For ground-contact surfaces in EnergyPlus, it is important
to specify appropriate ground temperatures. Do not use the
“undisturbed” ground temperatures from the weather data. These
values are too extreme for the soil under a conditioned
building. For best results, use the Slab or Basement program
described in this section to calculate custom monthly average
ground temperatures. This is especially important for
residential applications and very small buildings. If one of
these ground temperature preprocessors is not used, for
typical commercial buildings in the USA, a reasonable default
value is 2C less than the average indoor space
temperature.
There are two difficulties behind linking ground heat
transfer calculations to EnergyPlus. One is the fact that the
conduction calculations in EnergyPlus (and in DOE-2 and BLAST
previously) are one-dimensional and the ground heat transfer
calculations are two or three-dimensional. This causes severe
modeling problems irrespective of the methods being used for
the ground heat transfer calculation. The other difficulty is
the markedly different time scales involved in the processes.
Basically, the zone model is on an hour scale and the ground
heat transfer is on a monthly time scale. The basic heat
balance based zone model of EnergyPlus has to be considered as
the foundation for building energy simulation at the present
time and for some time in the future. Thus, it is necessary to
be able to relate ground heat transfer calculations to that
model.
The heat balance zone model considers a single room or
thermal zone in a building and performs a heat balance on it.
A fundamental modeling assumption is that the faces of the
enclosure are isothermal planes. A ground heat transfer
calculation usually considers an entire building and the earth
that surrounds it, resulting in non-isothermal face planes
where there is ground contact. While it is not impossible to
imagine multi-zone, whole building models that include the
surrounding earth and non-isothermal building surfaces, such
models will not be practical for some time in the future, and
their usefulness even then is not clear.
The EnergyPlus development team addressed the problem and
decided that the most reasonable first step would be to
partially decouple the ground heat transfer calculation from
the thermal zone calculation. The most important parameter for
the zone calculation is the outside face temperature of the
building surface that is in contact with the ground. Thus this
becomes a reasonable “separation plane” for the two
calculations. It was further decided that the current usage of
monthly average ground temperature was reasonable for this
separation plane temperature as well, since the time scales of
the building heat transfer processes are so much shorter than
those of the ground heat transfer processes.
Using the separation plane premise, the 3D ground heat
transfer programs for slabs developed by Bahnfleth (1989,
1990) were modified by Clements (2004) to produce outside face
temperatures. EnergyPlus permits separate monthly average
inside temperatures as input. The program produces outside
face temperatures for the core area and the perimeter area of
the slab. It is described in the section “Use of the Ground
Temperatures with Slabs” below.
A 3D basement program also is included with EnergyPlus.
This is described below in Using Ground Temperatures with
Basements. It uses the same principle as the slab procedure;
it determines the outside face (surface) temperature of the
walls and floor of a basement in contact with the ground.
It should be noted that either for slabs or basements the
ground heat transfer is usually small unless the building is
very small or has some special characteristics.
Multiple Ground Temperatures shows how the
OtherSideCoefficients object of EnergyPlus can be used to
supply multiple ground temperatures.
The Slab program produces temperature profiles for the
outside surface at the core and at the perimeter of the slab.
It also produces the average based on the perimeter and core
areas used in the calculation. This allows the user to apply
the Slab temperatures one of two ways in EnergyPlus:
Option 1 - Core and Perimeter Temperatures: The
EnergyPlus OtherSideCoefficients object can be used to provide
two sets of twelve monthly average ground temperature values.
In this way, both the perimeter and the core values from the
Slab program can be used in the succeeding EnergyPlus run.
This method assumes that the floor slab will be described
using at least two different heat transfer surfaces. The use
of OtherSideCoefficients to provide additional ground contact
surfaces is described in detail in the “Multiple Ground
Temperatures” section below.
Option 2 - Average Temperatures: Use the monthly
average temperatures produced by the Slab program in the
EnergyPlus GroundTemperatures object. This will provide an
average ground temperature at the outside face of any heat
transfer surface whose OutsideFaceEnvironment field is set to
“ground”.
