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Can one zone have multiple Air Distribution Units (multiple Airloops)?

asked 2021-07-13 00:58:28 -0500

Keigo's avatar

updated 2021-07-18 14:17:47 -0500

The following figure in I/O Reference suggests that we should select one Air Distribution Unit per zone, but can we select multiple Air Distribution Units per zone? In other words, can we add multiple Airloops per zone?

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I want to model HVAC system with PAU(Primary Air Handling Unit) +AHU. To do this, I think I need to add 2 Air Distribution Units to each zone. Below is my image of how I would model the HVAC system with PAU+AHU. The actual number of zones is more than 100 and there are many PAUs and AHUs, but this illustration is simplified.

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ZoneHVAC:EquipmentConnections has Zone Air Inlet Node or NodeList Name and Zone Air Exhaust Node or NodeList Name fields, so I guess I can input multiple Nodes to these fields by using NodeList, but I'm not so sure.

If there is an ExampleFile with such a system, it would be even more helpful.

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answered 2021-07-13 11:43:08 -0500

updated 2021-07-13 13:46:41 -0500


The air distribution unit is limited to one combined component-controller unit; because controls are normally based on the zone thermostat and can work in parallel or series in complex fashion. Since the control and the flow resolution can be complex, each air distribution unit is unique in addressing these combinations and therefore only one is allowed per zone.

Edit: actually yes. See example file 5ZoneAirCooledWithDOASAirLoop.idf.

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The second link is out of date. As of EnergyPlus v8.8, you can indeed have multiple air loops serving a single thermal zone. I use it all the time in our residential EnergyPlus models. That said, I've only ever used it for models with a single conditioned thermal zone; I don't know how EnergyPlus would behave if there are multiple air loops, each of which are serving multiple zones, and some of the zones are served by both air loops.

shorowit's avatar shorowit  ( 2021-07-13 12:40:35 -0500 )edit

Thanks @shorowit I thought something had changed from working with OpenStudio's ThermalZone::airLoopHVACs, but then it was right there in the documentation. And the docs are never wrong, right!?

ericringold's avatar ericringold  ( 2021-07-13 13:36:54 -0500 )edit

Thanks @Eric Ringold and @shorowit ! I looked in the ExampleFile. It has two Air Distribution Units per zone. This is what I was looking for. It also taught me that an airloop for DOAS does not necessarily require a Zone Return Air Node. And another big learning for me is that I/O Reference is not always exactly updated with the new version of EnergyPlus.

Keigo's avatar Keigo  ( 2021-07-14 21:53:21 -0500 )edit

If you see outdated documentation, please report the issue at The EnergyPlus developers are very good about fixing the documentation when they know there is a problem. Given how large the documentation is, there's no easy way to audit all thousands of pages for information that needs to be updated when a new feature (like support for multiple air loops) is added.

shorowit's avatar shorowit  ( 2021-07-14 22:38:20 -0500 )edit

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Asked: 2021-07-13 00:58:28 -0500

Seen: 348 times

Last updated: Aug 08 '21