Blog Post

Heat Pump Water Heaters in Cold Climates: Pros and Cons

While a heat-pump water heater will save significant energy on a year-round basis, be aware that in a cold climate the net performance (water heating plus space heating) will drop in the winter.

Electricity consumption by our GeoSpring heat pump water heater in February. Note the spike mid-month when I switched the mode to "boost." Click to enlarge.

Image: Alex Wilson
We chose a heat pump water heater for our new house, and as I've recently discussed here, there are a lot of reasons why you might be doing the same.

Using an air-source heat pump, heat pump water heaters (HPWHs) extract heat out of the air where they are located to heat the water.

That means that a HPWH cools the space where it is located. That’s a good thing in the summer—it doubles as air conditioning—but in the winter it’s not so helpful. That’s especially the case in a cold climate in a house without a standard heating system.

Cooling the space where they are located

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°F) and our basement has stayed pretty cool: typically 50°F–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. Our basement temperature would probably be considerably lower if my wife and I used a lot of hot water, but we're pretty frugal.

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Robbing from Peter to pay Paul

Minute-by-minute electricity consumption by the water heater over a two-day period. The line in green shows consumption through mid-morning on March 5th; consumption on March 4th is shown by the gray line.

Image: Alex Wilson
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 mini-split air-source heat pump wall-mounted unit 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.

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. This is a back-up in case the basement gets too cold, but we haven’t used that fan because of its noise.

So our 18,000 Btu/hour Mitsubishi air-source heat pump 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.

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 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 heater lowers the temperature in the basement by about 3 degrees, our wood floor has an R value of 2.75 and the basement ceiling is 1217 square feet.  This gives me heat loss of 1314 BTUs per hour.  Running 6 hours a day (probably a little high), 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 mini-split 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.

Slow recovery

Another thing to keep in mind is that HPWHs have a quite slow recovery rate—I think ours recovers at a rate of about eight gallons per hour. This is why larger water heaters often make sense with HPWHs, though I thought a 50-gallon model would be alright 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 WaterSense plumbing fixtures and a high-efficiency dishwasher, but our younger 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.

Noise

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.

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 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.

Published March 5, 2014

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Comments

November 4, 2019 - 12:41 pm

Michel, you're right the Kill-a-Watt only works with 110V loads. I installed an eMonitor system fro SiteSage when we renovated/rebuilt the house. You attach CTs (Current transducers) onto the wires going into specific loads in your electrical service panel (an electrician needs to do this) and the CTs measures current flow—or something like that. The problem is you need a system to capture that information and make it available. The eMonitor system had a costly annual fee to maintain access to the data, and I decided not to maintain that, especially after one of the components failed and I was going to have to buy a new one. There may well be other systems available for energy monitoring now that are cheaper.

November 2, 2019 - 2:55 pm

Hi. Just finished installing  myself a Rheem 50-gal HPWH in our unheated/ uninsulated attached garage.  Took more work than I thought, especially to install a drain for the condensates.  Glad I remembered how to glue PVC pipes and how to sweat copper pipes as well.  And that I still had the ramps I used to use for changing oil for my old cars; I used them to lift the HPWH in place on top of its stand.
Question for yoou: how do you keep track specifically of the power or energy (kWh) used by this one appliance, as it is hard-wired to a dedicated breaker in the electrical panel?  I have a Kill-a-Watt that I use regularly to keep track of ghost loads and new appliances, but that only works for plug-in 110V.
Thanks.

August 13, 2019 - 10:00 am

I live in New England looking to put in a Hot water heater.  I currently have an instant water heat inserted into the Oil furnace.  Our Electric company is offering a rebate for Heat Pump water heaters.  I am curious if you have any insights into if I should convert to electric heat pump Hot water.

April 22, 2019 - 11:16 am

You should be able to find a fuel cost comparison calculator online (BuildingGreen used to have one, but when we rebuilt our website, we lost that) and figure out which makes more sense on a cost-per-delivered-Btu basis. I'm guessing that even if your natural gas costs are fairly low, the heat pump water heater is a better deal. It's also safer, and it will make the transition to renewable energy easier.

April 22, 2019 - 11:12 am

Ok , I installed my 50 gallon Rheem platinum HPWH this past week. Bit of a job getting the oil boiler and doing the plumbing and electrical. I put my 12 year old son into forced labor for a bit....So far so good. I ducted the intake into the hot  attic-getting hotter every day this time of year- and for now I'm just going to exhaust into the storage room with a vent in the front door. When it gets cold I plan on pulling warm air from behind my refrigerator which is just on the other side of the wall and exhaust into the room. I might use that same winter intake to exhaust the cold air in the summer. I'm also thinking about installing a passive central air register that I can open or close to allow heat to enter the room in winter and also for pressure equalization ( figure drawing air out of the living area and exhausting into the sealed storage room could develop a pressure/vacuum situation). I'll also seal off the storage rooms exterior vent in the winter. 

April 20, 2019 - 3:57 pm

Forgot to add: we have an un-insulated garage. 

April 20, 2019 - 3:41 pm

Alex, Thank you for your article. 

We live in Oregon and have 50gal tanked electric water heater. Our house is all electric and we use space heaters in room during winter to keep warm. 

Would you recommend heat pump water heater for Oregon's climate or just add a gas line, which will cost $650 for access line + $900+ for "plumping" to run gas to a water heater ?

April 2, 2019 - 1:49 pm

Sounds like a good plan. Report back on how it works!

