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Controlling AHU for Outdoor Air and Temperature

asked 2015-08-05 11:17:21 -0500

updated 2017-05-17 12:36:05 -0500

I have a dedicated outdoor air system that I am modelling in EnergyPlus. It has a heat exchanger, OAM, Fan, Heating Coil and Cooling Coil. Its function is to supply a constant volume of air (16.625m3/s) between 6am and 8pm at a constant temperature (12.8°C). It is connected into series fan powered boxes in the zones. I have set up the following controls:

  1. OA Controller is set to no economizer, minimum limit type - Fixed minimum, Minimum outdoor air schedule is a fraction schedule set to the criteria above. Minimum and maximum air flow rates are both set at 16.625 m3/s.
  2. I am using the setpointManager:MultiZone:CoolingAverage to control my cooling coil, and it is set to minimum setpoint temperature 12.8, maximum setpoint temperature 12.8.
  3. The Constant volume fan has a fractional schedule that turns it on at 6am and off at 8pm.

When I check the run neither the OA is controlled nor the temperature coming off the cooling coil. The OA varies between 10 m3/s and 16.625m3/s but is at least turned off completely at night, and the air temperature coming off the cooling coil is anywhere between 12.8°C and 21°C.

My suspicion is that I have a series fan powered box connected to the AHU and I have put autosize in the maximum air flow rate box and 0 in the minimum primary air flow fraction box, I have put the correct OA flow rates in the maximum primary air flow rate box though.

Can anyone think of why my DOAS is not behaving itself, or anything I can try to make it work. (note, when we use singleductuncontrolled, and set up the DOAS in the same way the DOAS behaves itself perfectly).

Thanks as always for the help.

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answered 2015-08-05 17:06:36 -0500

I would suspect the minimum primary air flow rate as the limiting factor. Plot the mixed air and outdoor air flow rates. The outdoor air flow rate cannot be greater than the mixed air (supply air) flow rate. If the times when the OA flow rate falls below the expected minimum coincide with supply air flow rates also below 16.6725 m3/s, then the fan powered boxes are limiting the OA flow rate. Adjust the minimum primary air flow rate fraction upward to compensate.

Regarding the OA temperatures, report the coil outlet air temperature and air temperature set point to see if the coil is meeting the SP temperature. If a strict 12 C is required as the OA control temp, then use a scheduled SPM.

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Thanks @rraustad. The coil wasn't reaching setpoint because I had hard wired the flow rates in the pumps. As soon as I removed those the coil started to behave properly again. The flow rates are still off though so I will look at my mixed flow rates and make sure they are higher than the primary air flow rate! Thanks for answering!

Annie Marston gravatar image Annie Marston  ( 2015-08-05 17:17:22 -0500 )edit
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answered 2015-08-05 22:09:20 -0500

Thanks to @rraustad pointing me in the right direction, I found to control my Cooling coil, I had hard wired the flow rates on the pumps and by autosizing the cooling coil setpoint was met consistently. To get control of the flow rates was little more tricky, I had to put 1 in the minimum primary air flow fraction box (this gave me a constant supply of fresh air to each zone) AND I had to put hard values in the maximum flow rate box AND I had to put hard values in the fan. When I did all that my DOAS gave me the correct fresh air amounts at the correct times.

Thanks for pointing me in the right direction @rraustad

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Asked: 2015-08-05 11:17:21 -0500

Seen: 846 times

Last updated: Aug 05 '15