Branches, Connectors, and Nodes[LINK]

In EnergyPlus, the HVAC system and plant form a network (technically, a graph). The individual pieces of equipment - the fans, coils, chillers, etc. - are connected together by air ducts and fluid pipes. In EnergyPlus nomenclature, the air and fluid circuits are called loops. Specifying how an individual system and plant are connected is done in the EnergyPlus input (IDF) file. The overall structure of the network is defined with Branch and Connector objects. The detail is filled with components and their inlet and outlet nodes. A Branch consists of one or more components arranged sequentially along a pipe or duct. A Connector specifies how three or more branches are connected through a Splitter or Mixer. Nodes connect components along a branch: the outlet node of one component is the inlet node of the next downstream component. The nodes represent conditions at a point on a loop. Each component has one or more inlet and outlet nodes, depending on how many loops it interacts with. A fan, for instance, has one inlet node and one outlet node, since it interacts with a single air loop. A water coil will have 2 inlet and 2 outlet nodes, since it interacts with an air and a fluid loop. Figure 1 shows a diagram of an EnergyPlus HVAC input.

As an illustration of how such a network is built up on the IDF, here is the section of the IDF that describes the supply fan, splitter, and heating and cooling coil section of the dual duct air system.

BranchList,
    Dual Duct Air Loop Branches,  !- Name
    Air Loop Main Branch,    !- Branch 1 Name
    Heating Coil Air Sys Branch,  !- Branch 2 Name
    Cooling Coil Air Sys Branch;  !- Branch 3 Name

ConnectorList,
    Dual Duct Connectors,    !- Name
    Connector:Splitter,      !- Connector 1 Object Type
    DualDuctAirSplitter;     !- Connector 1 Name

NodeList,
    Zone Equipment Inlet Node List,  !- Name
    Main Hot Air Inlet,      !- Node 1 Name
    Main Cold Air Inlet;     !- Node 2 Name

NodeList,
    Air Loop Outlet Node List,  !- Name
    Heating Coil Outlet Node,!- Node 1 Name
    Cooling Coil Outlet Node;!- Node 2 Name

Branch,
    Air Loop Main Branch,    !- Name
    autosize,                !- Maximum Flow Rate {m3/s}
    Fan:ConstantVolume,      !- Component 1 Object Type
    Supply Fan 1,            !- Component 1 Name
    Supply Fan Inlet Node,   !- Component 1 Inlet Node Name
    Supply Fan Outlet Node,  !- Component 1 Outlet Node Name
    PASSIVE;                 !- Component 1 Branch Control Type

Branch,
    Heating Coil Air Sys Branch,  !- Name
    autosize,                !- Maximum Flow Rate {m3/s}
    Coil:Heating:Water,      !- Component 1 Object Type
    Main Heating Coil,       !- Component 1 Name
    Heating Coil Inlet Node, !- Component 1 Inlet Node Name
    Heating Coil Outlet Node,!- Component 1 Outlet Node Name
    ACTIVE;                  !- Component 1 Branch Control Type

Branch,
    Cooling Coil Air Sys Branch,  !- Name
    autosize,                !- Maximum Flow Rate {m3/s}
    Coil:Cooling:Water,      !- Component 1 Object Type
    Simple Cooling Coil,     !- Component 1 Name
    Cooling Coil Inlet Node, !- Component 1 Inlet Node Name
    Cooling Coil Outlet Node,!- Component 1 Outlet Node Name
    ACTIVE;                  !- Component 1 Branch Control Type

Connector:Splitter,
    DualDuctAirSplitter,     !- Name
    Air Loop Main Branch,    !- Inlet Branch Name
    Heating Coil Air Sys Branch,  !- Outlet Branch 1 Name
    Cooling Coil Air Sys Branch;  !- Outlet Branch 2 Name

Fan:ConstantVolume,
    Supply Fan 1,            !- Name
    FanAndCoilAvailSched,    !- Availability Schedule Name
    0.7,                     !- Fan Efficiency
    600.0,                   !- Pressure Rise {Pa}
    autosize,                !- Maximum Flow Rate {m3/s}
    0.9,                     !- Motor Efficiency
    1.0,                     !- Motor In Airstream Fraction
    Supply Fan Inlet Node,   !- Fan Inlet Node Name
    Supply Fan Outlet Node;  !- Fan Outlet Node Name

