OpenStudio: Cooling electricity increases significantly after allowing VAV's to control for outdoor air?

asked 2018-03-06 11:06:43 -0600

Gramnin
55 ●2

updated 2018-03-06 11:31:15 -0600

__AmirRoth__

4446 ●5 ●17 http://bleedinggreenna...

I've been working on a fairly simple model in OpenStudio, and for some reason, when I allow the VAV boxes to control for outdoor air, the electricity used by cooling increased very significantly in the colder months (January, February, November, December in particular). I can't seem to figure out why this is happening?

I have a fixed drybulb economizer in my AHU's that is set to a maximum limit temperature of 72F and a minimum limit temperature of 55F. I don't know if those values are affecting this either.

I can supply more information if need be, I just don't know what all could be affecting this.

answered 2018-03-07 11:05:31 -0600

CrankyOne
26 ●2

Try significantly reducing or eliminating that economizer low limit. It should not be needed unless there is humidification in the building. It appears that what is happening is not that "the VAV boxes are controlling OA", but the AHU OA damper is reducing OA% when the OAT is below 55F, so the MAT increases, so the AHU cooling coil uses more energy or CHW.

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Thanks! Thinking about it the way you said makes complete sense, and when I ran it again after lowering the minimum significantly, the cooling spike went away.

Gramnin ( 2018-03-07 14:17:54 -0600 )edit

Interesting. I have a simple VAV-with-Reheat system, but no economizer (at least I think so, because the OS:Controller:OutdoorAir's "Economizer Control Type" is set to "NoEconomizer"). When I tried the OS:AirTerminal:SingleDuct:VAV:Reheats' "Control for Outdoor Air" set to "Yes", I too observed much increased energy consumption. I ended up setting this back to "No", as ResultsViewer seems to show proper OA anyway, at least at the system level. Not sure what that means in the above context.