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How can I develop two different Air Loops in the OpenStudio model?

asked 2015-07-21 14:33:23 -0500

dr_orear gravatar image

updated 2015-07-21 19:03:49 -0500


I am attempting to create two different air loops for the same zones in OS. One loop is for the Energy Recovery Ventilation System. The other is for a standard Air-to-Air Heat Pump HVAC system. So far, if I link one loop to a zone and attempt to link that same zone to the other air loop, the zone drops out of the initial loop. Any suggestions?


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Your question would be more accurate if you said "develop two air loops in one thermal zone"

David Goldwasser gravatar image David Goldwasser  ( 2015-07-21 14:47:23 -0500 )edit

@amirroth I changed your EP "air-to-air-heat-pump" tag to the existing and more general "air-source-heat-pump". Feel free to adjust if you see fit.

MatthewSteen gravatar image MatthewSteen  ( 2015-07-21 19:05:38 -0500 )edit

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answered 2015-07-24 13:15:48 -0500

EnergyPlus constrains a given zone to only one AirLoopHVAC system through the ZoneHVACEquipmentConnections object, which allows only a single "Zone Return Air Node" as opposed to a list of nodes. A zone is added to the AirLoopHVAC based system through the "Zone Air Inlet Node" (of which there can be many) and the "Zone Return Air Node" (of which there can only be one). @rraustad might be able to give more detail about why EnergyPlus is structured this way.

Zone equipment connects to a zone through the "Zone Air Inlet Node" and the "Zone Air Exhaust Node" (defined in ZoneHVACEquipmentConnections) both of which are extensible through a node list. Because of this you can have as many zone hvac components as you want.

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I would suggest that a single return node in each zone was somewhat a lack of foresight and also the use of a single mixer/splitter pair for each air loop (which would only need a single return node). The availability of multiple inlet/exhaust nodes was to allow attachment of zone equipment.

rraustad gravatar image rraustad  ( 2015-07-24 13:51:33 -0500 )edit

It would also be difficult to determine the RA split between multiple return air nodes.

rraustad gravatar image rraustad  ( 2015-07-24 14:33:05 -0500 )edit

answered 2015-07-21 14:46:49 -0500

OpenStudio doesn't let you do this because that is a constraint in EnergyPlus. The most common solution for this would be to provide ventilation through an air loop and conditioning through zone equipment.

There is actually an old old video on YouTube that demonstrates this with a packaged terminal heat pump as zone equipment, but I think without the energy recovery.

Another technique I have heard of in different use cases when someone wants to model two air loops in one zone, is to model it as two zones with zone mixing objects between the zones. I don't have any experience doing that to know how well it works.

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I noticed that a while back when working on an EnergyPlus model. I had to rethink the way I was modeling the HVAC system, bummer... @David Goldwasser Would you happen to know why this constraint exists?

Jeremy gravatar image Jeremy  ( 2015-07-21 16:17:34 -0500 )edit

@JeremyL. I'm going to let someone from the EnergyPlus team answer this. My assumption is that the one zone per air loop is deeply ingrained in the code base, and that while it might be possible to support multiple air loops per zone, it would require pretty extensive code changes.

David Goldwasser gravatar image David Goldwasser  ( 2015-07-22 13:26:10 -0500 )edit

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Asked: 2015-07-21 14:33:23 -0500

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Last updated: Jul 24 '15