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Control of AirTerminal:SingleDuct:VAV:Reheat

asked 2017-01-05 09:58:17 -0600

updated 2017-01-05 23:34:24 -0600

I assigned a typical VAV system with reheat but there are many heating unmet hours in the core area. After I did further check for node temp as well as air cfm and found:

1) the air cfm is almost fixed all the time, like CAV 2) although I set the sizing air inlet/outlet temp for heating coil in reheat is 13C/35C. However, the heating coil can only heat the air to around 18C, even though the heating is not enough at that time. Therefore, it came out the unmet hours.

Anyone had same problem? Don't know what's wrong with the setup.

Here is the link for my sample model, which is a 5-floor small building with typical 5 zone in each floor: link

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Can you check node temperatures (and unmet hours) once you replace "Normal" Damper Heating Action with "Reverse" in all AirTerminal:SingleDuct:VAV:Reheat objects?

Ivan Korolija gravatar image Ivan Korolija  ( 2017-01-05 11:13:59 -0600 )edit

@Ivan Korolija, thanks for your reply. Actually I do check this. In this sample model, if I changed "Normal" to "Reverse", the unmet hours did go. Also, the hot water coil temp increase a little bit. But air flow rate is still the same which I thought should be different. And this action did not apply to other models. I mean, for this model, "Reverse" will remove unmet hours. However, for other models. it removed unmet hours but created more unmet hours in other zones. So I was wondering if this is the correct action I should take

Yan gravatar image Yan  ( 2017-01-05 11:19:44 -0600 )edit

2 Answers

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answered 2017-01-07 12:17:13 -0600

General Advice for Sizing Problems with VAV Systems

  1. Make sure Zone Heating Design Supply Air Temperature in Sizing:Zone matches the Maximum Reheat Air Temperature in the AirTerminal:SingleDuct:VAV:Reheat assigned to this zone. If the sizing temperature is higher than the maximum reheat temperature, then during operation the terminal isn't allowed to operate at the temperature it was designed for, meaning that it can't provide the designed heating capacity.
  2. Make sure that the Design Loop Exit Temperature in the Sizing:Plant for the Hot Water PlantLoop matches the Winter Design Day Schedule in the SetpointManager:Scheduled assigned to your Hot Water PlantLoop. If everything is sized for high temperature water but then operated at a lower temperature, sometimes the equipment can't meet the load.
  3. Make sure that the Rated Inlet Water Temperature and Rated Outlet Water Temperature of theCoil:Heating:Water Reheat Coils match the Design Loop Exit Temperature and Loop Design Temperature Difference in theSizing:Plant for the Hot Water PlantLoop. This ensures that the coils and their plant loop are sized with the same temperature and delta-T.

So for your particular IDF:

  1. Change Zone Heating Design Supply Air Temperature from 40C to 35C to match max reheat
  2. Change Design Loop Exit Temperature for Hot Water Loop from 82.2C to 67C to match HW setpoint
  3. Change Rated Inlet Water Temperature for Reheat Coils from 82.2C to 67C to match HW setpoint
  4. Change Rated Outlet Water Temperature for Reheat Coils from 71.1C to 55.9C to match HW delta-T

Result: Time Setpoint Not Met During Occupied Heating drops from 1145.00 to 234.33

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answered 2018-03-15 06:17:55 -0600

Dear All,

I assigned a typical VAV system (Baseline System 5) with reheat but there are many heating unmet hours in the zones served by the system. I did most of the quality checks (correct OA, zone setpoints, zone and system sizing etc.) to see where the problem lies. And i found ' AirTerminal:SingleDuct:VAV:Reheat' is NOT active or sizing is zero in HTML report. To further ascertain the cause of the problem, I plotted temperatures [ see attachment] at all the nodes to see the temperature regime. This also shows that reheat coil doesn't raise the air temperature to meet the room set-point.

This is being simulated for CZ5A.

I also tried to look out for some solutions discussed here on forum but unfortunately nothing works for me.

I would appreciate any help in this regard.

image description

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Asked: 2017-01-05 09:58:17 -0600

Seen: 736 times

Last updated: Mar 15 '18