multistack air to water heat Pump in eQuest

asked 2021-08-12 16:17:21 -0500

updated 2021-08-12 16:17:57 -0500

Have a model where a MultiStack Air-Source HP Chiller/Boiler is being considered. This unit is air-source and can operate in cooling only, cooling & heating, or heating only. Hot Water loop will have Electric Boiler as back-up/supplemental. Climate 4A. Air Side is a Multizone VAV with HW / CHW. Only resource I found online is quite dated, from Dana Troy in September 2011. However, that write-up was using a ground coupled system for loop imbalance, my potential design will not.

Within eQuest I found the Type: Heat Pump Chiller, but it must be tied to a single Chilled Water or a 2Pipe Loop. The first work around I tested was to create two loops (Chilled Water and 2Pipe) with double the normal number of chillers and use equipment controls for sequencing. The 2Pipe loop had a High Snap temperature to always keep it in heating mode & act as the Hot Water Loop. The Air-Side allowed Chilled Water loop assignment for cooling, and allowed the 2Pipe Assignment for Heating and Zone Heating. Though, this run resulted in 3000+ unmet hours and the Chilled Water Loop to attempt to operate at 70F. If the Heating source to changed to electric, the unmet hours decrease and the loops control as expected. If I change the Heating Source to a separate dummy/test Hot Water loop, this too allows proper operation. It would appear that when attempting to assign a 2Pipe loop to both Heating and Zone Heating causes the Chilled Water Loop to misbehave(?!). Anyone else experienced this or have other solutions?

In an effort to work around this issue, I swapped the Heat Pump to a Loop-to-Loop Heat Pump Option. This allowed the creation of a Hot Water & Chilled Water loop – but also requires a Lake/Well Loop. Luckily, a Ground Loop Heat exchanger can take a schedule of temperatures. Thus, I extracted the Outside Air temperature to an 8760 schedule and assigned this to the Ground Loop Heat Exchanger. This way the loop is ~to the OA temp, but having doubts that this is legitimate as I am seeing a significant delta in the hourly reports between the Loop Temperatures and the OA Temp for some hours.

Open to ideas. Thanks in advance.

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