The Oil Drum

Air Source Heat Pumps are designed to fall below a COP of 1.0 when the outdoor temperature gets near or below freezing

Hi Saratoga Peak,

Frankly, I don't see how this is possible and it certainly doesn't reflect my own experience. Air source heat pumps are rated by their HSPF (Heating Season Performance Factor), which is a ratio between how much heat they produce over the heating season versus the amount of energy they consume, in total; obviously, higher numbers are better. The Fujitsu 12RLQ has a HSPF of 10.55 (Zone 4). To convert this to its "seasonal COP", you divide this number by 3.4, which gives us a COP of 3.1. The York product I mentioned above has a HSPF rating of 8.0, so its seasonal COP is 2.4. Just to be clear, these numbers take into consideration the energy used to defrost the outside coils. I should also add that Halifax, N.S. is located in Zone 4 and our heating degree days number 7,800, which means our winters are as cold as those of Minneapolis, MN (not exactly tropical by any means).

Prior to installing my ductless heat pump, I used, on average, a little over 2,000 litres of heating oil a year for space heating and domestic hot water purposes. The following winter, this number dropped to 827 litres and last year it came in at 830 litres. My records show my DHW related consumption during the summer months averages 1.2 litres/day and about 1.5 litres/day during the winter months when water inlet temperatures are lower, I do more laundry (bulker and heavier clothing) and when longer (and hotter) showers are preferred. On this basis, I can reasonably assume 500 or so litres a year can be attributed to DHW needs, with the balance related to space heating. Thus, with the addition of my heat pump, my space heating consumption has fallen from about 1,500 litres/year to 330 litres, for a net savings of 1,170 litres/year. [My home, btw, is a 2,500 sq. ft., 40-year old Cape Cod that has been extensively upgraded in terms of its thermal efficiency, with a space heating demand that places it somewhere between a conventional new home and a R2000 equivalent.]

My oil-fired boiler has an AFUE of 82%, which means I net about 8.77 kWh of heat from each litre of heating oil. Multiplying 1,170 litres x 8.77 kWh/litre, tells me my heat pump is providing me with an average of 10,260 kWh of heat over the course of the heating season. My electrical consumption averages 17 kWh/day during the summer months and climbs to 40 to 43 kWh/day during the coldest winter months when the poor little guy is working flat out. Taking a look at my most recent power bill, here's how the numbers break out (total consumption / days in billing cycle / kWh/day):

For the period ending:

January 08 -- 2,540 kWh, 59 days, 43 kWh/day
November 07 -- 1,527 kWh, 62 days, 25 kWh/day
September 07 -- 1,136 kWh, 62 days, 18 kWh/day
July 07 -- 1,043 kWh, 62 days, 17 kWh/day
May 07 -- 1,848 kWh, 62 days, 30 kWh/day
March 07 -- 2,397 kWh, 58 days, 41 kWh/day
January 07 -- 2,332 kWh, 58 days, 40 kWh/day

Past Year: 10,491 kWh, 365 days, 29.7 kWh/day

Our heating season basically kicks off October 1st and eventually tapers off towards the latter part of May, so if we look at the consumption for just this period, I used a total of 7,548 kWh over the span of some 210 days, which is an average of 35.9 kWh/day. From that, I can subtract what would be used for other household needs (i.e., lighting and appliances) which, based on my summer usage, seems to be in the range of 17.5 kWh/day. If these numbers are more or less correct, my heat pump consumes a little less than 3,900 kWh/year but provides me with just over 10,000 kWh/year in heat. On this basis, my seasonal COP should be in the range of 2.5. Note too that my heat pump has a HSPF of 7.2 and the Fujitsu has a HSPF of 10.55, so the Fujitsu should be, in theory, 1.5 times more energy efficient than my own.

My apologies for the long post, but I wanted you to understand my own "real world" experience with air source heat pumps. I'd be happy to answer any questions you have and provide you with more information if it would be helpful.

Cheers,
Paul