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Zone temperature lower than outside

asked 2020-03-18 12:01:56 -0600

WannesVDV's avatar

updated 2020-03-18 12:50:32 -0600

I'm simulating a simple 2 zone building in EnergyPlus. I've noticed at night the temperature in the zones is lower than the outside temperature, how is this possible? There are non a HVAC equipment defined in the EnergyPlus model. C:\fakepath\Temperature zones.PNG

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answered 2020-03-18 12:56:32 -0600

updated 2020-03-19 11:07:44 -0600

Without looking at the rest of your IDF, it is likely because of the floor surfaces exchanging heat with the soil below. These temperatures can be colder than the outside air temperature. To confirm this, you can add an output variable column for the "Surface Outside Face Temperature" variable and key name input set to the name of the floor surface objects in the model (example below).

  <name of floor surface object>,        !- Key Name
  Surface Outside Face Temperature,      !- Variable Name
  timestep;                              !- Reporting Frequency


After looking at your IDF and EPW file (thanks for sharing a link in your comment below), the zone air temperature only falls below outside air drybulb temperature at night. This is because while there is conduction heat gain entering the zone (hotter drybulb air outside the zone than inside), there are still radiation heat exchanges between all surfaces to their surroundings. This means that the surfaces radiate to the surrounding sky temperature as well as to the surrounding ground surface temperature. In my screenshot below looking at these outputs from your IDF, we can see that both sky and ground surface temperatures are much colder than outside air drybulb. This causes greater heat losses leaving the surfaces via radiation than the heat gains entering the surfaces via conduction.

image description

The surface ground temperature is constant because that is set in the header section of the EPW file and not something that changes hourly. This is an "open field" measurement for undisturbed top soil/grass/etc. without any structures sitting on it.

EnergyPlus calculates sky temperature based on horizontal infrared radiation from sky, cloudiness factors and current temperature. You can set the sky temperature using the WeatherProperty:SkyTemperature object instead.

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My outside boundary condition is set to Outdoors for the floors. Is it possible that the ground temperature still effects these floor surfaces even if the boundary condition is set to outdoors?

WannesVDV's avatar WannesVDV  ( 2020-03-18 13:12:44 -0600 )edit

No, the "Outdoors" boundary condition should tell EnergyPlus to assign outside air temperature from the weather file and not interact with ground temperatures. The cold zone temperatures could be a result of high radiation heat transfer losses to the surrounding ground and sky that negate the conduction heat transfer gains coming into the zone from outdoor air. Or, there could be multiple other heat transfer/storage details that explain this behavior. The only way to know for sure is if you can share the IDF via DropBox, Google Drive, etc. so that others can review your model inputs.

Aaron Boranian's avatar Aaron Boranian  ( 2020-03-18 13:39:48 -0600 )edit
WannesVDV's avatar WannesVDV  ( 2020-03-18 13:48:55 -0600 )edit

Some surfaces are named in dutch. Dak = roof, Gevel = wall, vloer = floor

WannesVDV's avatar WannesVDV  ( 2020-03-18 13:58:17 -0600 )edit

Thank you for your time and answer

WannesVDV's avatar WannesVDV  ( 2020-03-19 14:38:38 -0600 )edit

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Asked: 2020-03-18 12:01:56 -0600

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Last updated: Mar 19 '20