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VAV minimum flow rate higher than what is specified in EnergyPlus

asked 2023-03-09 19:43:23 -0500

Abarker's avatar

updated 2023-03-17 14:51:49 -0500

I have a VAV system with a minimum flow fraction of 35%, however for some reason the minimum flow is limiting itself to around 49%.

I have verified that:

  • The Outdoor air rate is less than the minimum 35% flow, and set as Fixed Minimum
  • After quadruple checking all inputs, they all agree that 35% is the minimum requested flow rate, not 49%. The 35% is corroborated by reviewing the HTML output. It is only when looking at hourly reporting that I noticed that the flow never goes below 49%.

I have also verified that it is the percentage of total flow that is what is somehow being specified, not the actual flow. I verified this by modifying the maximum flow rate input and noticing that the minimum flow rate adjusted as well, maintaining the exact same percentage. This also leads me to believe it is not limiting itself to some demand that needs to be met.

This is a model right out of HVAC Template. What am I missing or forgetting, as it seems I have inadvertently indicated a minimum flow of ~49%. Is there some known reason a VAV system would do this?

IDF file is attached.

The AirLoops in question are labelled AHU101 and AHU201.

it can be seen from the HTML outputs that the minimum flow of these systems are 35% of their max flow, e.g. 4.64 m3/s for AHU101.

image description

Hourly output reports however confirm that AHU101 is only reducing its flow to ~6.6 m3/s.

image description

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Please upload your idf file to identify the cause.

Keigo's avatar Keigo  ( 2023-03-11 07:06:15 -0500 )edit

Hi @Keigo and thank you. I edited the original post to add more context and I thought I had added the IDF file by using the attach file tool. It doesn't seem to have worked as I don't see the uploaded IDF file anywhere. Any advice on how to do this?

Abarker's avatar Abarker  ( 2023-03-17 14:50:50 -0500 )edit

Please upload your idf file to the cloud (Google Drive, OneDrive, Dropbox, etc.), and please paste the file location link in your question or this Comments section.

More importantly, please attach the chart of airflow in winter rather than in summer if you want to check the minimum heating airflow rate.

Keigo's avatar Keigo  ( 2023-03-17 22:01:01 -0500 )edit

Thanks again @Keigo, I have pasted a Dropbox link to the IDF file below.

Abarker's avatar Abarker  ( 2023-03-20 15:17:02 -0500 )edit

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answered 2023-03-21 06:47:24 -0500

Keigo's avatar

You can check the err file first.

   ** Warning ** Calculated design cooling load for zone=106-N-TUNNEL-AHU101 is zero.
   **   ~~~   ** Check Sizing:Zone and ZoneControl:Thermostat inputs.
   ** Warning ** Calculated design heating load for zone=106-N-TUNNEL-AHU101 is zero.
   **   ~~~   ** Check Sizing:Zone and ZoneControl:Thermostat inputs.
   ** Warning ** Calculated design heating load for zone=100-N-COMMON-AHU101 is zero.
   **   ~~~   ** Check Sizing:Zone and ZoneControl:Thermostat inputs.

106-N-TUNNEL-AHU101 and 106-N-TUNNEL-AHU101 are zones supplied by AHU101. These 2 zones are not autosized correctly. I think that's the main cause.

Autosizing for cooling/heating load is not working well, but 106-N-TUNNEL-AHU101 and 106-N-TUNNEL-AHU101 have some airflow because the airflow rate of the AirTerminals are autosized according to the minimum outdoor air flow requirement. AHU101 is not a dedicated outdoor air system, so the outdoor air and the return air are completely mixed, and the mixed air is supplied to each zone. For these 2 zones, The supply airflow rate = The mixed air flow rate including the minimum outdoor air.

You can also check the Output:Variable Zone Air Terminal VAV Damper Position. The damper position (= Air Flow Fraction) of these 2 zones is always 1 while the damper position of other zones connected to AHU101 is 0.35.

There are some weird settings in your model, but I'm not familiar with HVAC system in Canada, so I won't go into detail. Just one example is inconsistency between Sizing:Zone and Sizing:System.

Zone Heating Design Supply Air Temperature Input Method is set to TemperatureDifference and Zone Heating Design SUpply Air Temperature Difference is 21deltaC in Sizing:Zone. However, AHU101's airloop does not have a reheat coil for each zone. AHU101 has a central heating coil, but the setpoint is fixed to 5ºC, which seems inconsistent with your Sizing:Zone settings.

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Thanks for checking It is certainly a strange system type and not a common thing here in Canada. The air is only tempered to 5C and has no active cooling, but as you noted there is a lot of recirculation. I am getting more info from the designer.

Fixing the Sizing:Zone and Sizing:System inconsistencies doesn't seem to change the outcome. I think the fact that those 2 zones never go below a VAV damper position of 1.0, even though 0.35 is specified, is a limitation of the system type not quite being a VAV system, but still having to be modelled as one as a workaround.

Abarker's avatar Abarker  ( 2023-03-21 18:27:06 -0500 )edit

If zones really do not have loads, you don't need to try to change the outcome. It's correct. The airflow is already minimum when the damper position is 1 (Again, The supply airflow rate = The mixed air flow rate including the minimum outdoor air). If the damper position becomes smaller than 1, the minimum outdoor air flow rate is not ensured.

Keigo's avatar Keigo  ( 2023-03-21 20:10:52 -0500 )edit

I agree, though not the outcome I wanted or initially expected it is correct. Thanks again for the help.

Abarker's avatar Abarker  ( 2023-03-22 08:15:26 -0500 )edit

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Asked: 2023-03-09 19:43:23 -0500

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Last updated: yesterday