !- High Humidity Control Flag
East Zone, !- Humidistat Control Zone Name
1.2, !- High Humidity Outdoor air Flow Ratio
Yes; !- Control High Indoor Humidity based on Outdoor Humidity Ratio
Curve:Cubic,
ElectronicEnthalpyCurveA, !- Name
0.01342704, !- Coefficient1 Constant
-0.00047892, !- Coefficient2 x
0.000053352, !- Coefficient3 x**2
-0.0000018103, !- Coefficient4 x**3
16.6, !- Minimum Value of x
29.13; !- Maximum Value of x
Group Controllers[LINK]
Controls (Air Loop and Zone Equipment)[LINK]
A controller mimics the function of an actual physical controller in a somewhat rudimentary way. It can sense one node variable, compare it with its setpoint, and determine the desired value for another node variable. It should be noted that a controller cannot span a loop manager boundary (but a Setpoint Manager can). Thus, in an air loop (AirLoopHVAC), the sensed node and the controlled device must be in the air loop simulation. This means that a single zone system cannot be modeled with a simple controller sensing zone temperature and controlling coil water flow. Instead this must be modeled as a Setpoint Manager sensing a zone temperature and resetting the supply air temperature setpoint each timestep. This is artificial but should simplify the control modeling task. It should also be noted that there are various types of controllers and that each controller might hook into loops in a slightly different way. As a result, each controller type is described separately below.
Controller:WaterCoil[LINK]
This controller is really a solution inverter. For a water coil the simulation cannot be inverted where the mass flow rate of the water through the coil can be solved directly given an air temperature. Thus, this controller will numerically step through all of the water flow possibilities by an interval-halving technique until the mass flow rate is determined to meet the specified outlet air temperature within a specified user tolerance.
As the reader probably noted when reading the descriptions of the coil syntax shown earlier in this section, there were no controls attached directly to a particular component. This is because the input can be simplified somewhat by entering node names to be controlled. This avoids having to search through multiple lists of component types for the sake of simply controlling components. The Controller:WaterCoil shown below is a way of controlling variables at one node based on conditions at another node. After the identifying name for the controller, the user must define which control variable the controller is managing. These options include Temperature, Humidity Ratio, TemperatureAndHumidityRatio, or Flow.
The next parameter in the input syntax is the Action of the control, which determines how the controlled variable (e.g., mass flow rate through a water coil) is changed based on the control signal. The following input parameter is the actuator variable, which is currently limited to mass flow through a water coil.
The next two parameters in the input syntax are node names corresponding to the node that is being sensed and the actuated node which controls the water mass flow through the coil. For example, in the case of a cooling coil the control variable might be the outlet air temperature of the coil while the actuated variable might be the water flow rate through the coil. These two parameters are followed by the controller convergence tolerance. Finally, the last two input parameters represent the maximum and minimum values allowed for the actuated variable at the actuated node.
Inputs[LINK]
Field: Name[LINK]
This is the unique name of the controller.
Field: Control Variable[LINK]
This was setup to be generic but to date has only been used for temperature control, or temperature and humidity ratio control, of a water coil in the air loop simulation. The keyword Temperature is used for air temperature control and is normally specified for the coils outlet air node. The keyword TemperatureAndHumidityRatio is used for controlling both air temperature and high humidity levels, and is normally specified for a cooling coils outlet air node. The keyword HumidityRatio is used for humidity control and would normally be specified for a dehumidifier outlet node. These two keywords require a ZoneControl:Humidistat object and a maximum humidity setpoint manager object (SetPointManager:SingleZone:Humidity:Maximum, SetPointManager:MultiZone:MaximumHumidity:Average or SetPointManager:MultiZone:Humidity:Maximum). If the coil is located in the outdoor air stream, it may also be necessary to use SetpointManager:OutdoorAirPretreat.
Field: Action[LINK]
The next input refers to the action of the control. This can be best described by an example. In a coil where water mass flow rate is to be controlled, a coil will increase the mass flow rate through the coil when more heating or cooling is requested. In a heating coil, this increases the value of heat transfer from the water to the air stream. As a result, this is considered a Normal action controller. In a cooling coil, an increase in water mass flow rate through the coil decreases the value of heat transfer from the water to the air stream (absolute value increases, but since cooling is traditionally described as a negative number, an increase in absolute value results in a decrease in the actual heat transfer value). Thus, the cooling coil controller has Reverse action since an increase in flow rate results in a decrease in heat transfer.
Field: Actuator Variable[LINK]
This was again meant to be more generic but currently has only been used to control the water mass flow rate of a heating or cooling coil. This actuator variable must be set to the keyword Flow to control the water mass flow rate.
Field: Sensor Node Name[LINK]
Name of the node where the temperature, humidity ratio or flow is sensed.
Field: Actuator Node Name[LINK]
Name of the actuated node that controls the water mass flow rate through the coil.
Field: Controller Convergence Tolerance[LINK]
The coil is controlled by knowing the outlet temperature and/or humidity ratio specified by the setpoint managers, and setting the outlet conditions from the coil to meet these setpoints. The hot and chilled water coils use complex models that cannot be inverted directly. Therefore, to determine the correct mass flow rate for the hot or cold water the models are inverted numerically using an iterative procedure. The iterative solution uses an interval-halving routine and needs a termination criteria that is set with the Controller Convergence Tolerance parameter. The convergence tolerance is the maximum difference between the actual temperature at the setpoint node and the setpoint temperature. This control offset is set to a small temperature difference, such as 0.01 to denote 1/100 degree C. The default is 0.1 degree C.