EnergyPlus accepts twelve separate monthly average inside
temperatures. In addition, it is possible to add an hourly
sinusoidal variation of the inside temperature with a 24 hour
period sine function. This was included to show the effect of
something such as night setback on the face temperature.
Generally, the effect is quite small.
First the results for a monthly specified constant average
inside temperature. The location is Minneapolis, and the slab
is insulated.
The resulting heat flux is shown below. The inside heat
transfer coefficient and slab thermal properties are specified
in the input file. For this example the total thermal
resistance from the inside air to the slab bottom surface was
0.27 (m\(^{2}\) C)/W. This
value is controlled by the user with the inside heat transfer
coefficient and slab thermal properties values in the slab
program input file.
Month
Average
Perimeter
Core
Inside
Perimeter Heat Flux W/m
Average Heat Flux W/m
1
17.67
16.11
18.03
18
7.00
1.22
2
17.45
15.92
17.81
18
7.70
2.04
3
17.43
16.07
17.74
18
7.15
2.11
4
19
17.82
19.27
20
8.07
3.70
5
19.24
18.23
19.48
20
6.56
2.81
6
19.31
18.42
19.52
20
5.85
2.56
7
20.92
20.14
21.11
22
6.89
4.00
8
21.17
20.44
21.35
22
5.78
3.07
9
21.22
20.45
21.4
22
5.74
2.89
10
21.21
20.26
21.44
22
6.44
2.93
11
19.62
18.54
19.88
20
5.41
1.41
12
19.35
17.99
19.67
20
7.44
2.41
Then for the same conditions, the results with a 2 degree C
amplitude 24-hour sine wave variation. Notice that the inside
temperatures are the same since they are monthly averages and
the daily variation oscillates about the mean. The core and
perimeter slab temperatures are affected slightly.
A plot of the daily profiles is shown below. Note that the
inside temperature change of 4 C produces only a small change
in the slab lower face temperature.
Daily Temperature Profiles
(Slab) [fig:daily-temperature-profiles-slab]
The resulting heat fluxes are shown below. They can be
compared with the fluxes shown above for the constant inside
temperature run. The changes resulting from a fairly large 4 C
daily temperature variation are probably not significant.
Caution[LINK]
For ground-contact surfaces in EnergyPlus, it is important to specify appropriate ground temperatures. Do not use the “undisturbed” ground temperatures from the weather data. These values are too extreme for the soil under a conditioned building. For best results, use the Slab or Basement program described in this section to calculate custom monthly average ground temperatures. This is especially important for residential applications and very small buildings. If one of these ground temperature preprocessors is not used, for typical commercial buildings in the USA, a reasonable default value is 2C less than the average indoor space temperature.
Introduction[LINK]
There are two difficulties behind linking ground heat transfer calculations to EnergyPlus. One is the fact that the conduction calculations in EnergyPlus (and in DOE-2 and BLAST previously) are one-dimensional and the ground heat transfer calculations are two or three-dimensional. This causes severe modeling problems irrespective of the methods being used for the ground heat transfer calculation. The other difficulty is the markedly different time scales involved in the processes. Basically, the zone model is on an hour scale and the ground heat transfer is on a monthly time scale. The basic heat balance based zone model of EnergyPlus has to be considered as the foundation for building energy simulation at the present time and for some time in the future. Thus, it is necessary to be able to relate ground heat transfer calculations to that model.
The heat balance zone model considers a single room or thermal zone in a building and performs a heat balance on it. A fundamental modeling assumption is that the faces of the enclosure are isothermal planes. A ground heat transfer calculation usually considers an entire building and the earth that surrounds it, resulting in non-isothermal face planes where there is ground contact. While it is not impossible to imagine multi-zone, whole building models that include the surrounding earth and non-isothermal building surfaces, such models will not be practical for some time in the future, and their usefulness even then is not clear.
