Heat-Pump Water Heaters in Cold Climates
Heat-pump water heaters save energy on a year-round basis — but during the winter, the energy savings will drop
In last week's blog I wrote about the GE GeoSpring heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. in our new house — first, why we decided to go with electric water heating over solar thermal (since we use solar to generate as much electricity as we will consume), and then how we decided on a heat-pump water heater instead of one of the other electric water heating options. This week, I’ll get into a little more about heat-pump water heaters and some of the issues that come into play when installing them in cold climates.
To review from last week: a heat-pump water heater (HPWH) extracts heat out of the air in the room where it is located to heat the water. I’ve described previously the basics of heat pumps and how they perform this counterintuitive process of taking heat from a colder space and heating something that’s warmer. Suffice it to say that that process (relying on the vapor-compression or refrigeration cycle) works and has been used for a long time in refrigerators, air conditioners, and heat pumps used for heating and cooling houses.
Cooling the space where the appliance is located
What this means for a HPWH in a house like ours, is that it cools the space where it is located. That’s a good thing in the summer — free air conditioning — but in the winter it’s not such a good thing. That’s especially the case in a cold climate in a house without a standard heating system.
In a typical New England house that has a furnace or boiler in the basement producing a lot of waste heat, a heat-pump water heater can use some of that waste heat and it’s not really very noticeable. The less efficient the heating system, the less noticeable is the effect of the HPWH.
But we don’t have a heating system in our basement. As a result, our HPWH cools the space. With the cold weather we’ve had (as I write this, it’s about 2 below), our basement has stayed pretty cool: typically 50°-54°F, though with the exceptionally cold weather we had a few weeks ago during a time of heavier hot water usage, the temperature dropped as low as 47°F. Right now, it’s 52°F.
I think our basement temperature would be considerably lower if my wife and I used a lot of hot water, but we’re pretty frugal, and my wife swims a lot, so she often showers at the pool where she swims.
Robbing from Peter to pay Paul
In cooling the space where it is located, a HPWH makes the heating system work harder. In our house the heating system is a single minisplit air-source heat-pump; the indoor cassette is mounted on a first-floor wall. That system delivers heat to the basement through the uninsulated floor and through the basement door, which we usually leave closed, but sometimes leave open.
We also have a fan and ductwork at the top of our stairs so that, if we need to, we can pull warm air from the heated space in the house and dump that into the mechanical room in our basement. Eli Gould suggested this as a backup in case the basement got too cold. We haven’t used that fan, though, because it’s too noisy.
So our 18,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /hour Mitsubishi air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. has to work harder (and use more electricity) because it’s also indirectly heating our water. With the really cold weather we’ve had since moving into the house in early January, our air-source heat pump has been working pretty hard to keep up. And I think the HPWH has contributed to our first floor being a little cool — especially near the floor. (Having heated mostly with wood for 30-plus years, this has been a very minor issue, but cooling our basement with the HPWH may be keeping our first floor a little cooler than I would like — especially near the floor level.)
My friend Lester Humphreys in Brattleboro, who also has a HPWH in his basement but has an oil-fired boiler there as well, has done some back-of-the-envelope calculations to estimate how much oil he’s using for his water heating, and he asked me to look over his numbers:
“I calculated the loss from our living space through the floor to the basement using the formula Area x 1/R x delta T; I figure our heat-pump water heater lowers the temperature in the basement by about 3 degrees, our wood floor has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 2.75, and the basement ceiling is 1,217 square feet. This gives me heat loss of 1,314 BTU per hour. Running 6 hours a day (probably a little high), with a delivered heat efficiency of 70% for our oil system, this equals 2.4 gallons a month (about $9), which is not bad.”
With our hot water usage, the electricity consumption directly by the HPWH isn’t that great: 56 kilowatt-hours (kWh) in February — or about $8 worth at 15¢ per kWh, which is Green Mountain Power’s current residential electric rate. Consumption averaged a little less than 2 kWh per day in February, jumping to over 8 kWh one day when both of our daughters were visiting from out of town and we had to switch the water heater to the “Boost” mode (in which an electric resistance heating element supplements the heat-pump mechanism).
Along with that 56 kWh used by the HPWH, though, some of the 814 kWh used in February by our minisplit air-source heat pump heating system (about $125 worth), was for water heating. I haven’t calculated what our total water heating cost was for February, but that should be possible to do.
Another thing to keep in mind is that HPWHs have a quite slow recovery rate; I think that ours recovers at a rate of about 8 gallons per hour. This is why larger water heaters often make sense with HPWHs, though I thought a 50-gallon model would be all right for our usage.
In mid-February, though, our younger daughter from New York City and our older daughter and financé from California were visiting, and we had a party. We still might have been all right with hot water, since we have WaterSenseProgram developed and administered by the U.S. Environmental Protection Agency to promote and label water-efficient plumbing fixtures. plumbing fixtures and a high-efficiency dishwasher, but our younger daughter decided to take a bath after we had all done a lot of party prep. She ran out of hot water before the tub was all the way filled.
Fortunately, most HPWHs, including our GE GeoSpring model, allow you to change the mode. I normally operate the water heater on Heat Pump Only mode, but switched it to Boost mode for a few hours that Saturday.
The jump in power draw was dramatic (shown by my eMonitor). In Heat Pump Only mode, the power draw peaks at about 500 watts, but that jumped to 5,000 watts in Boost mode.
The other issue to consider with HPWHs is that they have fans and compressors that are noisy. I don’t think I would consider a HPWH if we didn’t have a basement and had to place the water heater on the first floor. We have fairly good acoustic isolation between our basement and first floor — and the water heater is in a mechanical room to which we can retrofit ceiling and wall insulation if the noise proves annoying.
So far, the noise isn’t very noticeable, but in the summer (when the air-source heat pump is unlikely to be running) we may find that we can hear the water heater — in which case I’ll probably insulate the mechanical room. Noise did play into our product selection; when I was researching options, the GE GeoSpring was the quietest HPWH I found.
The bottom line
In summary, we’re happy with our heat-pump water heater, despite the cold climate and the fact that we don’t have a waste heat source in our basement. I’m guessing that, for half the year, we’ll save at least 60%, compared with a standard electric water heater, while we'll save only 10-20% in the cold months. Homeowners with a waste heat source in their basements will do better.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
- eMonitor data, Alex Wilson
Thu, 03/06/2014 - 10:56
Thu, 03/06/2014 - 11:48
Thu, 03/06/2014 - 12:49
Thu, 03/06/2014 - 17:08
Thu, 03/06/2014 - 17:11
Thu, 03/06/2014 - 18:29
Thu, 03/06/2014 - 21:38
Fri, 03/07/2014 - 12:18
Fri, 03/07/2014 - 13:06
Fri, 03/07/2014 - 21:16
Sun, 03/09/2014 - 08:06
Tue, 03/11/2014 - 10:50