Field: Maximum Actuated Flow[LINK]
This is the maximum water flow (m\(^{3}\)/sec) through the coil. Set to the maximum design water flow for the coil.
Field: Minimum Actuated Flow[LINK]
Set to the minimum design water flow (m\(^{3}\)/sec) for the water coil, normally a shut off valve that is set to zero.
An example of this object in an IDF, along with appropriate setpoint managers, is shown below:
Controller:OutdoorAir[LINK]
A mixed air box has its own controller type called Controller:OutdoorAir. The purpose of the outdoor air controller is to provide outdoor air for ventilation and also provide free cooling (through additional outdoor air and/or bypassing an air-to-air heat exchanger) whenever possible. The outdoor air controller includes a number of user-selectable limit controls. If any of the selected limits are exceeded, the outdoor airflow rate is set to the minimum.
If all the limits are satisfied, the outdoor air controller does the following for continuous air flow systems: if the outdoor air temperature is greater than or equal to the mixed air temperature setpoint, the outdoor air flow rate is set to the maximum; if the outdoor air temperature is less than the mixed air temperature setpoint, the outdoor air controller will modulate the outdoor air flow so that the mixed air temperature will match the mixed air setpoint temperature.
A time-of-day schedule may also be used to simulate an increase in outdoor air flow rate for push-button type economizer applications. When the schedule permits (i.e., schedule values are greater than 0), the outdoor air flow rate is increased to the user-specified maximum outdoor air flow rate.
The outdoor air controller can also account for changes in the outdoor air flow rate during times when indoor humidity levels are high. A zone humidistat must be used with this control option. During high indoor humidity, the outdoor air flow rate is modified in response to a high indoor humidity condition. If high humidity control is based on the outdoor air humidity ratio and the outdoor humidity ratio is greater than the indoor humidity ratio, high humidity control is terminated. When the economizer is used in conjunction with the high humidity control option, high humidity control has priority and controls the change in air flow rates. The Night Ventilation Availability Manager has priority over high humidity control and will use the controllers maximum outdoor air flow rate when this Availability Manager cycles the fan on (Ref. System Availability Managers AvailabilityManager:NightVentilation)
The mixed air box connections are defined separately in the OutdoorAir:Mixer object.
Although the mixer will commonly be connected directly to the outdoor air, other components may be placed on the outdoor air path upstream of the mixer. When this is the case, any modulation will be determined by the conditions at the inlet node of the mixer rather than the outdoor air. This means that the controller will account for any heat recovery or other preheating/precooling components that may modify the condition of outdoor air before it reaches the mixer.
If all the limits are satisfied, the outdoor air controller does the following for cycling fan systems: the outdoor air flow rate is set to the maximum when the fan cycles on. If the limits are not satisfied, the outdoor air flow rate is at the minimum when the fan cycles on.
There are various minimum and maximum limits on the outdoor air flow rate which are applied in the following order. Limits that fall later in the list can override earlier limits.
OA flow rate = Minimum Outdoor Air Flow Rate * Minimum Outdoor Air Schedule
Apply economizer controls
OA flow rate \(\ge\) Controller:MechanicalVentilation OA flow rate
OA flow rate \(\ge\) System exhaust flow rate
(OA flow rate)/(Current mixed air flow rate) \(\ge\) Minimum Fraction of Outdoor Air Schedule
(OA flow rate)/(Current mixed air flow rate) \(\le\) Maximum Fraction of Outdoor Air Schedule (even if this reduces the OA flow rate, can override all minimums and economizer)
OA flow rate \(\le\) Maximum Outdoor Air Flow Rate
Apply OA flow rate specified by demand limiting
Apply OA flow rate specified by EMS
OA flow rate \(\le\) Current mixed air flow rate (system flow rate)
Inputs[LINK]
Field: Name[LINK]
The unique user-assigned name for an instance of an outdoor air controller. Any other object referencing this outdoor air controller will use this name.
Field: Relief Air Outlet Node Name[LINK]
The name of the relief air node of the outdoor air mixer associated with this outdoor air controller.
Field: Return Air Node Name[LINK]
The name of the return air node of the outdoor air mixer associated with this outdoor air controller.
Field: Mixed Air Node Name[LINK]
Name of the node where the mixed air setpoint is set. The outdoor air controller senses the temperature at this node and attempts to control that temperature to the node setpoint.
Field: Actuator Node Name[LINK]
The name of the node that is associated with the outdoor air damper. This should be the outermost air node on the outdoor air path connected to the outdoor air stream for the mixer associated with this outdoor air controller.
Field: Minimum Outdoor Air Flow Rate[LINK]
Input for this field is the minimum outdoor air flow rate for the system in cubic meters per second. This field may be autosized. If a Controller Mechanical Ventilation Name is specified, note that this value times the Minimum Outdoor Air Schedule is a hard minimum that may override DCV or other advanced outdoor air controls.
Field: Maximum Outdoor Air Flow Rate[LINK]
Input for this field is the maximum outdoor air flow rate for the system in cubic meters per second.
Field: Economizer Control Type[LINK]
The options for this field are
FixedDryBulb
DifferentialDryBulb
FixedEnthalpy
DifferentialEnthalpy
ElectronicEnthalpy
FixedDewPointAndDryBulb
DifferentialDryBulbAndEnthalpy
NoEconomizer
Choosing NoEconomizer means the economizer will not operate and the outdoor airflow rate will be at the minimum for the entire simulation.
