Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
In EnergyPlus, the zone equipment (ZoneHVAC:*) do not model an outside air system. Instead they model outdoor air mixers integral with the zone equipment model. This means there is only a single fan object and an outdoor air mixer (no OA system as in AirloopHVAC equipment).
ZoneHVAC:TerminalUnit:VariableRefrigerantFlow,
Fan:ConstantVolume, !- Supply Air Fan Object Type
TU1 VRF Supply Fan, !- Supply Air Fan Object Name
OutdoorAir:Mixer, !- Outside Air Mixer Object Type
TU1 OA Mixer, !- Outside Air Mixer Object Name
Since your terminal unit equipment uses constant fan operation, the amount of outdoor air will not vary while the terminal units are operating. Outdoor air will turn off when the terminal unit turns off (based on availability), but will be constant otherwise.
You could use a cycling fan (Fan:OnOff) to model varying outdoor air proportional to the terminal unit coil PLR, however, this will also vary the terminal unit supply air flow which may not be your intention. It's not a bad idea to compare energy use of cycling versus constant fan operation mode scenarios just so you have an idea of the (fan) energy impacts.
VRFFanSchedule, !- Supply Air Fan Operating Mode Schedule Name
| | 2 | No.2 Revision |
In EnergyPlus, the zone equipment (ZoneHVAC:*) do not model an outside air system. Instead they model outdoor air mixers integral with the zone equipment model. This means there is only a single fan object and an outdoor air mixer (no OA system as in AirloopHVAC equipment).
ZoneHVAC:TerminalUnit:VariableRefrigerantFlow,
Fan:ConstantVolume, !- Supply Air Fan Object Type
TU1 VRF Supply Fan, !- Supply Air Fan Object Name
OutdoorAir:Mixer, !- Outside Air Mixer Object Type
TU1 OA Mixer, !- Outside Air Mixer Object Name
Since your terminal unit equipment uses constant fan operation, the amount of outdoor air will not vary while the terminal units are operating. Outdoor air will turn off when the terminal unit turns off (based on availability), but will be constant otherwise.
You could use a cycling fan (Fan:OnOff) to model varying outdoor air proportional to the terminal unit coil PLR, however, this will also vary the terminal unit supply air flow which may not be your intention. To do this, use a 0 in the Supply Air Fan Operating Mode Schedule to denote cycling fan operation, and a 1 to denote constant fan operation. You could use constant fan during occupied hour and cycling fan during unoccupied hours. It's not a bad idea to compare energy use of various cycling versus fan operation schemes to one that uses constant fan operation mode scenarios just so you have an idea of the (fan) energy impacts.
VRFFanSchedule, !- Supply Air Fan Operating Mode Schedule Name