April 2, 2019 - 1:09 pm

First of all thanks a lot as I'm not as skilled in the subject. Even though it would require seasonal adjustments as you say I also thought that I could easily run the intake duct up into my attic which gets very hot in the summer that way it would pull down that hot air and then the cool air could just exhaust into the storage room. Obviously in the winter that wouldn't be an option. Then I could vent the cold exhaust to the outside and seeing that there's currently an oil boiler in there I have a vented exterior door but doing this would suck cold outside air into the room so I think then I would have to basically cover over that door vent and when it's really cold run it in either hybrid or just resistive mode and if the room gets too cold supplement it with some other heat source. Again I thought about a vent behind my refrigerator that could be closed in the summer and open it in the winter having my mini split and my refrigerator which conveniently blows the hot air out the back heat the room to an extent.

April 2, 2019 - 12:38 pm

Chris, I worry a little about the room freezing—or dropping below the minimum recommended temperature for a HPWH (which I think with mine was about 40°F). You might look into a Rheem HPWH, which allows you to duct both the incoming air and the outgoing air. Doing this would be a little more work and require some seasonal adjustments, but you could dump the chilled air outside in the winter months—though I'm not sure what the impact would be on the pressure balance in the house. Providing a connection between the house and unheated room would be another way to keep the temperature from dropping too low. Be aware that a HPWH makes about the same amount of noise as a refrigerator.

April 2, 2019 - 10:23 am

I live in the suburbs of Philly. My main concern is that even though the storage room I would be putting the HPWH in is attached to the house it doesn't have heat. This hasn't been a problem as the boilers waste heat has kept the pipes from freezing.. I was thinking about a vent that can be closed in summer and opened in winter and insulating the room well with spray foam.

April 2, 2019 - 10:06 am

Chris, you don't say where you live, but clearly you have a heating season. Unless you're in a fairly severe heating climate (say over 7,000 degree-days per year), I think a heat pump water heater will, on average, save you energy if it operates on heat-pump mode all year. You might consider a model that can be switched from heat pump mode to electric-resistance mode for the winter months. I am in the same situation at my house, and I'm thinking next winter of doing just that and seeing if it makes a big difference in keeping my basement warmer.

April 2, 2019 - 8:45 am

I recently installed Mitsubishi mini splits to replace my old oil boiler for heat but still use the boiler for domestic hot water.. I also have it as backup heat (hydronic baseboard) if the Mitsubishi's broke. Lately I've been thinking about getting rid of the boiler , which along with the tank is in a storage room that's attached to the house. I'd love to put a HPWH in there but it's an unconditioned space. It's be great in the summer but in the winter I'd have to run some sort of heat in there to keep the pipes from freezing or maybe put a vent in the wall the room shares with the living space? There's an ideal spot behind my fridge but wondering if it's worth it. I thought in the summer maybe I can connect the intake to the attic which is a great source of hot air. Any thoughts?

May 18, 2018 - 7:04 am

I search a lot on the web and saw that a gas tankless water heater consumes less energy than an electric tankless water heater. Is it true or false? I read lots of reviews and analytics but than again I felt confused in my mind. Please help me the find the answer. I know you can easily solve this questions that I asked because I read your article where you analyze many technical terminologies

February 27, 2018 - 7:24 am

If the shed is kept warm due to waste heat/heat loss from the hot water system, there isn't a source of heat for the heat pump water heater. It won't be able to heat the water with waste heat from the system that it is trying to heat. Are all your water pipes insulated? Can you insulate the shed better? Or is there a location (basement or closet?) that is closer to your water fixtures that you could relocate your water tank to, so that you don't need the circ pumps at all?

December 18, 2016 - 12:56 pm

I am considering switching to HPHWH here in S Oregon.

I have a solar water heater sytem with electric back up built into a mech room/garden shed. The shed is so far from the house fixtures that I had to install a circ pump to provide hot water to taps in reasonable time. The circ pump radiates enought heat into the insulated shed to keep it warm in winter.

Which is to say I have a stupid amount of heat loss built into my  system. I turn off the pump in warm months.

 In winter I could get the little room to heat up even more  if I  installed a skylight or window. The added heat could go to the HPHWH. Then the circ pump and the proposed solar window could be heating water . this might lower the guilt factor though spending on the new water heater and window might never be recouped.

How do I find the numbers to decide if this makes sense? I see the posts here derive from installation and observation, very, very helpful but how does one plan such installations ?

How much ambiant heat produces how much hot water? In what time frame? Who shares this sort of information?

July 22, 2014 - 7:27 pm

Would a gas condensing tank heater perhaps run about even in terms of cost with a HPHWH in VT?

Hard to figure out without really knowing what numbers they derive the Energy guide figures which list about 1800 KWH for the year for the HPHWH. A Rheem gas condensor EnergyGuide puts it at 200 gallons. So at first glance the HPHWH costs less, but if it runs much less than advertised they will even out (assuming you get a good price on propane).

March 22, 2014 - 1:43 pm

If you insulate the mechanical room, your HPWH will certainly become quiter to you upstairs. But it will have less heat available from upstairs to draw on, so the Mech Room will get colder and its efficiency will drop. During the summer you may want to vent the HPWH into the house to harvest the coolth. You might also bring in a duct to it to bring it warm air from the house. But these ducts may also send noise around.

In a desgn I had had for a HPWH in an unheated mechanical room here in Oregon, I included a buried air supply to prewarm the air for the heater. Trenching was not a big issue because I already had a trench from the pumphoue.