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Heat pump water heaters (HPWH) provide an efficient way to heat water with electricity. Like refrigerators, air conditioners, heat pumps, and dehumidifiers, HPWHs use the vapor-compression cycle to move heat from one location to another. In the case of HPWHs, heat is absorbed from the surrounding air and transferred to water in the tank.
If the air moving through the HPWH cools to its dew point temperature, water condenses on the evaporator coil and drips into a drain pan. From there, the condensate can be piped or pumped to a drain. Because HPWHs can remove water vapor from the surrounding air, manufacturers and efficiency programs have sometimes touted their dehumidification benefits. But how much do HPWHs actually dehumidify? Should they ever be considered as a substitute for an actual dehumidifier?
These questions come up periodically in the GBA Community Q&A forum (see, for example, these threads from 2018 and 2019). Here, I’ll look at a few studies and anecdotal reports of HPWH dehumidification. I’ll also share results from my own home. Using the same tests I described in an earlier article on dehumidifier performance, I’ll answer the following questions:
- Can my HPWH, on its own, keep my basement at a healthy humidity level?
- How much condensate does my HPWH produce? How does this result depend on whether my dehumidifier is in use?
- What do air-side measurements tell us about a HPWH’s dehumidification capacity under different ambient conditions?
Published reports on HPWH dehumidification
The literature on HPWH dehumidification is very limited. The most detailed study I found was a 2016 paper, “Fifteen Years of Dehumidification Results from Heat Pump Water Heaters,” by William E. Murphy. Murphy monitored humidity and condensate production daily for two HPWHs in the garage of his home in Kentucky.
From 2003 through 2007, he…
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5 Comments
Thanks for the article. I think this settles the question nicely. I could see greater dehumidification in commercial mechanical rooms assuming proper staging/controls to maintain base load run time.
I greatly appreciate the measurements of how much water is being removed from the air at different rH levels. But I think you are being modest about being a low hot water usage home. Looking back at runtimes for my HPWH in Emporia, my household of three had run times closer to four hours per (week)day (with a pre-teen daughter).
I'm in a 1945 home in central Indiana. Nearly every home with a basement around us should have a dehumidifier running in the summer time. I've done a bit of air sealing (2400 cfm@50 Pa for ~2400 sqft home) and some moisture mitigation. Along with an appropriately sized central A/C, I've been able to stop using my dehumidifier in the basement since the HPWH was installed. There might be a week or two during the summer after a longer wet spell where it feels like a little more dehumidification would be nice (rH ~66%). But for the most part, the HPWH is able to handle the leftover latent load the central A/C doesn't handle during more moderate temperature days and keep the basement rH around 55% +/-5%.
Maybe I'm the exception due to not much additional latent load and longer HPWH run times, but not needing to run an additional dehumidifier in the basement has helped the value proposition in my case.
This is interesting. Thank you. I think you are right, I may be on the low-use tail of the hot water curve. I'll take another look at HPWH run times when family is here over the winter holidays. By then, I wouldn't expect to see much condensate from the winter air, but I should see more 3-4 hour blocks when the system is running.
My experience tracks with the author's, though I'm in the D.C. area with a household of four. I haven't measured the condensate from the HPWH, but I can see it drain into the laundry sink. There isn't much compared with the dehumidifier. My HPWH runs about 2 hours/day (rough approximation from ~2 months of Emporia energy monitor data) and is set to 130F during the day and 125F overnight.
Great article and great findings.
A few things to consider is that ANY dehumidification experienced via a HPWH is FREE dehumidification. Further the biproduct (dry & cool air) can be ducted anywhere that it makes sense to have/use this air. In our deep-energy-reno'ed 1945 bungalow, I ducted the biproduct air from the basement (where I do not need any cooler air) to one floor up (helps with stratifications) letting out below our refrigerator coils (where it can use cool air) which is located in our south-facing kitchen (all characteristics that typically welcome cooler dry air). Thus while it may not replace a full blown dehum, the air it provides (albeit infrequently) still should be a selling point. A HPWH is a now brainer for me.
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