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Exhaust Air Heat Pumps in Equest

asked 2023-07-12 15:05:05 -0500

Oumina's avatar

updated 2023-07-13 14:29:26 -0500

I am trying to model an exhaust air heat pump using equest. Anyone had previous experience modeling this system in equest? Thanks for any help you could provide!

The system recovers heat from exhaust to increase the ASHPs efficiency. HW boiler will work as a backup during outside cold temperatures. The cooling will be provided mainly using chilled water but the ASHPs will operate to satisfy a portion of the cooling load.

Equest users, please help! Thank you!!

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answered 2023-07-18 10:06:08 -0500

Drew Morrison's avatar

updated 2023-07-18 16:50:48 -0500


That's quite an interesting system you have! Unfortunately, I'm not aware of any way to model a heat pump that uses a building exhaust stream as a heat source/sink in eQuest. At the risk of understatement, it is not a commonly employed HVAC system. If you have to move forward I think your best approach would be to use eQuest's hourly reports to print out the hourly heating and cooling loads for your heat pump, the outside air temperature, and the temperature and flow rate of your exhaust stream. Then, using the heat pump performance curves provided in eQuest, calculate the cooling (or heating) COP for both the outdoor air temperature and the exhaust air temperature. Finally, use those two COPs to calculate the heat pump's electricity consumption for each scenario; the difference will be your (very roughly estimated) energy savings.

From a design perspective, I think this strategy carries some risks. Even if the exhaust flow rate is huge, a building exhaust will still have a very limited capacity to absorb or provide heat compared to all the outside air in the immediate vicinity of a heat pump condenser located outdoors. I could easily envision a scenario where the condenser raises or lowers the exhaust stream temperature beyond the heat pump's operational limits, which could "choke" the heat pump and limit its capacity. I don't mean to discourage you from investigating the idea; I just want to make you aware of the risks. You might be better served with another energy recovery strategy like using an energy recovery ventilator to preheat or precool makeup air.

Good luck!

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This sounds similar to an optional energy code pathway for laboratories in MA. Large airflows 24/7. The HP condensing coil would be on the outlet side of the exhaust stream which will always be warmer than ambient for heat recovery purposes. I modeled this in energy plus and had to post process all the hourly data similar to how you describe above. Not aware of any commonly used tool that can simulate this directly; equest, E+, HAP, IES etc....

ashopinion's avatar ashopinion  ( 2023-07-19 17:47:24 -0500 )edit

The fact that an exhaust air-source heat pump is an optional code compliance path is quite interesting, @ashopinion - thanks for bringing it to my attention! My first reaction was to consider this a niche system, so I'd be interested to hear how often designers do it. I agree this is a job for postprocessing using hourly model output of exhaust temperature and flow rate. Could be a very nifty spreadsheet or script...

Drew Morrison's avatar Drew Morrison  ( 2023-09-08 11:07:47 -0500 )edit

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Asked: 2023-07-12 15:05:05 -0500

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Last updated: Jul 18 '23