Are you considering replacing your current water heater with a heat pump water heater but worry that it’ll freeze your basement in winter? Well, enough homes have this new technology that we now have some data on this topic. A new paper by Slipstream is just out with some results that may interest you. Titled “Installed Performance of Heat Pump Water Heaters in a Cold Climate,” the paper covers many facets of heat pump water use. I’m going to discuss only one here: the effect on air temperature.
The homes in the study
First, note that I’m going to show data below from two different parts of the study. One was a field study with 9 homes. The other part was a survey that included the 9 field study homes plus another 72 homes, for 81 total.
The 9 homes in the field study were in rural Michigan. The heat pump water heaters were all in partially conditioned basements. Seven were in the main part of the basement; two were in a separate room. All of the water heaters were 50-gallon models from Bradford White, Rheem, and AO Smith. The Slipstream researchers monitored water temperature (hot and cold), water flow rates, electricity consumption, inlet air temperature, and indoor and outdoor temperatures.
The objectives of the study were:
- HPWH Performance: Characterize measured energy factor and effective capacity at different operating modes.
- Economics: Calculate cost and energy savings compared to an electric resistance water heater.
- Space heating impacts: Quantify changes in basement temperature and estimate HPWH’s impact on space heating and cooling operation.
- Customer satisfaction: Survey on experience and satisfaction with installation and performance.
(I encourage you to read the whole study, if you’re interested in this topic. There’s a lot in it!)
Effect on basement temperature in the field study
First, the field study monitored the basement temperature at four heights at a distance of 5 to 10 feet from the heat pump water heater. The graph below shows the cycles of the heat pump water heater at the top. In this case, there were three cycles during that 28-hour period.
The lower part shows the air temperature in the basement at the four temperature sensors. One interesting feature of the temperature data is what they show about the cycling of the water heater and the furnace. The floor sensor shows the water heater cycles most clearly—the big dips coincident with the water heater cycles. The ceiling sensor shows the furnace cycles most clearly—the choppiness of the curve, especially between about 1 a.m. and 6 p.m.
In terms of the effect on basement temperature, the data above show a drop of about 4°F near the floor as the heat pump water heat operates. Then the temperature recovers pretty quickly. For the 9 homes overall, the average temperature drop during a cycle was 2.3°F. After a 4-hour recovery period, the average basement temperature was 0.1°F lower than it was before the cycle. The study didn’t measure the long-term effect on basement temperature over the course of a whole winter, but that would be interesting to see.
Note that these basements aren’t really cold. The range of temperatures at the beginning of a heat pump water heater cycle was 62 to 65°F. My unheated, uninsulated (for now!) basement in Atlanta has been about 60°F for the past 6 weeks. The basements in the field study, recall, are partially conditioned, but they aren’t independently heated. In other words, the thermostat is upstairs.
Satisfaction with effect on temperature and humidity
The other part of the study was a survey, and one of the questions participants were asked was, “Do you like the changes to temperature and humidity?” The results, separately for summer and winter, are shown in the chart below. In both seasons, the majority had no opinion. Seventeen people liked the changes in winter. Only 9 people didn’t like the cooler basements.
I have no idea why anyone would like their basement to be cooler in winter. Then again, I don’t understand why someone would choose to live in a cold climate that’s not near a mountain with 2,000 vertical feet of ski runs.
It’s easier to understand the summer results for this question. If a heat pump water heater keeps a basement cooler and drier in summer, that’s a great thing. One of the survey respondents wrote: “One added benefit: I do not have to run my dehumidifier in the basement anymore.”
Factors to consider
If you’re on the fence about putting a heat pump water heater in your basement (or elsewhere inside your house), here are some things that can affect how much impact it will have.
- Amount of hot water use. The more hot water you use, the more the heat pump water heater will run and cool off the house.
- Water heater mode. These things come with a heat pump and electric resistance heating (strip heat). You can operate them in one of three modes: all heat pump, all strip heat, or a hybrid mode that supplements the heat pump with strip heat as needed. If you use heat pump only, it will do more cooling.
- Size of the basement. The bigger the basement, the more air there is to dilute the cooling effect.
- Basement use. If you have an office in the basement, you may need a way to heat the space independently of the upstairs. If your main use of the basement is for exercise, you may welcome the cooling.
The Slipstream heat pump water heater study shows that fears of freezing your basement are overblown. And if you find that your basement does get too cold, you can always turn it to strip heat mode.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.
Get building science and energy efficiency advice, plus special offers, in your inbox.