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

asked 2021-04-25 06:17:49 -0500

Keigo gravatar image

updated 2021-04-27 12:08:52 -0500

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. image description

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.

image description

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.

image description

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? image description

P.S. I found this post. This is what I want to do with HVACTemplate:Zone:VAV and HVACTemplate:System:VAV. The post was 5 years ago, and I don't know if we still ... (more)

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answered 2021-04-26 18:05:23 -0500

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 ?

Keigo gravatar image Keigo  ( 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.

ericmartinpe gravatar image ericmartinpe  ( 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?

Keigo gravatar image Keigo  ( 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.

Keigo gravatar image Keigo  ( 2021-05-04 04:38:17 -0500 )edit

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Asked: 2021-04-25 06:17:49 -0500

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Last updated: Apr 26