HPHW Heater in line with Gas HW tank… which first?
I have a 2 family, converting to 1-family living. I’m discontinuing one gas HW tank heater pre-emptively due to age, replacing with HPHW Heater and will leave the other gas HW tank for now. In short, I’d like to put them in line to save energy costs and to make sure we have a constant supply of how water. Also there’s a need to keep that one gas HW tank so that I don’t need to change my chimney liner.
Is it correct that I should put the HPHW Heater first (always in Heat Pump mode) to supply the gas HW tank for storage? This provides
1) efficient HP heating of cold water, and 2) even if HP runs low on hot, the gas HW tank will quickly kick it up to temp.
Am I on the right track?
Lastly, would there be a benefit to also adding a mixing valve to the HPHW heater, heating HP to 140F, and mixing down to 120F?
I’m in MA, where electric costs are almost $0.29/kW.
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Replies
This would be the way to do it, but provide a way to have hot water if one of them is out of commission. I think you'll find that this is a lot more capacity then you'll end up needing. I don't see a benefit to the mixing valve if energy costs are expected to remain close.
The HPWH will always win on carbon emissions, but money wise will probably lose out somewhat. Therefore if you want to save the most money - put the gas tank first. If you want to save the most emissions - put the HPWH first.
Either way, the second water heater will not be used that much. More for a natural draft water heater because they have huge standby losses due to having a giant column in the middle of the tank exposed to ambient air.
I think it’ll be close cost wise? Gas isn’t that cheap either and a typical big box tank is very low efficiency.