Choosing FixedDryBulb means the economizer will set the outdoor airflow rate at minimum if the outdoor air temperature is higher than a specified dry-bulb temperature limit.
Choosing DifferentialDryBulb will trigger the outdoor airflow to minimum when the dry-bulb temperature of outdoor air is higher than the dry-bulb temperature of the return air.
FixedEnthalpy checks the upper limit of the enthalpy given as a field input against the enthalpy content of outdoor air and will set the outdoor airflow rate to minimum if the latter is greater than the former.
DifferentialEnthalpy does the same thing but compares the return air enthalpy with the enthalpy of outdoor air. When the enthalpy of outdoor air is greater than the enthalpy of the return air, the outdoor air flow rate is set to minimum.
Choosing ElectronicEnthalpy enables the simulation to calculate the humidity ratio limit of outdoor air based on the dry-bulb temperature of outdoor air and a quadratic/cubic curve, and compare it to the actual outdoor air humidity ratio. If the actual outdoor humidity ratio is greater than the calculated humidity ratio limit, then the outdoor airflow rate is set to minimum.
Choosing FixedDewPointAndDryBulb compares both the outdoor dewpoint temperature and the outdoor dry-bulb temperature to their specified high limit values. If either outdoor temperature exceeds the high limit value, the outdoor airflow rate is set to minimum.
Another option DifferentialDryBulbAndEnthalpy enables the control strategy to be based on both the DifferentialDryBulb and DifferentialEnthalpy economizer control strategies.
In addition to all economizer control types listed above, each control type checks for user-entered values for the upper limit of dry-bulb temperature, enthalpy limit, humidity ratio limit and dewpoint limit. The outdoor air flow rate is set to minimum if any of these entered limits are exceeded.
The default for this field is NoEconomizer.
Field: Economizer Control Action Type[LINK]
There are two choices for this Field: MinimumFlowWithBypass and ModulateFlow, with the default being ModulateFlow if this input field is left blank.
ModulateFlow means the outdoor air flow rate will be increased to meet the mixed air setpoint temperature, subject to the limits imposed via other inputs for this object (e.g., Economizer Maximum Limit Dry-Bulb Temperature, Maximum Outdoor Air Flow Rate, etc.).
MinimumFlowWithBypass is used exclusively in conjunction with air-to-air heat exchanger:objects (Ref. HeatExchanger:*) for providing free cooling operation in the absence of a conventional air-side economizer (i.e., when outdoor air flow rate is not increased during economizer mode). The MinimumFlowWithBypass choice forces the outdoor air flow rate to always remain at the minimum. However, when high humidity control is used, the outdoor air flow rate is set to the product of the maximum outdoor air flow rate multiplied by the high humidity outdoor air flow ratio. The heat exchanger uses the limit checking in the outdoor air controller to decide whether or not to bypass the outdoor air around the heat exchanger or turn off the wheel motor in the case of a rotary heat exchanger. Heat exchange is also suspended when high humidity control is active.
The ModulateFlow option can also be used with the HeatExchanger:AirToAir:FlatPlate or HeatExchanger:AirToAir:SensibleAndLatent objects.
Field: Economizer Maximum Limit Dry-Bulb Temperature[LINK]
Input for this field is the outdoor air temperature high limit ( °C) for economizer operation. If the outdoor air temperature is above this limit, the outdoor airflow rate will be set to the minimum. This field is required if Economizer Control Type FixedDryBulb or FixedDewPointAndDryBulb has been specified.
No input (blank) in this field means that there is no outdoor air temperature high limit control. This limit applies to the conditions at the Actuator Node regardless of whether or not there are any other components in the outdoor air path upstream of the mixer. If non-blank, this limit is applied regardless of the specified Economizer Control Type.
Field: Economizer Maximum Limit Enthalpy[LINK]
Input for this field is the outdoor air enthalpy limit (in J/kg) for economizer operation. If the outdoor air enthalpy is above this value, the outdoor airflow rate will be set to the minimum. This field is required if Economizer Control Type FixedEnthalpy has been specified.
No input (blank) in this field means that there is no outdoor air enthalpy limit control. This limit applies to the conditions at the Actuator Node regardless of whether or not there are any other components in the outdoor air path upstream of the mixer. If non-blank, this limit is applied regardless of the specified Economizer Control Type.
Field: Economizer Maximum Limit Dewpoint Temperature[LINK]
Input for this field is the outdoor air dewpoint limit (°C) for economizer operation. If the outdoor air dewpoint temperature is above this value, the outdoor airflow rate will be set to the minimum. This field is required if the Economizer Control Type FixedDewPointAndDryBulb has been specified.
No input (blank) in this field means that there is no outdoor air dewpoint limit control. This limit applies to the conditions at the Actuator Node regardless of whether or not there are any other components in the outdoor air path upstream of the mixer. If non-blank, this limit is applied regardless of the specified Economizer Control Type.
Field: Electronic Enthalpy Limit Curve Name[LINK]
Input for this field is the name of a quadratic or cubic curve which provides the maximum outdoor air humidity ratio (function of outdoor air dry-bulb temperature) for economizer operation. If the outdoor air humidity ratio is greater than the curve’s maximum humidity ratio (evaluated at the outdoor air dry-bulb temperature), the outdoor air flow rate will be set to the minimum. This limit applies to the conditions at the Actuator Node regardless of whether or not there are any other components in the outdoor air path upstream of the mixer. No input (blank) in this field means that there is no electronic enthalpy limit control. If non-blank, this limit is applied regardless of the specified Economizer Control Type.