The EnergyPlus development team addressed the problem and decided that the most reasonable first step would be to partially decouple the ground heat transfer calculation from the thermal zone calculation. The most important parameter for the zone calculation is the outside face temperature of the building surface that is in contact with the ground. Thus this becomes a reasonable “separation plane” for the two calculations. It was further decided that the current usage of monthly average ground temperature was reasonable for this separation plane temperature as well, since the time scales of the building heat transfer processes are so much shorter than those of the ground heat transfer processes.
Using the separation plane premise, the 3D ground heat transfer programs for slabs developed by Bahnfleth (1989, 1990) were modified by Clements (2004) to produce outside face temperatures. EnergyPlus permits separate monthly average inside temperatures as input. The program produces outside face temperatures for the core area and the perimeter area of the slab. It is described in the section “Use of the Ground Temperatures with Slabs” below.
A 3D basement program also is included with EnergyPlus. This is described below in Using Ground Temperatures with Basements. It uses the same principle as the slab procedure; it determines the outside face (surface) temperature of the walls and floor of a basement in contact with the ground.
It should be noted that either for slabs or basements the ground heat transfer is usually small unless the building is very small or has some special characteristics.
Multiple Ground Temperatures shows how the OtherSideCoefficients object of EnergyPlus can be used to supply multiple ground temperatures.
Use of the Ground Temperatures with Slabs[LINK]
The Slab program produces temperature profiles for the outside surface at the core and at the perimeter of the slab. It also produces the average based on the perimeter and core areas used in the calculation. This allows the user to apply the Slab temperatures one of two ways in EnergyPlus:
Option 1 - Core and Perimeter Temperatures: The EnergyPlus OtherSideCoefficients object can be used to provide two sets of twelve monthly average ground temperature values. In this way, both the perimeter and the core values from the Slab program can be used in the succeeding EnergyPlus run. This method assumes that the floor slab will be described using at least two different heat transfer surfaces. The use of OtherSideCoefficients to provide additional ground contact surfaces is described in detail in the “Multiple Ground Temperatures” section below.
Option 2 - Average Temperatures: Use the monthly average temperatures produced by the Slab program in the EnergyPlus GroundTemperatures object. This will provide an average ground temperature at the outside face of any heat transfer surface whose OutsideFaceEnvironment field is set to “ground”.
EnergyPlus accepts twelve separate monthly average inside temperatures. In addition, it is possible to add an hourly sinusoidal variation of the inside temperature with a 24 hour period sine function. This was included to show the effect of something such as night setback on the face temperature. Generally, the effect is quite small.
First the results for a monthly specified constant average inside temperature. The location is Minneapolis, and the slab is insulated.
The resulting heat flux is shown below. The inside heat transfer coefficient and slab thermal properties are specified in the input file. For this example the total thermal resistance from the inside air to the slab bottom surface was 0.27 (m\(^{2}\) C)/W. This value is controlled by the user with the inside heat transfer coefficient and slab thermal properties values in the slab program input file.
Then for the same conditions, the results with a 2 degree C amplitude 24-hour sine wave variation. Notice that the inside temperatures are the same since they are monthly averages and the daily variation oscillates about the mean. The core and perimeter slab temperatures are affected slightly.
An example of a 24-hour inside temperature profile for this case is shown below. The sine wave amplitude was 2 C.
A plot of the daily profiles is shown below. Note that the inside temperature change of 4 C produces only a small change in the slab lower face temperature.
The resulting heat fluxes are shown below. They can be compared with the fluxes shown above for the constant inside temperature run. The changes resulting from a fairly large 4 C daily temperature variation are probably not significant.
Documentation content copyright © 1996-2026 The Board of Trustees of the University of Illinois and the Regents of the University of California through the Ernest Orlando Lawrence Berkeley National Laboratory. All rights reserved. EnergyPlus is a trademark of the US Department of Energy.
This documentation is made available under the EnergyPlus Open Source License v1.0.