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What air terminal unit types are allowed on a AirLoopHVAC:Unitary system?

asked 2018-03-01 12:40:07 -0500

Molly Curtz gravatar image

I see that the diagrams in the section of the I/O reference on AirLoopHVAC:UnitarySystemshow only AirTerminal:SingleDuct:Uncontrolled. I have a file built by a colleague (which runs) that uses AirTerminal:SingleDuct:VAV:NoReheat for a zone served by an AirLoopHVAC:UnitarySystem. I am wondering if this is problematic.

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answered 2018-03-01 12:47:53 -0500

updated 2018-03-02 07:55:38 -0500

The AirloopHVAC:UnitarySystem can model load, set point or single-zone VAV control.

AirLoopHVAC:UnitarySystem,
  A2,  \field Control Type
     \type choice
     \key Load
     \key SetPoint
     \key SingleZoneVAV
     \default Load

When load control is used the UnitarySystem will set the operating flow rate so the terminal units do not need to be VAV. I would expect that if VAV were used there would be fighting between the components as to what flow rate to operate at.

When set point control is used, the model expects constant fan mode to be used. So since the UnitarySystem is not cycling the fan, it really doesn't care what flow rate is used, just that the flow rate is constant throughout the time step. So yes, I would expect VAV boxes to work with the UnitarySystem model.

See the Eng. Ref. for a description of fan speed and capacity modulation for SZVAV control.

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Asked: 2018-03-01 12:40:07 -0500

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Last updated: Mar 02 '18