Field: Economizer Minimum Limit Dry-Bulb Temperature[LINK]
Input for this field is the outdoor air temperature low limit ( °C) for economizer operation. If the outdoor air temperature is below this limit, the outdoor airflow rate will be set to the minimum.
No input (blank) in this field means that there is no outdoor air temperature low limit control. This limit applies to the conditions at the Actuator Node regardless of whether or not there are any other components in the outdoor air path upstream of the mixer. If non-blank, this limit is applied regardless of the specified Economizer Control Type.
Field: Lockout Type[LINK]
Choices for this field are NoLockout, LockoutWithHeating, and LockoutWithCompressor. This field is used for packaged systems with DX coils. LockoutWithHeating means that if the packaged unit is in heating mode, the economizer is locked out i.e., the economizer dampers are closed and there is minimum outdoor air flow. LockoutWithCompressor means that in addition to locking out the economizer when the unit is in heating mode the economizer is locked out when the DX unit compressor is operating to provide cooling. In other words, the economizer must meet the entire cooling load; it is not allowed to operate in conjunction with the DX cooling coil. This option (LockoutWithCompressor) is sometimes called a nonintegrated economizer.
When LockoutWithHeating or LockoutWithCompressor is selected, the lockout may also be applied to non-packaged systems for heating. If any air loop heating coil is operating, the lockout control compares the mixed air temperature at minimum outdoor air flow without heat recovery (if any) to the mixed air temperature set point. If the mixed air temperature at minimum outdoor air flow is less than the mixed air temperature set point, then the economizer is locked out and the outdoor air flow rate is set to the minimum. When the economizer is locked out, the heat recovery bypass control will be set to activate heat recovery (no bypass), if present. This action is meant to minimize heating energy (this action may also disable the heating coil on subsequent iterations, see output variable Air System Outdoor Air Heat Recovery Bypass Heating Coil Activity Status).
The default is NoLockout.
Field: Minimum Limit Type[LINK]
Choices for this field are FixedMinimum or ProportionalMinimum. FixedMinimum means that the minimum outdoor airflow rate is fixed no matter what the actual system flow rate is. ProportionalMinimum means the minimum outdoor airflow rate varies in proportion to the total system air flow rate. The default is ProportionalMinimum.
Field: Minimum Outdoor Air Schedule Name[LINK]
The name of a schedule which uses decimal values (e.g., 0.0 or 1.0). These values are multiplied by the minimum outdoor air flow rate. This schedule is useful for reducing the outdoor air flow rate to zero during unoccupied or start up hours. If this field is not entered, the minimum outdoor air flow rate either remains constant during the simulation period (Minimum Outdoor Air Control Type = FixedMinimum) or varies in proportion to the supply air flow rate (Minimum Outdoor Air Control Type = ProportionalMinimum).
Field: Minimum Fraction of Outdoor Air Schedule Name[LINK]
The name of a schedule with decimal values which are limited between 0.0 and 1.0. The current schedule value is multiplied by the current system mixed air flow rate to set the minimum outdoor air flow rate. If this schedule name is blank, then it is not applied.
If this schedule is 1.0 (and there is no Maximum Fraction of Outdoor Air Schedule Name), then the system works at 100% outdoor air regardless of any other condition as long as the Maximum Outdoor Air Flow Rate is \(\ge\) the system supply air flow rate.
Field: Maximum Fraction of Outdoor Air Schedule Name[LINK]
The name of a schedule with decimal values which are limited between 0.0 and 1.0. The current schedule value is multiplied by the current system mixed air flow rate to set the maximum outdoor air flow rate. This schedule is applied after all other limits (except EMS and demand-limiting overrides). This schedule can override all minimums and economizer opearation. For example, if this schedule is zero, then the outdoor air flow rate will be zero, regardless of any other settings in Controller:OutdoorAir or Controller:MechanicalVentilation. If this schedule name is blank, then it is not applied.
Field: Mechanical Ventilation Controller Name[LINK]
This optional field is the name of the mechanical ventilation controller object to be used in conjunction with this outdoor air controller. The Controller:MechanicalVentilation object allows the user to define the minimum outdoor air flow rate based on air flow per unit floor area and air flow per person (occupant) for the zones being served by the air loop that utilizes this controller.
This feature allows the user to perform a first-order evaluation of carbon dioxide(CO\(_{2}\))-based demand controlled ventilation (outdoor ventilation varied according to occupancy levels).
If a valid name for a Controller:MechanicalVentilation object is entered in this field, the minimum outdoor air flow rate delivered will be the greater of:
the minimum outdoor air flow rate calculated by the fields Minimum Outdoor Air Flow Rate, Minimum Limit Type, and Minimum Outdoor Air Schedule Name as defined for this outdoor air controller, or
the outdoor air flow rate calculated using the Controller:MechanicalVentilation object named in this input field.
Leaving this input field blank will bypass the Controller:MechanicalVentilation object calculations and the minimum outdoor air flow rate will be based on the other inputs associated with this outdoor air controller object. Actual outdoor air flow rates may be higher than the minimum if free cooling is available and the object inputs are properly selected. Regardless, the maximum outdoor air flow rate is limited by the field Maximum Outdoor Air Flow Rate.
Note that the Minimum Outdoor Air Flow Rate times the Minimum Outdoor Air Schedule is a hard minimum that may override DCV or other advanced outdoor air controls specified in the Controller:MechanicalVentilation object. The Maximum Fraction of Outdoor Air Schedule (if specified) times the current system supply air flow rate may limit the outdoor air flow rate set by the Controller:MechanicalVentilation.
