Blog Post

Cold Weather Tests the Limits of Our Mini-Split Heat Pump

January 29, 2014

Testing the limits of the air-source heat pump in our new house with this cold weather

The interior unit of our Mitubishi air-source heat pump. Click photos to enlarge.Photo Credit: Alex Wilson


It’s been pretty chilly outside. A number of people have asked me how our air-source heat pump is making out in the cold weather. I wrote about ths system last fall, well before we had moved in to our new home. Is it keeping us warm?

First, if you want to get up to speed on the surprising and counterintuitive nature of how an air-source heat pump works, check out our primer on the topic—which includes a great diagram.

We’ve only been living in the house for a few weeks, but so far, so good. Our 18,000 Btu/hour Mitsubishi mini-split heat pump (MSZ FE18NA indoor unit and MUZ FE18 outdoor unit) is doing remarkably well in keeping us comfortable. We don’t have any oil or gas heating in the house, only the electric heat pump and a small wood stove that we’ve fired up twice so far.

The indoor heat pump unit is mounted on a wall next to our kitchen, and it’s been operating pretty steadily in this cold weather. (Even though we’ve heated with wood for decades and have all the wood we could ever use, I’ve been curious how the house will do just on electricity, so have refrained from using the wood stove.)

A thermometer in a bookcase on an outside wall diagonally across the kitchen-dining-living space from the heating unit is reading 66°F as I write this, with the outside temperature about 12°F. A thermometer in our upstairs bedroom read 70° when I got up this morning, and has typically been about 68°—and remarkably uniform.


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Interior unit placement on a kitchen wall.Photo Credit: Alex Wilson


When the mercury dropped to –6°F a few days ago, the house got colder. I saw one reading on the outside wall downstairs as low as 61°F and our bedroom got down to about 65°F. It was chilly enough that I fired up our small wood stove for the first time, and that fairly quickly raised the downstairs temperature to a comfortable 68°F. With our tight construction there are few drafts.

Monitoring our energy consumption

We have an eMonitor (made by PowerWise Systems of Blue Hill, Maine) installed to track the home’s overall electrical consumption as well as the consumption of a number of individual loads. The monitor has clips that clamp onto different circuits in the electrical panel as well as the electrical main coming into the panel, and it somehow measures electricity flow through those cables. We’re tracking consumption separately for our heat pump heating system, our heat-pump water heater, and our heat-recovery ventilator.

Most of the time the air-source heat pump has been drawing about 2,500 watts, with very brief spikes up to about 3,400 watts (I suppose those spikes occur when a pump or fan kicks on). To put this in perspective, the 2,500 watts in the standard heating mode is about twice what our KitchenAid toaster draws (1,200 watts), though of course the toaster operates for only short periods of time.

So far we haven't had to do any snow clearing from the outdoor unit, but in a heavy snow we likely will have to.Photo Credit: Alex Wilson


Since we hooked up the eMonitor and started collecting data (five days ago), our Mitsubishi heat pump has used 221 kWh of electricity—during a fairly cold stretch. This is about what the entire solar-electric system on our barn cranked out during this period—and roughly three times the output of that portion of our PV system allocated to the house. (It’s a “group-net-metered” system, with two-thirds of the output going to neighboring homes.)

It will be interesting to look at this data over the course of months and years to see how the electricity consumption averages out over time and  how that compares to our solar production.

Heat distribution with point-source heating

Because our heat source is on a downstairs wall, I had been very curious how effectively heat would be distributed throughout our 1,600-square-foot house. The main kitchen-dining-living space keeps a fairly even temperature in the high-60s. A downstairs study or guest room at the far corner of the house and separated from the heat pump by a hallway and doors (with the door open) stays a little cooler, though watching a movie there last night was fine with a sweater.

Upstairs, the our bedroom on the north side of the house has maintained a remarkably constant 68-70°F on all but the coldest nights. When the outside temperature dipped to minus-6°F, our bedroom dropped to the mid-60s. Last night, with the outside temperature down to 7.5°F, we actually closed our door to keep the bedroom a bit cooler, and the temperature dropped from 70°F to 67.8 by morning.