Coil:Cooling:Water,
    Simple Cooling Coil,     !- Name
    CoolingCoilAvailSched,   !- Availability Schedule Name
    autosize,                !- Design Water Flow Rate {m3/s}
    autosize,                !- Design Air Flow Rate {m3/s}
    autosize,         !- Design Inlet Water Temperature {C}
    autosize,         !- Design Inlet Air Temperature {C}
    autosize,         !- Design Outlet Air Temperature {C}
    autosize,     !- Design Inlet Air Humidity Ratio {kg-H2O/kg-air}
    autosize,     !- Design Outlet Air Humidity Ratio {kg-H2O/kg-air}
    Cooling Coil Water Inlet Node,  !- Water Inlet Node Name
    Cooling Coil Water Outlet Node,  !- Water Outlet Node Name
    Cooling Coil Inlet Node, !- Air Inlet Node Name
    Cooling Coil Outlet Node,!- Air Outlet Node Name
    SimpleAnalysis,          !- Type of Analysis
    CrossFlow;               !- Heat Exchanger Configuration

AirLoopHVAC:ZoneSplitter,
    ZoneColdAirSupplySplitter,  !- Name
    Main Cold Air Inlet,     !- Inlet Node Name
    Zone 1 Dual Duct Cold Inlet,  !- Outlet 1 Node Name
    Zone 2 Dual Duct Cold Inlet,  !- Outlet 2 Node Name
    Zone 3 Dual Duct Cold Inlet;  !- Outlet 3 Node Name

AirLoopHVAC:ZoneSplitter,
    ZoneHotAirSupplySplitter,!- Name
    Main Hot Air Inlet,      !- Inlet Node Name
    Zone 1 Dual Duct Hot Inlet,  !- Outlet 1 Node Name
    Zone 2 Dual Duct Hot Inlet,  !- Outlet 2 Node Name
    Zone 3 Dual Duct Hot Inlet;  !- Outlet 3 Node Name

AirLoopHVAC:ZoneMixer,
    ZoneReturnAirMixer,      !- Name
    Return Air Mixer Outlet, !- Outlet Node Name
    Zone 1 Outlet Node,      !- Inlet 1 Node Name
    Zone 2 Outlet Node,      !- Inlet 2 Node Name
    Zone 3 Outlet Node;      !- Inlet 3 Node Name

Coil:Heating:Water,
    Main Heating Coil,       !- Name
    FanAndCoilAvailSched,    !- Availability Schedule Name
    autosize,                !- U-Factor Times Area Value {W/K}
    autosize,                !- Maximum Water Flow Rate {m3/s}
    Heating Coil Water Inlet,!- Water Inlet Node Name
    Heating Coil Water Outlet,  !- Water Outlet Node Name
    Heating Coil Inlet Node, !- Air Inlet Node Name
    Heating Coil Outlet Node,!- Air Outlet Node Name
    UFactorTimesAreaAndDesignWaterFlowRate,  !- Performance Input Method
    autosize,                !- Nominal Capacity {W}
    82.2,                    !- Design Inlet Water Temperature {C}
    16.6,                    !- Design Inlet Air Temperature {C}
    71.1,                    !- Design Outlet Water Temperature {C}
    32.2;                    !- Design Outlet Air Temperature {C}

Controller:WaterCoil,
    Main Cooling Coil Controller,  !- Name
    Temperature,             !- Control Variable
    Reverse,                 !- Action
    FLOW,                    !- Actuator Variable
    Cooling Coil Outlet Node,!- Sensor Node Name
    Cooling Coil Water Inlet Node,  !- Actuator Node Name
    0.001,             !- Controller Convergence Tolerance {deltaC}
    autosize,                !- Maximum Actuated Flow {m3/s}
    0.0;                     !- Minimum Actuated Flow {m3/s}

Controller:WaterCoil,
    Main Heating Coil Controller,  !- Name
    Temperature,             !- Control Variable
    Normal,                  !- Action
    FLOW,                    !- Actuator Variable
    Heating Coil Outlet Node,!- Sensor Node Name
    Heating Coil Water Inlet,!- Actuator Node Name
    0.01,              !- Controller Convergence Tolerance {deltaC}
    autosize,                !- Maximum Actuated Flow {m3/s}
    0.0;                     !- Minimum Actuated Flow {m3/s}

Obviously, the creation of such a system/plant network description is best handled by a graphical user interface (GUI). However, for testing purposes a developer may have to create the input for a component by hand and insert it into an existing IDF. Then the developer must be careful to choose unique names for the branches and nodes and make sure the entire network makes physical sense.