Field: Time of Day Economizer Control Schedule Name[LINK]
This alpha field is the name of a schedule which controls the outdoor air flow rate based on a time-of-day economizer. Schedule values equal to 0 disable this feature. Schedule values greater than 0 cause the outdoor air flow rate to increase to the maximum. When an economizer is used in conjunction with the high humidity control option, high humidity control has priority.
Field: High Humidity Control[LINK]
This choice field establishes whether or not the outdoor air flow rate is modified in response to high indoor relative humidity. Valid choices are Yes and No. If Yes is selected, the outdoor air flow rate may be modified when the indoor relative humidity is above the humidstat setpoint. If No is selected, this option is disabled and the following three fields are not used.
Field: Humidistat Control Zone Name[LINK]
This input defines the zone name where the humidistat is located. This is the same name used in the ZoneControl:Humidistat object. This field is required when the input field High Humidity Control is specified as Yes.
Field: High Humidity Outdoor Air Flow Ratio[LINK]
This input is the ratio of the modified outdoor air flow rate to the maximum outdoor air flow rate. When the high humidity control algorithm determines that the outdoor air flow rate will be changed (i.e., increased or decreased), the operating outdoor air flow rate is equal to the maximum outdoor air flow rate multiplied by this ratio. The minimum value for this field is 0. If this field is blank, the default value is 1. This field is used only when the input field High Humidity Control is specified as Yes. When an economizer is used in conjunction with the high humidity control option, high humidity control has priority.
Field: Control High Indoor Humidity Based on Outdoor Humidity Ratio[LINK]
This choice field determines if high humidity control is activated based on high indoor relative humidity alone or is activated only when the indoor relative humidity is above the humidstat setpoint and the indoor humidity ratio is greater than the outdoor humidity ratio. Valid choices are Yes and No. If No is selected, high humidity control is active any time the zone humidistat senses a moisture load. If Yes is selected, the model also verifies that the outdoor humidity ratio is less than the humidistat s zone air humidity ratio. This field is used only when the input field High Humidity Control is specified as Yes. The default value is Yes.
Field: Heat Recovery Bypass Control Type[LINK]
This choice field determines if specialized control is used to optimize the use of heat recovery. Valid choices are BypassWhenWithinEconomizerLimits and BypassWhenOAFlowGreaterThanMinimum. If BypassWhenWithinEconomizerLimits is selected, heat recovery is disabled any time the controller determines that the economizer is active (i.e., all controls are within limits). If BypassWhenOAFlowGreaterThanMinimum is selected, the model first verifies that the economizer is active and then checks to see if the outdoor air flow rate is greater than the minimum. If it is greater than minimum, then heat recovery (if any) is set to bypass. When this option is used with Time of Day Economizer Control or High Humidity Control, this option has priority. The default value is BypassWhenWithinEconomizerLimits.
An Example IDF specification:
Outputs[LINK]
Note that the key value for these outputs is the AirLoopHVAC name, not the name of the Controller:OutdoorAir.
HVAC,Average,Air System Outdoor Air Economizer Status []
HVAC,Average,Air System Outdoor Air Heat Recovery Bypass Status []
HVAC,Average,Air System Outdoor Air Heat Recovery Bypass Heating Coil Activity Status []
HVAC,Average,Air System Outdoor Air Heat Recovery Bypass Minimum Outdoor Air Mixed Air Temperature [C]
HVAC,Average,Air System Outdoor Air High Humidity Control Status []
HVAC,Average,Air System Outdoor Air Flow Fraction []
HVAC,Average,Air System Outdoor Air Minimum Flow Fraction []
HVAC,Average,Air System Outdoor Air Mass Flow Rate [kg/s]
HVAC,Average,Air System Mixed Air Mass Flow Rate [kg/s]
HVAC,Average,Air System Outdoor Air Maximum Flow Fraction []
HVAC,Average,Air System Outdoor Air Mechanical Ventilation Requested Mass Flow Rate [kg/s]
Air System Outdoor Air Economizer Status [][LINK]
Reports the average operating status of an air economizer over the reporting interval. The economizer status is set to 1 when the conditions are favorable for the economizer to operate (i.e., none of the control limits have been exceeded). While conditions may be favorable for economizer operation, it does not guarantee that the air-side economizer has increased outdoor air flow above the minimum level since the actual outdoor air flow rate is also governed by other controls (e.g., mixed air set point tempeature, time of day economizer control, maximum humidity setpoint, etc.). This variable is set to 0 if conditions disable economizer operation or NoEconomizer (Economizer Control Type) is specified.
Air System Outdoor Air Heat Recovery Bypass Status [][LINK]
This indicates if the controls have determined if the bypass mode for heat recovery is in effect or not.
Air System Outdoor Air Heat Recovery Bypass Heating Coil Activity Status [][LINK]
Reports the operating status of any heating coil in the air loop. If the heating coil is active, the heat exchanger will be activated (no air bypassed) and the heating energy will be reduced or eliminated. While conditions may be favorable for economizer operation, it does not guarantee that the air-side economizer has increased outdoor air flow above the minimum level since the actual outdoor air flow rate is also governed by other controls (e.g., mixed air set point temperature, time of day economizer control, maximum humidity setpoint, etc.). This variable is set to 0 if conditions disable economizer operation. This output variable is only available when using Heat Recovery Bypass Control Type = BypassWhenOAFlowGreaterThanMinimum.
