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How can HVACTemplate:Zone:VAV ensure Minimum OA Flow Rate?

Any model is fine, but let me explain the current issue by using an ExampleFile in E+ v9-4-0 for simplicity.

This idf file is a slightly revised version of HVACTemplate-5ZoneVAVFanPowered.idf from ExampleFiles. Originally there were 5 zones, but I deleted 4 HVACTemplate:Zone:VAV:FanPowered because they worked fine. Now there is only one zone (SPACE5-1) with HVACTemplate:Zone:VAV and HVACTemplate:System:VAV. I want to focus on them.

Besides, I added Zone Air Terminal Outdoor Air Volume Flow Rate in Output:Variable, and just ran the simulation. I got the results for 2 days in winter and summer. The charts below show outdoor air flow rate.

I don't know why the actual outdoor air flow rate is not constant and below the design outdoor air flow rate.

In HVACTemplate:Zone:VAV, Outdoor Air Method is flow/person and the value is fixed to 0.00944 m3/s per person. In People, Number of People is fixed to 20, so I think the outdoor air flow rate should be constant at 0.00944 x 20 = 0.1888m3/s (the orange dashed lines in the charts). But the actual outdoor air flow rate is quite different from 0.1888m3/s, especially when heating.

At first, Minimum Outdoor Air Flow Rate in HVACTemplate:System:VAV was set to autosize, so I changed it to the exact value 0.1888, but in vain.

Economizer Type is set to DifferentialDryBulb in HVACTemplate:System:VAV, but even when I changed it to NoEconomizer, the result didn't change.

Minimum Outdoor Air Control Type is set to FixedMinimum, and the fraction of Minimum Outdoor Air Schedule is 1 from 9:00 to 21:00, so there seems to be nothing strange about it. When I tried changing this control type to ProportionalMinimum, The outdoor air flow rate further decreased (see the charts below). The change in outdoor airflow rate by time does not seem to correspond to the occupancy schedule either.

People has an occupancy schedule with different fractions for different time zones, but I believe that flow/person for Outdoor Air Method is not affected by occupancy schedule.

Constant Minimum Air Flow Fraction is set to 0.3. When I tried changing this fraction to 1, the outside airflow rate became constant, but the supply airflow rate also became constant (see the charts below). This is not what I expect. This is not VAV, but CAV. I want to keep outdoor airflow rate, and I want supply airflow rate to vary according to cooling/heating load.

I know that acrording to Engineering Reference, VAV doesn't work as VAV when heating (although I'm not convinced why this is the default setting). But is there any way to keep minimum outdoor air flow rate as I want?

For your reference, below is the input for HVACTemplate:Zone:VAV and HVACTemplate:System:VAV.

HVACTemplate:Zone:VAV,
SPACE5-1,                !- Zone Name
VAV Sys 1,               !- Template VAV System Name
All Zones,               !- Template Thermostat Name
autosize,                !- Supply ...
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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.

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If you want to maintain a constant OA flow rate while varying total flow in a VAV system, you need to make 2 changes:

1. Specify the DesignSpecification:OutdoorAir object name in AirTerminal:SingleDuct:VAV:Reheat.
2. Make a Controller:MechanicalVentilation object, set Demand Controlled Ventilation to "No", VentilationRateProdecure system Outdoor Air Method, and reference your zone name and corresponding DSOA and DSZAD objects.

I don't believe you can make either of these changes using HVACTemplate; you probably need to edit the IDF directly.

With these changes made, you can set Constant Minimum Air Flow Fraction (in AirTerminal:SingleDuct:VAV:Reheat) and Minimum Outdoor Air Flow Rate (Controller:OutdoorAir) back to autosize (or not, it'll still maintain steady OA flow).

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Hi @ericmartinpe, thank you for your response. I followed these changes. OA flow is steady, but the total flow is also steady. The result is the same as the chart I attached at the bottom of my question (Constant Min Air Flow Fraction = 1). Could you please check where the problem is in my idf file ?

( 2021-05-02 00:07:30 -0500 )edit

@Keigo you changed the Economizer Type (in Controller:OutdorAir) to NoEconomizer, so the OA damper is fixed. Change this back to DifferentialDryBulb and it works. Also, simulate for more than just the 2 design days so you can see the fan flow vary throughout the year. The total flow and OA flow plots look like this. Here's the before and after IDFs sorted so you can see what changed.

( 2021-05-03 11:53:40 -0500 )edit

@ericmartinpe Thank you for fixing my idf file. However, the result is not what I expect. Your OA flow plots are from September. They just show that economizer is working when OA temperature is very low. Economizer is a completely different function from VAV. When OA is higer than 19°C (Economizer Maximum Limit Dry-Bulb Temperature) such as Jul and Aug, the total airflow is constant. That's what doesn't make sense to me. Supply air temperature is constant at 12.8°C and internal load changes hourly according to occupancy schedule, then supply airflow should change hourly. Why doesn't it change?

( 2021-05-04 03:01:16 -0500 )edit

@ericmartinpe Another thing that doesn't make sense is that reheat coil is almost always operating in summer. In order to prevent overcooling, the supply airflow should be reduced first, but it is kept constant and it is reheated instead. As a result, the zone air temperature in summer is kept at heating setpoint of 23°C instead of cooling setpoint of 24°C. All these are far from what I expect.

( 2021-05-04 04:38:17 -0500 )edit