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Air Loop Water to Air DX Coil?

asked 2017-12-06 08:06:38 -0500

jmoody's avatar

updated 2017-12-06 13:21:14 -0500

We typically use DOAS units on our goethermal projects that have water to air coils (typically cooling and HGRH). I noticed all of the DOAS with GSHP measures model the DOAS units with chilled water coils and a chiller. I am relatively new to Energy Plus and Open Studio but it appears that you cannot directly model a water to air DX coil in an air loop at least not one that I can find.

Am I correct in this assumption? Is modeling it with a chilled and hot water coils the only way to go?

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answered 2017-12-06 10:14:15 -0500

updated 2017-12-06 10:16:10 -0500

You can model water to air DX coils on an air loop by using an AirLoopHVACUnitarySystem. In the Library menu, if you expand the 'Air Loop HVAC Unitary System' selection, at the bottom you'll see 'Unitary - WaterToAir Heat Pump - Cycling - Elec reheat', which you can drag to the supply side of an empty air loop. That object is a composite system containing a Fan:OnOff, Coil:Cooling:WaterToAirHeatPump:EquationFit, Coil:Heating:WaterToAirHeatPump:EquationFit, and Coil:Heating:Electric. You can connect the coils to your ground source loop via the chain icons at the top, similar to how you would connect water coils.

You'll probably also want a OutdoorAirSystem on the air loop.

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Eric, thank you for this. I have done several projects where I modeled the geothermal with zone water to air heat pumps and just split out the ventilation in post. I have three >200,000 ft^2 schools in design right now and I really wanted to account for the DOAS that we will be using on the geothermal loop load.

jmoody's avatar jmoody  ( 2017-12-06 10:24:28 -0500 )edit

answered 2017-12-06 09:48:43 -0500

updated 2017-12-06 09:49:39 -0500

You do want to use a Coil:Cooling:Water and a Coil:Heating:Water, served by your geothermal object.


I think you might have a problem with the terminology here, and that's what creating the confusion (or I have a problem with your terminology and didn't understand, that's a possibility too)

DX stands for direct expansion. It means that it's the refrigerant that is in the coil placed in the air stream. The refrigerant is expanded, evaporates, and therefore provides useful cooling to the air stream.

Coil:Cooling:Water is a coil is placed in an air stream and has a fluid in it (typically water, or water + glycol). This coil is therefore also placed on the demand side of a plant loop. Said plant loop can have whatever you want on the supply side: a chiller for example (like the measures you're talking about), but it could be a geothermal object too.

Similarly, the Coil:Heating:Water can be served by a plantLoop with a geothermal object.

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Sounds like OP might actually want an air loop containing water-to-air heat pump coils that reject heat to a ground loop. Kind of like this situation (except with a geothermal object on the plant loop instead of tower-boiler).

Eric Ringold's avatar Eric Ringold  ( 2017-12-06 10:02:16 -0500 )edit

That is exactly what I am looking for and yes my terminology might be off a bit as I dig through the energyplus documentation to narrow it down a bit.

jmoody's avatar jmoody  ( 2017-12-06 10:11:44 -0500 )edit

It all worked out in the end, so I'm glad even if I was off topic ;)

Julien Marrec's avatar Julien Marrec  ( 2017-12-06 11:15:25 -0500 )edit

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Asked: 2017-12-06 08:06:38 -0500

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Last updated: Dec 06 '17