Air System Outdoor Air Heat Recovery Bypass Minimum Outdoor Air Mixed Air Temperature [C][LINK]
Reports the outdoor air mixer s mixed air node temperature at minimum outdoor air flow rate when the heat exchanger is disabled (off). This temperature is calculated as the return air temperature multiplied by the return air mass flow rate plus the mixer s inlet node temperature multipled by the minimum outdoor air flow rate. This quantity is then divided by the mixed air mass flow rate. If this temperature is less than the outdoor air mixer s mixed air node set point temperature, and the Heat Recovery Bypass Control Type = BypassWhenOAFlowGreaterThanMinimum, the outdoor air flow rate is set to the minimum. This output variable is only available when using Heat Recvoery Bypass Control Type = BypassWhenOAFlowGreaterThanMinimum.
Air System Outdoor Air High Humidity Control Status [][LINK]
Reports the average operating status of the controller s high humidity control over the reporting interval. The high humidity control status is set to 1 when the controller determines that a zone high humidity condition exists according to the settings specified in the controller. This variable is set to 0 if conditions disable high humidity control operation or High Humidity Control is specified as No.
Air System Outdoor Air Flow Fraction [][LINK]
Reports the average actual outdoor air fraction for the outdoor air controller over the reporting interval.
Air System Outdoor Air Minimum Flow Fraction [][LINK]
Reports the average minimum limit of the outdoor air fraction for the outdoor air controller over the reporting interval.
Air System Outdoor Air Mass Flow Rate [kg/s][LINK]
Reports the average outdoor air mass flow rate introduced by the outdoor air controller over the reporting interval.
Air System Mixed Air Mass Flow Rate [kg/s][LINK]
Reports the average mixed air mass flow rate of the HVAC air loop associated with this outdoor air controller over the reporting interval.
Air System Outdoor Air Maximum Flow Fraction [][LINK]
Reports the average maximum limit of the outdoor air fraction for the outdoor air controller over the reporting interval. The maximum flow fraction is used to prevent DX cooling coils from freezing, specified by ASHRAE Stadard 90.1. This output variable is available when a corresponding SetpointManager:MixedAir object specifies optional inputs of Cooling Coil Inlet Node Name, Cooling coil Outlet Node Name, and Minimum Temperature at Cooling Coil Outlet Node.
Air System Outdoor Air Mechanical Ventilation Requested Mass Flow Rate [kg/s][LINK]
Reports the average outdoor air mass flow rate requested by the Mechanical Ventilation Controller (Controller:MechanicalVentilation, if specified) over the reporting interval.
Controller:MechanicalVentilation[LINK]
This object is used in conjunction with an outdoor air controller (Ref. Controller:OutdoorAir, Field: Mechanical Ventilation Controller Name) to establish the minimum outdoor air flow rate provided by a mixed air box.
Ventilation standards provide guidance on appropriate levels of outdoor ventilation air required for acceptable indoor air quality. The Ventilation Rate Procedure (VRP) of ASHRAE Standard 62.1-2007/2010 (www.ashrae.org) requires outdoor ventilation rates to be determined based on the floor area of each occupied zone plus the number of people in each zone and considers the zone air distribution effectiveness and system ventilation efficiency. The outdoor air ventilation rate can be reset dynamically as operating conditions change (e.g., variations in occupancy). The Controller:MechanicalVentilation object implements the VRP for calculating these outdoor air ventilation requirements and resetting them based on varying occupancy levels and zone diversification. This is particularly useful for large air distribution systems that serve a number of different zone types with varying occupancy levels. This object can also be used to model the Indoor Air Quality Procedure (IAQP) as defined in Standard 62.1
The first five inputs for this object are the name, the availability schedule, the zone outdoor air method, the system outdoor air method, and the zone maximum outdoor air fraction. The next three input fields define the zone name (or zone list name), the design specification outdoor air object name, and the design specification zone air distribution object name to be applied to this zone (or zone list). The last three fields are extensible
Inputs[LINK]
Field: Name[LINK]
The unique user assigned name for an instance of mechanical ventilation. Any other object referencing this mechanical ventilation object will use this name.
Field: AvailabilitySchedule Name[LINK]
The name of a schedule whose values are greater than 0 when mechanical ventilation, as calculated by this object, is desired. If the schedule s value is 0.0, then mechanical ventilation is not available and flow will not be requested. If the schedule s value is > 0.0 (usually 1 is used), mechanical ventilation is available. If this field is blank, the schedule has values of 1 for all time periods. This schedule is useful for purging the building of contaminants prior to occupancy (i.e., ventilation rate per unit floor area will be provided even if the occupancy is zero).
Field: Demand Controlled Ventilation[LINK]
This field indicates whether the air loop is capable of doing demand controlled ventilation (DCV) to vary the amount of outdoor air based on actual number of occupants in spaces. Two choices: Yes and No. Default is No.
