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EnergyPlus ignores the design values on my AirloopHVAC

asked 2016-07-19 04:33:24 -0600

I have an AirLoopHVAC with a fan constant volume, a DX cooling coil and a gas heating coil. Multiple zones are connected with ATUs of type AirTerminal:SingleDuct:Uncontrolled (autosized)

I want my loop to work at $X$ CFM of supply air constantly.

Here's what I've done:

I was expecting the loop to run at this value, but it doesn't.

Even weirder, I had set the minimum and the maximum outdoor airflow rate of the Controller:OutdoorAir attached to the AirLoopHVAC:OutdoorAirSystem to a value $Y$, and the loop ends up running at a constant value that is even below this $Y$ value...

I've found that to have it run at $X$ CFM, I have to either:

  • Modify the Sizing:System object attached to the AirLoopHVAC to have a Cooling/Heating Design Air Flow Method set to Flow/System and to enter $X$ for the Cooling/Heating Design Air Flow Method
  • Hardsize the ATUs
  • Set a different DesignSpecification:OutdoorAir (DSOA) for the zones

So it seems that E+ is just simple ignoring the Design Supply Air Flow Rate that I've entered for the AirLoopHVAC, and calculating another value that will meet the ATUs need (My Sizing:System is set to Type of Load to Size On at Sensible, though I've tried VentilationRequirement). Why is that? Is this the expected behavior?

For the record, I'm using OpenStudio 1.12.1, as I think this could be linked to the way OpenStudio uses and forward translates the Controller:MechanicalVentilation set to ZoneSum with all the DSOA for the zones served by the AirLoopHVAC

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I experienced the same thing with respect to outdoor air flow rate. The DesignSpecification:OutdoorAir sum seems to supercede whatever OA values the system is 'constrained' to. The only solution I found was to set the DesignSpecification:OutdoorAir to 0 or just delete them completely, the Controller:MechanicalVentilation then adheres to it's defined parameters.

I remember seeing something somewhere about how the 'Design Supply Air Flow Rate' wasn't actually used for loop sizing, but I can't seem to find where/what exactly I read. I could just be crazy.

Interested to see answers.

Adam Hilton's avatar Adam Hilton  ( 2016-07-19 09:19:38 -0600 )edit

I'm experiencing the same issue with a LEED Baseline building model. I have about (5) VAV units, each with roughly 20 zones and as might be imagined, manually entering the requirements for each zone's terminal unit/DesignSpecification:OutdoorAir will be very cumbersome. Has this issue been sufficiently addressed so that there is a way to simply specify the total outdoor air required at each baseline system level?

mikesweeney's avatar mikesweeney  ( 2017-03-30 13:28:22 -0600 )edit

A vv air loop will always operate at what the terminal equipment is requesting. You can't increase the supply to zones unless the terminals request it.

Adam Hilton's avatar Adam Hilton  ( 2017-03-30 13:59:21 -0600 )edit

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answered 2016-07-19 12:51:33 -0600

Archmage's avatar

The explanation lies with the zone sizing and the autosizing of the air terminals. The flow in an air loop is usually going to be the sum of the flows requested by the air terminals on that loop. As you point out you need to also hardsize the air terminals for this to work as expected. Otherwise you are mixing autosizing and hardsizing in the same air loop and this is the sort of thing that happens.

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Asked: 2016-07-19 04:33:24 -0600

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Last updated: Jul 19 '16