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VRF System & Terminal as DOAS for another VRF Terminal?

asked 2021-06-04 17:24:53 -0600

mattkoch gravatar image

updated 2021-06-05 17:48:49 -0600

So, I am reasonably familiar with VRF and DOAS at this point, I believe.

The way I understand it, the DOAS is injecting OA directly into the thermal zone, instead of into the VRF terminal unit intake. If the OA conditions coming from the DOAS were identical to the RA conditions, I could see there might not be a difference, but if the OA conditions coming from the DOAS were different from the RA conditions, then injecting OA into the thermal zone vs. injecting it into the VRF terminal unit intake might have different results.

I believe there is a AirTerminal:SingleDuct:Mixer that might be able to accomplish injection into the VRF terminal unit intake, but I am afraid I do not know how to apply it in the context of VRF (there is no "air loop" to manipulate). Would you have any ideas? One of the challenges would be to identify the various nodes, I believe.

The VRF terminal unit (ZoneHVAC:TerminalUnit:VariableRefrigerantFlow) does have OA provisions (for example Cooling Outdoor Air Flow Rate), but I believe this then represents straight OA, i.e. not pre-conditioned OA from a DOAS.

Further, the above DOAS really is a standard air loop. What if the DOAS itself were a VRF outdoor unit with VRF terminal unit? If there was only one such VRF terminal unit, which seems "typical", one might still be able to force this into an air loop by "simply" applying the VRF performance curves to its coils and fans. However, it might be possible that one VRF outdoor unit drives two or more VRF terminal units, all of which take in 100% OA. What would one do then?

All this in the context of OpenStudio measures, by the way.

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Hmm, I just noticed that there may not be an "AirTerminal:SingleDuct:Mixer" equivalent available in OpenStudio. I found a ConnectorMixer equivalent, which does seem to have many inlets and one outlet, and an AirTerminalSingleDuctInletSideMixer equivalent, which however only seems to have a secondary inlet (no primary inlet). What gives?

mattkoch gravatar image mattkoch  ( 2021-06-04 18:35:48 -0600 )edit

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answered 2021-06-06 14:41:21 -0600

updated 2021-06-06 14:44:50 -0600

You can connect a ZoneHVAC:TerminalUnit:VariableRefrigerantFlow as a supply component on an AirLoopHVAC (since v9.3), and in OpenStudio v3.2:

require 'openstudio'
m =
ahu =
tu =

You can also use the AirTerminalSingleDuctInletSideMixerto deliver air directly from an air loop to the inlet of a zone VRF terminal. Some sample code to do that in OpenStudio is shown in this issue, which has been fixed in v3.2:

You can also add a VRF terminal to the newish AirLoopHVAC:DedicatedOutdoorAirSystem, which can pre-treat outdoor air to supply multiple other AirLoopHVACs, though I've found some odd results with that system setup.

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Thank you kindly, Eric, and apologies for my delayed response. The progress EnergyPlus makes never fails to amaze me. Your reply now makes me consider abandon my stance on "OS 2.9.1 and no further", lest I loose free access to SketchUp. I did implement the AirTerminal:SingleDuct:Mixer in an EnergyPlus measure in OS 2.9.1, and it seems to work quite well. Unfortunately, it still cannot do the ZoneHVACTerminalUnitVariableRefrigerantFlow on the DOAS since this is only EP 9.2. So, I just installed OS 3.2.0 to see if I can implement what you stated, even though now I am without SketchUp.

mattkoch gravatar image mattkoch  ( 2021-06-11 22:11:25 -0600 )edit

Incidentally, is AirTerminalSingleDuctInletSideMixer the OS equivalent of EP's AirTerminal:SingleDuct:Mixer? The SD documentation (your link) only shows a secondary node - how do I tell it the primary node? And, what is the difference between "node" and "port"? Thanks.

mattkoch gravatar image mattkoch  ( 2021-06-11 22:14:42 -0600 )edit

Yes, OS's AirTerminalSingleDuctInletSideMixer makes the EnergyPlus AirTermina:SingleDuct:Mixer object. ahu.addBranchForZone(, at) will connect the terminal primary node to the demand side of the air loop, and connect the secondary air stream to the zone air node. vrf_tu.addToNode(at.outletModelObject.get.to_Node.get) will connect the terminal unit to the TU outlet node.

Eric Ringold gravatar image Eric Ringold  ( 2021-06-12 12:20:55 -0600 )edit

Sorry for the long delay. Can't afford to put as much time into this as I would like to. However, did spend some hours on it this weekend, but could not make it work (yet). Getting numerous EnergyPlus node connection errors. I have a couple of question: 1) It seems that tu.addToNode(ahu.supplyInletNode) DOES NOT add "tu" to the diagram on the OpenStudio 1.1.0 app's HVAC tab. 2) It seems that vrf_tu.addToNode(at.outletModelObject.get.to_Node.get DOES add "vrf_tu" to that diagram. Is that the expected behavior?

mattkoch gravatar image mattkoch  ( 2021-07-11 19:10:17 -0600 )edit

Duh, I still had OpenStudio Application 1.1.0 installed with OpenStudio 3.1.0. I just upgraded to OpenStudio Application 1.2.0 with OpenStudio 3.2.1, and now it all works as advertised above. The one exception may be to replace tu.addToNode(ahu.supplyInletNode) with tu.addToNode(ahu.supplyOutletNode). I also had to add a setpoint manager, of course. Not sure about the results, but at least it all runs to the end now.

mattkoch gravatar image mattkoch  ( 2021-07-14 21:01:46 -0600 )edit

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Asked: 2021-06-04 17:24:53 -0600

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Last updated: Jul 14