Field: System Outdoor Air Method[LINK]
The method used to calculate the system minimum outdoor air flow. Several choices are allowed: ZoneSum, VentilationRateProcedure, IndoorAirQualityProcedure, ProportionalControlBasedonOccupancySchedule, ProportionalControlBasedonDesignOccupancy, ProportionalControlBasedOnDesignOARate, and IndoorAirQualityProcedureGenericContaminant. ZoneSum sums the outdoor air flows across all zones served by the system. VentilationRateProcedure (VRP) uses the multi-zone equations defined in 62.1-2007 to calculate the system outdoor air flow. VRP considers zone air distribution effectiveness and zone diversification of outdoor air fractions. IndoorAirQualityProcedure (IAQP) is the other procedure defined in ASHRAE Standard 62.1-2007 for calculate the amount of outdoor air necessary to maintain the levels of indoor air carbon dioxide at or below the setpoint defined in the ZoneControl:ContaminantController object. Appendix A of the ASHRAE 62.1-2010 user’s manual discusses another method for implementing CO\(_{2}\)-based DCV in a single zone system. This method (Proportional Control) calculates the required outdoor air flow rate which varies in proportion to the percentage of the CO\(_{2}\) signal range and has two choices to calculate occupancy-based outdoor air rate. The ProportionalControlBasedonOccupancySchedule choice uses the real occupancy at the current time step to calculate outdoor air rate, while the ProportionalControlBasedonDesignOccupancy uses the design occupancy level to calculate outdoor air rate. The former choice is a good approach to estimate outdoor air rate. However, for practical applications, the zone controller usually does not have the real time occupancy information, and the design occupancy level is assumed. The latter choice is used in the design stage. The ProportionalControlBasedOnDesignOARate uses design outdoor air flow rate to calculate demand outdoor air flow rate. The IndoorAirQualityProcedure-GenericContaminant method calculates the amount of outdoor air necessary to maintain the levels of indoor air generic contaminant at or below the setpoint defined in the ZoneControl:ContaminantController object.
Note: When System Outdoor Air Method = IndoorAirQualityProcedure or IndoorAirQualityProcedureGenericContaminant is specified, only the Zone <x> Name fields are used. The other field inputs described below are not used.
Field: Zone Maximum Outdoor Air Fraction[LINK]
This positive numeric input is the zone maximum outdoor air fraction. For VAV systems, when a zone requires outdoor air higher than the user specified Zone Maximum Outdoor Air Fraction, the zone supply air flow will be increased (if damper not fully open yet) to cap the outdoor air fraction at the maximum value. This allows the system level outdoor air flow to be reduced while the total supply air flow increases to meet zone outdoor air requirement. Valid values are from 0 to 1.0. Default is 1.0 which indicates zones can have 100% outdoor air maintaining backward compatibility. This inputs work for single and dual duct VAV systems.
Field Set (Zone Name, Design Specification Outdoor Air Object Name, and Design Specification Zone Air Distribution Object Name)[LINK]
The following three fields are needed to define the parameters for the ventilation. This object is extensible by duplicating these three fields.
Field:Zone <x> Name[LINK]
The zone name or zone list to apply the ventilation rates specified in the DesignSpecification:OutdoorAir object.
Field: Design Specification Outdoor Air Object Name <x>[LINK]
The name of the DesignSpecification:OutdoorAir object, defining the amount of outdoor air, that applies to the zone or zone list. If this field is blank, the corresponding DesignSpecification:OutdoorAir object for the zone will come from the DesignSpecification:OutdoorAir object referenced by the Sizing:Zone object for the same zone. If no such zone match is found, default values from the IDD will be used for the DesignSpecification:OutdoorAir object which is 0.0094 m3/s-person.
If an Outdoor Air Schedule Name is specified in the DesignSpecification:OutdoorAir object, the schedule will be applied to all types of outdoor air calculations for the corresponding zone, regardless of the System Outdoor Air Method selected. If the schedule value is zero, then the zone will be completely removed from the system outdoor air calcaulations.
Field: Design Specification Zone Air Distribution Object Name <x>[LINK]
The name of the DesignSpecification:ZoneAirDistribution object, defining the air distribution effectiveness and secondary recirculation air fraction, that applies to the zone or zone list. If this field is blank, the corresponding DesignSpecification:ZoneAirDistribution object for the zone will come from the DesignSpecification:ZoneAirDistribution object referenced by the Sizing:Zone object for the same zone. If no such zone match is found, default values from the IDD will be used for the DesignSpecification:ZoneAirDistribution object which are effectiveness = 1.0 and recirculation = 0.0.
As described previously, the Controller:MechanicalVentilation object works in conjunction with Controller:OutdoorAir. As such, the minimum quantity of outdoor air delivered via the mixed air box will be the greater of:
the minimum outdoor air flow rate calculated by the fields Minimum Outdoor Air Flow Rate, Minimum Limit and Minimum Outdoor Air Schedule Name in the associated Controller:OutdoorAir object, or
the outdoor air flow rate calculated by this object.
The actual outdoor air flow rate may be higher than the minimum if free cooling is available. Regardless, the outdoor air flow rate will not exceed the Maximum Outdoor Air Flow Rate specified in the associated Controller:OutdoorAir object or the Maximum Fraction of Outdoor Air Schedule (if specified) times the current system supply air flow rate.
An example input for this object is shown below:
ZoneHVAC:EnergyRecoveryVentilator:Controller[LINK]
This controller object is used exclusively by the stand alone energy recovery ventilator (ZoneHVAC:EnergyRecoveryVentilator, see Figure [fig:schematic-of-the-zonehvac]). The purpose of this controller is to simulate economizer operation for the stand alone ERV and provide free cooling whenever possible or modify the outdoor air flow rate during high indoor humidity conditions. During economizer operation, if all of the limits are satisfied, the controller activates economizer mode (fully bypassing the fixed-plate air-to-air heat exchanger or stopping the rotation of a rotary heat exchanger). If any of the selected limits are exceeded, economizer operation is terminated. A time-of-day schedule may also be used to simulate a push-button type economizer controller.
In addition, the outdoor air flow rate may be modified and heat exchange suspended in response to high indoor relative humidities. When the indoor relative humidity exceeds the zone humidistat s relative humidity set point, high humidity control is activated. When activated, if high humidity control is based on the outdoor air humidity ratio and the outdoor humidity ratio is greater than the indoor humidity ratio, high humidity control is terminated.