The open stairwell does a great job at distributing heat upstairs. Our north bedroom stays 68-70°F.Photo Credit: Alex Wilson


I don’t have a thermometer in the south bedroom, which is being used as a home office by my wife, but it feels about the same. There are two double-hung windows instead of a single casement window, so there is certainly more air leakage, but there is also solar gain through those windows.

Bottom line

All in all, we are very satisfied with the air-source heat pump. It works well, in large part, because our house is so energy efficient. This is a superb heating option (and cooling, by the way) for a house with a very well-insulated building envelope. Once we install the low-e storm windows on the double-hung windows on the south and east sides of the house, we should do somewhat better. (With our superinsulated house, the south and east windows are a weak point, both relative to air leakage and R-value.)

And on a cost per delivered Btu basis, with the air-source heat pump we’re spending just 58% of what we would spend on oil heat (assuming an Energy Star oil boiler operating at 83% efficiency with #2 heating oil at $3.91 per gallon vs. electric heat in an air-source heat pump with a coefficient of performance of 2.25 and electricity costing 15¢/kWh). (You can plug in your own assumptions and compare fuels on BuildingGreen’s online calculator.)

A summary of energy consumption in our house over the past week with this cold weather. The HRV had to work harder than it generally should due to moisture that got into it before we hooked up the condensate drain. Click to enlarge.Photo Credit: eMonitor data from Alex Wilson


Plus, on an annual basis we should be producing as much electricity with solar as we consume—net-zero-energy. So we’re pretty happy. Warm and happy.

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.

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February 19, 2014 - 9:36 am

The kWh use strikes me as very high!

221 kWh for five days is much higher than others (P. Talmage) report. Yes it was minus 6 outside but that is not uncommon in VT where we live and have a PV system that at peak performance generates 20 kWh per day. We were all set to install a mini-split system to support our wood stove but now I am worried that we will generate a large electric bill!

Am I missing an important part of the puzzle or are my concerns justified?

thanks for your thoughts on this.

February 19, 2014 - 12:51 pm

I would agree, seems high, 44 kwh/day x 30 = 1326 kwh/ month or around $200/month- not including the rest of Electrical use beyong heat pump.  cannot draw any firm conclusions until there is a year or two of whole house data, converted to Btu/sf.  Just looking at my Passive House cheat sheet, annual heating allowance must be under 11.1 kwh/ sf, if house is 1600 sf that is 17760 kwh/yr / 12 mo = 1480 kwh/mo, which also seems high- is my math right ! ?  Our E use on 2000 sf ranch is around 375 kwh/mo year round, minimal AC use and minimal electric resistance heat use, gas forced air primary heat.

June 19, 2014 - 6:40 pm

We have developed and patented a solar thermal system (with evacuated tubes) to improve heat pump performance. We are just now building prototypes and doing tests. We envisioned this system having it's greatest utilization in the Southeast and Southwest due to more sunlight in the winter. You seem to be getting decent PV power in the winter in Vermont . Do you think there is sufficient sun to warm the evacuated tubes? I would be curious to know what the compressor discharge temperature is on a 0-10 degree F day.

June 20, 2014 - 4:45 pm

Be sure to read the warranty on any solar thermal products.

November 8, 2014 - 3:55 pm

What caught my eye from the eMonitor screenshot was that the heat pump water heater used only 12kwh in 8 days. Alex, what brand and size is that and where in the house is it located to get such low electricity usage?

November 8, 2014 - 5:17 pm

Answering my own question... I just found your articles on the heat pump water heater:

February 15, 2015 - 8:33 am

I recently had a Misubishi mini split system installed in our home. We installed (3) indoor units...(1) on main floor, (1) in upstairs master bedroom and (1) in finished basement. The house has 1700 square ft. I have the upstairs, master bedroom unit set at 64 degrees and woke up this morning to a room temperature of 45 degrees! The outside temperature was 8 degrees. We have had to turn off the unit on the main floor and the basement level in order to conserve electricity. Our readings were up to 45 kw per day expenditure with all three units running at a set temperature of 59 degrees! So far, we are very unhappy with the Mitsubishi ductless system. What temperature was your unit set on when you provided the thermometer temperature readings for your rooms? Also, how many interior units have you installed and where are they located? (I am under the impression you only have (1) unit installed on your main this correct?) Thanks for your great article.

