January performance snapshot

Living in a super-insulated house, I get very excited when extreme cold weather arrives. It is the only way to see how well the house performs in low temperatures. This January was the coldest month that we’ve lived in the house.

We recorded 1380 HDD for January, 16% colder than January 2013. Our coldest day was January 3rd, with 69.7 HDD. Temperatures on this day ranged from a high of 2.8°F to a low of -8.5°F. Indoor temps ranged from 60°F at night to 70.5°F during the day.

I was particularly interested in how our air-source heat pump (ASHP) would perform in these long stretches of cold weather. Our unit (Mitsubishi MSZ/MUZ-FE18NA) is rated to keep producing heat down to -15°F. According to the specifications, the unit can produce 10,300 btu/hr at 5°F.

In order to determine how well the house performed thru the month I decided to keep the backup heat (electric resistance) off. This meant we relied only on our ASHP and the sun to heat the house for the month.

Looking at the data for January, it looks like our heat pump starts to fall behind demand when temperatures slip below zero. This appears to happen mainly at night. During the day, a small amount of sunlight can raise indoor temps even in the extreme cold. January 1-3 offer a good example.

I normally turn the heat down at night or off. I did this the night of January 1st. Night temps were not particularly cold, but continued to drop throughout the next day. By 7am Jan 2nd, the inside temp got down to 60°F and the outside temp had reached zero. By the time I turned on the heat again in the morning, the ASHP was not able to make up the difference for most of the day. A little sun helped get the inside temp up to a high of 64°F. On Jan 3rd, I considered turning on the backup heat, but I held out because the forecast said clear skies. By the afternoon inside temps were back up to 70 while outside temps hovered around 2°F. By Jan 4th, outdoor temps were well over the zero mark.

The January 2nd experience and a fortuitous conversation with Mike Duclos that day, convinced me to leave the thermostat set to 68°F when temperatures are forecast to be in single to negative digits overnight. This makes it easier for the ASHP to keep up with the demand and lowers the temperature differential it has to make up.

In total, our ASHP used 529 kWh in January. If we paid for electricity, our heat would have cost us just under $80.

2014-01-02-daychart 2014-01-03-daychart 2014-01-04-daychart

Check out the interactive version of these charts on my other site, Netplusdesign.

4 Responses to “January performance snapshot”

  1. 1 Larry 6-February-2014 at 1:12 pm

    Temperature Trivia. Was just talking to someone at the Better Building by Design Conf in Burlington, VT about our experience with our ASHP. These are a hot topic in Vermont right now. We were talking about how often you’d need backup heat. So I pulled out my laptop and ran the database to find for January, we experienced 75 hours at or below zero. That is about 10% of the month. In comparison, there were only 27 hours at or below zero the entire year of 2013.

  2. 2 Patrick Campbell 6-August-2014 at 6:42 am

    I think that you mentioned somewhere that your heat pump cannot keep up on the coldest nights. What is your heat pumps output on the coldest nights compared to your homes heat loss at the same temperature? I am trying to figure out how to size these things. My 99% design temp is -13F (Vermont). The rating of the units is about 62% of max output at -13 F. This means a newer FH15 puts out 11k BTU/hour at -13 F. Two of these would more than cover my homes heat loss (about 18k BTU/hour at -13F with 70F indoor temps which is much higher than where we would sleep). So in theory, it will keep up. Is that not your practical experience?

    • 3 Larry 6-August-2014 at 9:04 am

      Hi Patrick,

      I don’t think I can fully answer your question. My energy consultants predicted a max heating load of 12.2 kBTU/hr at -10°F. Our ASHP rated capacity at 5°F is 10.3 kBTU/hr. I’m not sure what the output is at -10. We installed about 16 feet of electric resistance baseboard as our backup which we have not used yet.

      In our experience, a single unit providing all our heat has worked fine. If we had a week or more of sunless temps below zero we would need to use the backup heat. But the house is able to hold enough heat to coast through a few days like we experienced in January, especially if I don’t lower the thermostat at night.

      Depending on the layout of your home and solar access, you may want to investigate electric resistance baseboard to augment your heat pump output on the coldest nights. In the long run it may be less expensive than purchasing, installing, running and maintaining a second indoor unit.

      Let me know what you decide.


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