Inputs[LINK]
Field: Name[LINK]
A unique user-assigned name for the stand alone ERV controller. Any reference to this controller by another object will use this name.
Field: Temperature High Limit[LINK]
The input for this field is the outdoor air temperature high limit ( °C) for economizer operation. If the outdoor air temperature is above this limit, economizer (free cooling) operation is terminated. No input (blank) in this field means that there is no outdoor air temperature high limit control.
Field: Temperature Low Limit[LINK]
The input for this field is the outdoor air temperature low limit ( °C) for economizer operation. If the outdoor air temperature is below this limit, economizer (free cooling) operation is terminated. No input (blank) in this field means that there is no outdoor air temperature low limit control.
Field: Enthalpy High Limit[LINK]
The input for this field is the outdoor air enthalpy limit (in J/kg) for economizer operation. If the outdoor air enthalpy is above this value, economizer (free cooling) operation is terminated. No input (blank) in this field means that there is no outdoor air economizer limit control.
Field: Dewpoint Temperature Limit[LINK]
Input for this field is the outdoor air dewpoint limit ( °C) for economizer operation. If the outdoor air dewpoint temperature is above this value, the outdoor airflow rate will be set to the minimum. No input (blank) in this field means that there is no outdoor air dewpoint limit control. This limit applies to the conditions at the Actuated Node regardless of whether or not there are any other components on the outdoor air path upstream of the mixer.
Field: Electronic Enthalpy Limit Curve Name[LINK]
Input for this field is the name of a quadratic or cubic curve which provides the maximum outdoor air humidity ratio (function of outdoor air dry-bulb temperature) for economizer operation. If the outdoor air humidity ratio is greater than the curve s maximum humidity ratio (evaluated at the outdoor air dry-bulb temperature), the outdoor air flow rate will be set to the minimum. This limit applies to the conditions at the Actuated Node regardless of whether or not there are any other components on the outdoor air path upstream of the mixer. No input (blank) in this field means that there is no electronic enthalpy limit control.
Field: Exhaust Air Temperature Limit[LINK]
This input establishes whether or not there is a limit control on the exhaust air temperature. The choices are ExhaustAirTemperatureLimit or NoExhaustAirTemperatureLimit. If ExhaustAirTemperatureLimit is chosen, the controller deactivates economizer mode whenever the outdoor air temperature is greater than the exhaust air temperature. If NoExhaustAirTemperatureLimit is chosen, no limit check on the exhaust air temperature is performed.
Field: Exhaust Air Enthalpy Limit[LINK]
This input establishes whether or not there is a limit control on the exhaust air enthalpy. The choices are ExhaustAirEnthalpyLimit or NoExhaustAirEnthalpyLimit. If ExhaustAirEnthalpyLimit is chosen, the controller deactivates economizer mode whenever the outdoor air enthalpy is greater than the exhaust air enthalpy. If NoExhaustAirEnthalpyLimit is chosen, no limit check on the exhaust air enthalpy is performed.
Field: Time of Day Economizer Flow Control Schedule Name[LINK]
This alpha field is the name of a schedule which controls the change in air flow rate based on time-of-day. Schedule values equal to 0 disable this feature. Schedule values greater than 0 activate the economizer. Note that heat exchange between the air streams is suspended when the economizer is active. This schedule can be used with or without the high humidity control option. When an economizer is used in conjunction with the high humidity control option, high humidity control has priority.
Field: High Humidity Control Flag[LINK]
This optional choice field establishes whether or not the supply and exhaust air flow rates are modified in response to high indoor relative humidity. Valid choices are Yes and No. If Yes is selected, the supply and exhaust air flow rates may be modified when the indoor relative humidity is above the humidstat set point. If No is selected, this option is disabled and the following three fields are not used. Note that heat exchange between the air streams is suspended during times when high humidity control is active. The default value is No.
Field: Humidistat Control Zone Name[LINK]
This optional input defines the zone name where the humidistat is located. This is the same zone name used in the Zone Control:Humidistat object. This field is required when the High Humidity Control Flag is specified as Yes.
Field: High Humidity Outdoor Air Flow Ratio[LINK]
This optional input is the ratio of the modified supply (outdoor) air flow rate to the supply air flow rate specified in the Energy Recovery Ventilator:Stand Alone ERV object. When the high humidity control algorithm determines that the supply air flow rate will be changed (i.e. increased or decreased), the operating supply air flow rate is equal to the supply air flow rate specified in the Energy Recovery Ventilator:Stand Alone ERV object multiplied by this ratio. The minimum value for this field is 0. This field is used to modify both the supply and exhasut air flow rates when high humidity control is active. The supply and exhasut air fan volumetric flow rates must be able to account for the increase in air flow when this input is greater than 1. The default value is 1.
Field: Control High Indoor Humidity based on Outdoor Humidity Ratio[LINK]
This optional choice field determines if high humidity control is activated based on high indoor relative humidity alone or is activated only when the indoor relative humidity is above the humidstat set point and the outdoor humidity ratio is less than the indoor humidity ratio. Valid choices are Yes and No. If No is selected, high humidity control is active any time the zone humidistat senses a moisture load. If yes is selected, the model also verifies that the outdoor humidity ratio is less than the humidistat s zone air humidity ratio. This field is used only when the High Humidity Control Flag is specified as Yes. The default value is Yes.
Following is an example input for this stand alone ERV controller object:
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This documentation is made available under the EnergyPlus Open Source License v1.0.