Today is the perfect day to talk about low temperature heat pump performance! If you read my past blogs about decarbonizing my house, you might remember that I was lamenting the fact that we didn’t really get any cold weather in Toronto last winter. Well, you can thank me for provoking the winter gods into providing this frigid, snowy January 2022.
How much colder has it been this winter? Well, last January we had an average high temperature of 0 Celsius and an average low of -5C. This January, the average high and low have been -3C and -13C respectively. All of last winter we only reached -13C once, whereas this January we’ve reached that temperature every day on average! BRR! I knew last winter was pretty warm, so although my calculations showed I probably wouldn’t need supplemental heat down to -20C, I did buy some plug in radiators just in case.
So, how did the heat pump fare on those cold days? I measured the output using my trusty food thermometer:
Not sure if you can see it there on the thermostat, but it shows outside air temperature of -19C with my supply air coming out at 33.7C and keeping the house at 20 degrees. On January 15th this year, we reached -21C here in Toronto and the average temperature in the house was 19.5C during that time. Pretty amazing stuff! I cherry picked the highest output temperature for the photos, but I also took a few other random measurements to expand the performance charts I previously made.
The chart below shows that as temperatures drop below -20C (-5F), the heat pump is running almost continuously. If temperatures stayed below -20 for an extended period of time I would probably need to turn on an additional heat source. As of now, those plug in rads I bought are still in the box.
The next chart shows that at extremely low temperatures, the heat pump is matching the manufacturers performance specification. At higher outdoor temperatures, I suspect the average output was impacted by short run times at higher temperatures and more frequent defrosting which occurs mostly around 0 to -10C.
This data proves to me that heat pumps work in the cold and can be a primary heat source in the Toronto climate. And it can be done at a lower overall cost than natural gas heating.
Rainbows and Unicorns
Heat pumps in extreme cold aren’t all rainbows and unicorns though. There are a few key points I’d like to highlight to make sure you get the full picture if you are planning to decarbonize your house. If you want to achieve higher indoor temps like 22 or 24C you might need another source of heat. The primary trade off would be higher cost. For example, you could add an electric coil to your furnace (higher electricity cost) or keep your gas furnace as a hybrid system (high cost of paying gas account charges all year for few hours of operation). These alternatives are specific to my scenario of retrofitting a central heat pump in place of an older AC while keeping the existing furnace/fan (AHU). If your system has a specifically matched AHU or you chose to install mini split heat pumps you can likely achieve higher temperature setpoints without another heat source. Those systems are typically more expensive to install.
In colder months such as this January (20% colder than the 5-year average), utility costs will be high compared to gas. My utility cost in January was $196 (without the COVID electricity subsidy it would have been $210) compared to an estimated $180 back when my house was on gas. It is important to remember that these rare high cost months are more than offset by the savings during the other seasons.
To back that up, we can look at a full calendar year of utility costs. Looking at gas and electricity only, I spent $1638 in 2020 and $1376 in 2021, savings of $262. If I factor in the savings from canceling my hot water tank rental, total savings year over year are $721. I also spent $89 in gas account charges before canceling the account in 2021, so I can tack that onto the savings in 2022. Here’s a chart showing year over year costs:
Another key point is ensuring a good location for your heat pump – make sure the unit is not located in a spot where melt water can drip on it. The outdoor coil is significantly colder than outside air, so water dripping on the unit will cause ice to build up and can cause damage. My property isn’t very big, so I had very limited choice in deciding where to install the unit. Unfortunately, the large snowfall last week followed by a day of warm temperatures created a situation where water was dripping off my roof right on to the heat pump. This caused a bunch of ice and I had to MacGuyver something to deflect water away:
When you get a big ice build up, the heat transfer is worse and the usual defrost cycle might not be able to melt all the ice off. In this case, I actually ran the system in air conditioning mode for about 20 minutes to melt all the ice off (there wasn’t a significant change to indoor temperature even though it was -9C outside at the time). I plan to build a small awning over the heat pump to prevent this in the future as I would not want to have ice buildup occurring if I am away from home.
Now that I know this heat pump can get me through the coldest weather Toronto has to offer, I can stop thinking about that and get back to doing normal winter things:
Bonus – Heat Pumps at UHN
Given, the cold weather, I also wanted to provide a quick update on the heat pump heat recovery system at Bickle Centre. The BAS screenshot below shows that at -10C outdoor temperature, we are able to provide most of our heating for this particular air handler from the heat recovery system – the cold outside air has been warmed to 12C before reaching any fossil fuel derived heating. The valves supplying hot water from the boiler plant are at only 7% (heating valve), 7% (reheat valve), and 2% (reheat valve). This data shows that we are diverting almost all the heating loads on this system from high emitting, inefficient fossil fuels to low-emitting, efficient heat recovery/electricity. The heat pump is critical in the process because it significantly boosts the amount of heat that is being recovered compared to a traditional run-around heat recovery loop. Very cool stuff and we are looking to expand the use of this method of heat recovery at UHN in order to further reduce our GHG emissions.