Ducted Heat-Pump Water Heater
Hello. I recently purchased a small home near Tillamook, OR that badly needs a new water heater. I’m very interested in a HPWH to recue energy usage, however I believe the only configuration that would make sense is to duct the intake and exhaust to the outside.
The reason: the installation location will be a tiny utility closet, so it will have to be vented to somewhere. The house is heated by a terribly inefficient open-circuit electric furnace–yes, my furnace pulls intake air from my unconditioned crawlspace, heats it on electric coils, and then the house is passively vented to the outside (not to the crawl). I didn’t know such a wretched design exists, but I have it. That furnace will certainly be replaced as soon as I can, but for now that’s what I’m dealing with. So, if I were to pull the HPWH heat from inside the conditioned space, that’s where the replacement heat would come from. And due to my location, I never need air cooling, so discharging inside also is a penalty. Thus my conclusion that I’d need to duct outside.
Questions: does it make sense to use a HPWH in this scenario? The air temperature is cool but rarely cold; hardly ever approaches freezing. I’ve read the articles and discussions here (e.g. Saving Sustainably: Installing a Heat-Pump Water Heater – GreenBuildingAdvisor) and I’m not sure. Dana seems skeptical, but it looks like I’ll still achieve something like a COP of 2 (versus close to 4) based on a study he linked to. If I did go this route, would pulling the intake from the unconditioned crawl make sense? I’m thinking it’d be warmer than ambient air due to ground heat.
Thanks for your time,
Gary
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Replies
You already mentioned the hit in COP by ducting it outside. They can also be quite loud, I would not put one in a utility closet in a small home unless there is special consideration to isolate the noise. Your house might be a good candidate for a Sanden hpwh though given you don’t need cooling. It can provide ~8 btu/hr radiant heat as well. I’m sure it would be a future project, but something to consider.
If you have crawlspace, duct the intake and exhaust of the water heater there. As long as the volume of the crawlspace meets the minimum size required by the manufacturer it will work well.
Ducting will also eliminate a fair bit of the noise from the HPWH, most complaints are from noisy fans on some units.
Would this approach create any concern with the quality of the air? Smell like crawlspace, etc.? I'm installing an HPWH in a small utility closing under the stairs (there is enough vertical space for the HPWH), but trying to determine the best way to duct.
Looks like your climate has highs above 50 most of the year. If you use outside air and you put it on a timer to run in the afternoon, that will be plenty warm to get good benefit from the heat pump. And even if you had 3 months when you didn't use the heat pump and just used the electric heater in it, you'd still win vs. straight electric all year.
I have a HPWH that I’ve ducted multiple ways but generally pull air from inside the house. During the warmer months it’d make sense to duct it to the interior. If you get a hybrid unit you can switch it to less efficient resistive mode. My hpwh is in a storage room that’s attached to the house but separated by a wall. I can barely hear it when it’s running
If your hot water needs are lower, a regular electric tank might be the easiest option. Yes, it’s less efficient but likely you’ll come out ahead financially and since it’s run on electricity, you can offset it with on or off site green power. If you have access to the shower drain pipe, a drain water heat recovery pipe would save you about as much as a lower COP heat pump water heater with none of the compromises: it would be silent, long lasting and with zero ductwork involved.
As for the cost, it depends which model you get. I guess that's pretty obvious though. I got a Rheem hybrid and it was $1,200, but the local electric utility was giving a $300 rebate at the time. Their regular electric resistive model in the same class with $600 so it ended up only costing me an extra $300 and I figure I made that back in about a year. I installed it in 2019. Here's a screenshot of last week where I switch from heat pump mode to electric resistive because it's really cold here these last few weeks and the condensate line that drains to the outside has frozen on me twice before over the last 2 and 1/2 years. Then the water backs up and there's a problem I don't need. The solid bar shows where the daily usage is in resistive mode and the graph with the dots is heat pump mode drawing hot air from the house.