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How can I model an ASHP water heater for DHW along with an ASHP for HVAC similar to the one described here using detailed HVAC option

asked 2021-10-19 15:40:09 -0600

Simran gravatar image

Hi everyone! I'm trying to simulate a residential retrofit project with the new proposed HVAC with both - an ASHP for HVAC with heat recovery and - an Heat Pump Water heater for DHW (domestic hot water) similar to this one here:

For the HVAC I've been using a "Air Loop Unitary HP Air to Air" with heat recovery turned on. This is supplied to all of the zones. For the Heat pump water heater for DHW I've put in a "Water Heater Heat Pump" and connected it to the DHW loop. The "water Heater Heat Pump" is located within a zone of the building since it'll be an indoor equipment using indoor air.

When I try to run this file, I get a severe error which mentions: "Plant temperatures are getting far too cold, check controls and relative loads and capacities"

I've tried to run this same file previously with a different DHW water heater system (sourced from electricity) and it has been working but I'm not sure how to model the system I'm referencing from the website mentioned above in conjunction with the ASHP HVAC system.

It'd be great to get some help with this issue as soon as possible. Thank you!

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answered 2021-10-21 06:09:55 -0600

Filipe Sousa gravatar image

If the system has worked with another plant source then the issue lies with the ASHP. This equipment is quite complex since it is an assembly of different HVAC objects. However, I would suggest you to run a test and hard set the "Internal heating element" capacity of the ASHP Water heater. Based on my experience for some unknown reasons the heating element autosizing is not working correctly in EnergyPlus and hard setting this field fixes the plant loop severe warnings.

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Hi Filipe, Thank you for your response, I've tried this method, however the results are a bit alarming. When looking at the monthly fuel breakdown, there is a heating load in the summer time and the DHW energy use is inconsistent throughout the different months. I'm not sure how to interpret these results or what is causing this. The only thing I've changed is the DHW hot water heater. It'd be great to know if you have any input on this.

Simran gravatar image Simran  ( 2021-11-29 18:03:51 -0600 )edit

answered 2021-11-18 17:33:49 -0600

Eli Gould gravatar image

I hope the answer from Filipe fixed the problem; I actually started using this combo on some pilot projects 8 years ago and had to use some experience-based rules of thumb to figure out how they'd all interact. I ended up comparing the effect of the HP hot water heater to a small room-sized window air conditioner running five hours a day, and it worked out well to predict a lot. The ducted units added another level, so it will be interesting to hear back if your model lines up well in this scenario first.

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Hi Eli, Thanks for your answer. It did manage to get the model running but there are some inconsistencies in the results with a summer heating load and an inconsistent DHW load between the different months. If you know anyway to resolve this issue that'd be great.

Simran gravatar image Simran  ( 2021-11-29 18:05:24 -0600 )edit

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Asked: 2021-10-19 15:40:09 -0600

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Last updated: Oct 21