January 6, 2017 - 5:48 pm

Hi - we have been using mini splits for two years now. We have a small house (less than 1500 sq ft, plus a repurposed basement room). Windows/doors/roof are all newer so insulation is okay, I believe. We have two ceiling-mounted units upstairs and a wall-mounted unit in the basement. The first month we ever used them (two years ago) we used almost 2000 kwh of electricity keeping them on at about 74 degrees, constantly (it was a very cold January, also). The utility company actually threatened us with an off-peak on-peak system and needless to say we were shocked. After that we started keeping the splits down at 66 or 68 and when it got too cold, turned them to 74 ot 76, but we rarely had all three going at once. The company that sold us the splits and installed them also told us (later that summer when I was still asking about their efficiency due to our high electric bills) that they aren't very efficient below 14 degrees so it's better to go back to our oil heat on those extra-cold days. We live in Connecticut, so it gets cold. The next winter was mild, and we never used close to 2000 kwh again in a month, but we kept the splits so low the house always seems cold. This past year we had solar installed to help with the splits' cost. We banked about 2000 kwh right through October, and I noticed that for the November/December bill we had to use almost half of them to cover the electric bill, so it had to have been about 1000 kwh. This is during fairly mild weather, keeping only two going most of the time and usually keeping them low (66, or even below). 

Is it even helping to turn the thermostats on them down if we have to turn them back up to heat the rooms sufficiently when we go in them? Is it better just to keep them at a constant temp and keep them on always? Remember, keeping all three on and at 74 resulted in that 2000 kwh usage. I'm reading here about people in even colder climates heating slightly larger homes with mini splits from October to June with under 2000 kwh.  What's going on that ours uses 2000 kwl in a month??? These are Fujitsu models.

This isn't the first time I have written too Building Green about this - two years later and I'm still concerned about this. Even the solar company was shocked at the 2000 kwh usage for that one month two years ago when they reviewed our usage. Why do we seem to be getting very little efficiency from our system? I have a cold I can't shake because it's so cold in here all the time...and we are using a thousand kwh for it??? It's as inefficient as baseboard electric heat at that rate.

Thanks for any suggestions!

November 2, 2017 - 8:11 pm

 I built a home this past year in southern Vt. I am an HVAC tech and looked into every option. It’s important to research and understand how these products work. The location of the outdoor units. Keeping them covered so they are still well ventilated but, there isn’t water dripping on them constantly that they have to thaw out. I installed 8 mini splits in our home. I’ve only got 5 of them up and running and heat about 2000 sq ft. When we finish off the rest of the house I’ll be able to heat 4000 sq ft with every room individually zoned. Further more..... you can’t expect to heat with these units if you don’t have a really well insulated house . They just won’t work. They will be ok for supplement heat. But to be100% air source heat pump you need to be super insulated. I’ve yet to go a full winter with these units. I chose Daikin cold climate 1 to 1 heat pumps. They are super awesome. Quiet, and efficient. Mitsubishi will rave about their super heat mode and all there little grills. But in the end it’s smoke in mirrors  quite frankly don’t buy a product that heats your home that also makes cars, televisions and small appliances. They spread out to far. Daikin only makes refrigeration products. And that’s why they are the best. My kWh is rate for April of this year was 730. And throughout the summer it averages 500 kWh. And that was keeping it relatively cool but not cooling the house if we weren’t around. Why waste money. And with a well insulated house you don’t really need to crazy with the a/c. But for those who are asking why they are using 2000 kWh of electricity. I can’t answer that but I will say that it probably is because you went with the cheaper unit. Not to say your unit might be buried in snow and starving for air. But there is something to say about quality and engineers knowing how to squeeze every last bit of efficiency out of -13 degree air . It’s incredible and it gives us the consumer a way to net zero and  have a very comfortable home. I’m very sorry for the ones on here who aren’t having very good luck with their mini splits, but that doesn’t go for all of us. Research research research. . Cheers and God bless.