It’s been pretty chilly outside, if you haven’t noticed. A number of people have asked me how our air-source heat pump is making out in the cold weather. 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 Btu/hour Mitsubishi mini-split heat pump 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 only 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.

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

When the mercury dropped to minus 6 degrees Fahrenheit a few days ago, the house got colder. I saw one reading on the outside wall downstairs as low as 61 degrees and our bedroom got down to about 65 degrees. 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 degrees. 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.

With the eMonitor I’ve been able to keep an eye on how much electricity we’re using. 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 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 great looking 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 to 70 degrees on all but the coldest nights. When the outside temperature dipped to minus 6 degrees, our bedroom dropped to the mid-60s. Last night, with the outside temperature down to 7.5 degrees, we actually closed our door to keep the bedroom a bit cooler, and the temperature only dropped from 70 degrees 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.

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 system (and cooling, by the way) for a house with a very well-insulated building envelope.

And on a cost per delivered Btu basis, with the air-source heat pump we’re spending just 58 percent of what we would spend on oil heat (assuming an Energy Star oil boiler operating at 83 percent efficiency with No. 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 cents per kWh).

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

Tonight (Tuesday) there is an open forum on "Large-Scale Biomass Energy Opportunities in Windham County" at the River Garden in Brattleboro from 5:30 to 7:30 pm. Two of Vermont’s leading authorities on biomass energy and forest utilization will be speaking: Adam Sherman of the Biomass Energy Resource Center and Paul Frederick, a wood utilization forester at the Vermont Agency of Natural Resources. The forum is free and open to the public. It is sponsored by the Sustainable Energy Outreach Network. For details, contact Guy Payne (guy@seon.info) or call 802-376-9262.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to alex@buildinggreen.com.