Air Temperature Distribution

HVAC tab in model data

By default EnergyPlus assumes that air temperature within a zone is completely uniform (i.e. the air is fully mixed). However a number of other modelling options are provided to account for non-uniform room air temperatures that may occur within the interior air volume of a zone.

Although there are many types of analyses (comfort, indoor air quality, etc.) that might benefit from localized modelling of how room air varies across space, only the temperature distribution of room air within the zone is currently addressed in EnergyPlus. This allows surface heat transfer and air system heat balance calculations to be made taking into account natural thermal stratification of air and different types of intentional air distribution designs such as under-floor and side-wall displacement ventilation that purport to extract room air at higher-than-mean temperatures. Note that EnergyPlus does not have completely general methods of modelling room air that are applicable to every conceivable type of airflow that might occur in a zone. Such models (e.g. RANS-CFD) are too computationally expensive to use with EnergyPlus for the foreseeable future. The models that are available in EnergyPlus offer only limited modelling capabilities for select room airflow configurations. Also note that because the complete mixing model for room air has long been the standard in building energy simulation, there is not currently a consensus on how to best model non-uniform air temperatures in buildings. Therefore, it is up to the user to have a good understanding of when, where, and how to apply the room air models available in EnergyPlus. The rest of this section provides some guidance in the way of examples and further discussion of the models available in EnergyPlus.

The following room air temperature distribution models are provided:

• 1-Mixed the default option where the air temperature is modelled as being uniform through the zone.
• 2-Dynamic gradient which allows air temperature stratification within a zone to be modelled by setting up a temperature gradient which varies dynamically depending on any of: outside temperature, inside temperature, inside-outside temperature difference, heating load or cooling load.
• 3-Three node displacement ventilation which uses 3 nodes to model spaces that are served by a low velocity floor-level displacement ventilation air distribution system and the dominant sources of heat gain are from people and other localized sources located in the occupied part of the room.
• 4-Underfloor air distribution interior which uses upper and a lower sub-zone nodes to model interior spaces that are served by an underfloor air distribution system and the dominant sources of heat gain are from people, equipment, and other localized sources located in the occupied part of the room.
• 5-Underfloor air distribution exterior which uses upper and a lower sub-zone nodes to model exterior spaces that are served by an underfloor air distribution system. The dominant sources of heat gain should be from people, equipment, and other localized sources located in the occupied part of the room, as well as convective gain coming from a warm window.