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What's the correct way to model central air-to-water heat pump with backup electric boiler and hydronic baseboards?

asked 2020-12-18 13:10:31 -0500

ProbingParticle gravatar image

updated 2021-04-07 13:12:52 -0500

I'm modeling a 2-floor school building in eQuest with multiple zones and planning to use one VAV AHU per floor. I have created a 2-pipe loop with chiller of type = Heat pump to represent air-to-water heat pump. Also added is an electric boiler to this 2-pipe loop. I have created an equipment control and load management strategy to only use the boiler below 15F when the heat pump chiller no longer produces heat. This loop is referenced by the central VAV AHU hot water and chilled water coils.

My question is regarding the zone heat source for the above system? Do I use classroom reheat coils or zone baseboards?

Do I create another loop with a separate electric boiler to connect zone baseboards or use this loop to add reheat coils to the airside system?

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answered 2020-12-22 09:08:10 -0500

bfountain gravatar image

Using a chiller of type Heat Pump forces you to use the loop type 2-pipe as that defines whether the heat pump is in heating or cooling mode. The challenge as you have observed is that when the loop is in cooling mode it is not available for reheat.

One solution that was recommended to me was to have two 2-pipe loops -- one with forced to heating mode with heating loads on it and the other with cooling loads on it forced to cooling -- each with its own copy of the air-source heat pump chiller and with a supplementary boiler on the heating 2-pipe loop.

Notes: 1) I have found that this works when the loops are commanded to heating and cooling using Snap on Outdoor Air Temperature (very high for heating loop, very cold for cooling loop). It did not work for me when I tried scheduling the loop operation. (Not sure why.) 2) I was unable to add a heating coil to the airside system but could add a preheat coil and reheat coils. Adding a heating coil shifted the cooling load to the heating loop which results in the error that there was no load on the chilled water loop.

Thank you to Bill Bishop for this recommendation.

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Asked: 2020-12-18 13:10:31 -0500

Seen: 145 times

Last updated: Jan 11