Intro
Last week’s temperatures were indeed tortuous (recorded as low as -35.5C) but also perfect conditions to test the limits of a system of cold climate heat pumps. Air source heat pumps are units which extract the heat from outside air and move it to the inside of the house. Yes, there is lots of heat in the air at these low temperatures, and heat pumps continue to get more efficient at extracting that heat.
First, the test house
The house that we are analyzing is a “Net Zero Ready” building with two dwellings. Here are some of the elements that raised the energy performance of this building to the highest standard:
1. R-16 under the basement slab
2. R-40 basement walls
3. R-30 first and second floor walls
4. R-45 ceiling
5. Mostly triple glazed windows
6. Heat pump hot water tank
7. Drainwater heat recovery system
8. Two mini-split heat pumps with three heads. One ducted, two non-ducted.
9. Condensing clothes dryers.
10. 1.0 Air Changes/hr (very airtight)
The specifications above that pertain to the building envelope go a long way toward capturing and maintaining the heat inside the building, which has a huge effect on the sizing and specification of the heat pump system.
The heat pump system by Fujitsu is designed to produce about 23K BTUs/hour at a design temperature of -26C. It has 2 back up heating sources/floor…..a radiant style of resistance electric heat. In the picture below, the heat pump head is shown near the top of the picture while the back up heat is shown near the bottom left hand corner.
How did the system perform?
Here are the low temperatures of several cold days:
December 19 -23.5C
December 20 -26.6C
December 21 -31.1C
December 22 -35.5C
December 23 -29.2C
December 24 -21.0C
In spite of the design temperature of the heat pump system at -26C, the only time the back up heat was required was the morning of Dec 22nd when the temperatures dipped to -35.5C and didn’t climb higher than -24.4C all day.
This is a graph of the actual energy consumption. We can see the spike in energy on December 22nd when expensive back up heat was required (circled in orange), then we see the energy consumption drop precipitously as the temperatures warmed up to 0C by December 28th and the whole system is barely sipping power for heat, water, lights and other uses of two dwellings.
We can’t determine for certain as to why the system outperformed its design temperatures. Theoretically, we should have required some back up heat for several days. It is either that the system was slightly over designed and/or the fact that the building envelope captures heat so well that when outside temps drop below the design temp, the building retains the heat overnight such that inside temps don’t drop an noticeable amount.
(Edit) Here is an updated chart of electrical use vs temp to Feb 10th, 2023. The single day that supplementary heat was needed stands out. The fact that we haven’t experienced -30C since 2004 until 2022, it is possible that supplementary heat may never be needed again, or at least on extremely rare cases of a deep dip of Arctic vortex.
Regardless, the system has proven several things:
- Heat pumps do work exceedingly well in our cold climate (and provide summer air conditioning).
- Had we used a Mitsubishi or Daikin heat pump that is effective down to -30C, it is unlikely that back up heat would ever be used, particularly considering global warming which will reduce such cold temperature events. Even in this system, the extra cost of back up heat was about $4, and that is likely going to be the total excess cost for the whole winter.
- Heat pumps are very low cost to operate. A typical winter day in our area tends to be in the 0C to -10C temperature range and these heat pumps are extremely efficient and cost less to operate than a high efficiency gas furnace. At 0C in the above chart, full energy cost for two dwellings is about $4-5/day.
What about the cost of the system?
These heat pump systems cost more than a gas furnace alone, but since it also provides air conditioning, the cost of a heat pump system vs gas furnace + air conditioning, the upfront cost is remarkably similar. The back up system added some cost…..about $800 with no electrical upgrades.
Conclusion
We are ready for low emission energy for homes even in our cold climate. These systems are completely suitable for all of southern BC at least up to the 52nd latitude and all the way up the coastal regions. Mitsubishi says that they are close to introducing a heat pump effective down to -35C. When that happens, there will be few places in Canada where a heat pump won’t be the best alternative. For now, they work extremely well in all the populous regions of Canada when designed properly.
mindmuser 