Combatting long recovery times during cold weather with a heat-pump water heater
we moved into a new home (border of CZ 3&4) this past July, and one of the first improvements we made was to junk the 20yr old, poorly vented gas water heater (it was in the hall coat closet!) and replace it with a new state 50gal heat pump hot water heater located in the vented garage attic. We would have preferred the 66 or 80 gal, but due to space constraints and the fact that we’ve been fine in the past on a 40 electric, I figured 50 heat pump would be good enough.
Through the summer and fall it has performed very well, in hot weather, when the attic is over 100degF, we have very nearly unlimited hot water. As the weather has gotten colder, we’ve begun to have issues with long recovery times, with the heat pump running for up to 2-3 hours after someone has taken a shower, which can make the morning rush somewhat unpleasant. I’ve already bumped up the output temp from 120 to 125, and I’m hesitant to go higher without creating a scald risk.
I thought I’d cast for opinions on the ways to combat this, in general order of preference:
1) pipe insulation – there are some long copper runs through a vented crawlspace that need to be insulated in any case, but I don’t expect this to solve the issue entirely. ~$30-40 in pipe foam
2) Electric booster – rheem makes one for about $300 and they claim it gives a 50 gal the capacity of an 80gal. Only kicks on when the temp coming out of the main WH drops below a set point, so it likely wouldn’t run in the summer at all. I like this option because it’s simple to install, and $300 is about the price difference of the 50gal and 66gal heat pump models.
3) Air seal the garage. Currently it’s pretty well vented, with a gable vent, ridge vent, and soffit vent. Temps currently runs about 3-8 degrees above outside air, and the garage is not used for chemicals or vehicles so I’m not concerned about air quality, as long as it’s easy to open the vents up later if we move. This also probably doesn’t get all the way there, but may help if I can keep the garage 20deg or so above ambient.
4) Drainwater heat recovery – This was my first choice until I realized how much I’d have to re-route drain and supply pipes. Drain runs down an outside wall and supply is about 10 ft away through an interior wall. May have an option for horizontal install in the crawl, but still quite a bit of work. About $900 for all needed materials, and a lot of time, possibly including patching and painting. Probably too expensive and too much work.
5) passive booster – a little pipe setup that mixes 140degree hot with cold to output 120 Degree hot water. extends capacity but means the heat pump is making 140degree water even on days when 120 would suffice. Cheap and easy install.
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