Report on Our Ductless Minisplit Heat Pump
Testing the limits of the air-source heat pump in our new house with this cold weather
It’s been pretty chilly outside, if you haven’t noticed. A number of people have asked me how our 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. is making out in the cold weather. I wrote about the system last fall, well before we had moved in. Is it keeping us warm? We’ve only been living in the house for a few weeks, but here’s a quick report.
So far, so good. 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 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.
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 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. , and our heat-recovery ventilator(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. .
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.
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 PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. 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 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 -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.
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.
The 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 envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. Once we install the low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. 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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. .)
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 StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. 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¢ per kWh). (You can plug in your own assumptions and compare fuels on BuildingGreen’s online calculator.)
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.
- Alex Wilson
- eMonitor data from Alex Wilson
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