Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
After hundreds of trials and errors, I came up with a very simple answer.
Inputting the specific OA flow rate (i.e., 0.1888 in this case) into Fixed Minimum Air Flow Rate in HVACTemplate:Zone:VAV provides constant OA flow rate with variable supply air flow rate.
Zone Minimum Air Flow Input Method needs to be FixedFlowRate, and also Minimum Outdoor Air Control Type in HVACTemplate:System:VAV needs to be FixedMinimum (default is ProportionalMinimum).
In my first model, when the required supply air (SA) flow rate determined by cooling/heating load was smaller than the set OA flow rate, actual OA flow rate dropped to the required SA flow rate (i.e., actual SA = required SA = actual OA).
By changing the setting as described above, even when the required SA flow rate is smaller than the set OA flow rate, the actual SA is maintained at the set Fixed Minimum Air Flow Rate, and all of the actual SA seems to be supplied from OA (i.e., actual SA = fixed minimum SA = actual OA).
The charts below show the simulation results. Please note that since the zone (SPACE5-1) does not face the outdoors, the heating load is very small, and the required SA flow rate exceeds the set OA flow rate only at the start-up.
I also tried to use Controller:MechanicalVentilation, other Classes (e.g., AirTerminalSingleDuct:VAV:HeatAndCool:Reheat, SetpointManager:WarmsetTemperatureFlow) and EMS, but it was not possible to achieve both constant OA and variable SA simultaneously in both heating and cooling.
If there are a lot of zones, inputting specific OA flow rates may take some time, but for now this is the easiest solution for me. I put the fixed IDF file here just in case.
| | 2 | No.2 Revision |
After hundreds of trials and errors, I came up with a very simple answer.
Inputting the specific OA flow rate (i.e., 0.1888 in this case) into Fixed Minimum Air Flow Rate in HVACTemplate:Zone:VAV provides constant OA flow rate with variable supply air flow rate.
Zone Minimum Air Flow Input Method needs to be FixedFlowRate, and also Minimum Outdoor Air Control Type in HVACTemplate:System:VAV needs to be FixedMinimum (default is ProportionalMinimum).
In my first model, when the required supply air (SA) flow rate determined by cooling/heating load was smaller than the set OA flow rate, actual OA flow rate dropped to the required SA flow rate (i.e., actual SA = required SA = actual OA).
By changing the setting as described above, even when the required SA flow rate is smaller than the set OA flow rate, the actual SA is maintained at the set Fixed Minimum Air Flow Rate, and all of the actual SA seems to be supplied from OA (i.e., actual SA = fixed minimum SA = actual OA).
The charts below show the simulation results. Please note that since the zone (SPACE5-1) does not face the outdoors, the heating load is very small, and the required SA flow rate exceeds the set OA flow rate only at the start-up.
I also tried to use Controller:MechanicalVentilation, other Classes (e.g., AirTerminalSingleDuct:VAV:HeatAndCool:Reheat, SetpointManager:WarmsetTemperatureFlow) and EMS, but it was not possible to achieve both constant OA and variable SA simultaneously in both heating and cooling.
If there are a lot of zones, inputting specific OA flow rates may take some time, but for now this is the easiest solution for me. I put the fixed IDF file here just in case.
P.S. I have another thing to notice. When we use AirTerminal:SingluDuct:VAV:Reheat or AirTerminal:SingluDuct:VAV:NoReheat instead of HVACTemplate:Zone:VAV, Design Specification Outdoor Air Object Name in AirTerminal needs to be blank even though we have it. If Design Specification Outdoor Air Object Name is filled, SA flow rate unnaturally increases up to maximum flow rate as shown in the charts below.
