To schedule or not to schedule your heat-pump usage?

There is some verbiage online suggesting you should not use a schedule on your heat-pump (source). While we have largely suggested that you do. Lets get into the science a bit and see the arguments on either side. Briefly, as heat-pumps generally produce less heat, they will struggle to raise the temperature in your house when its cold outside. On the other hand, electricity prices vary greatly depending on the time of day. Hence by scheduling your heat-pump, you could greatly reduce your power bill.

Some basic thermodynamics. Your home heating system works by adding (or removing when its hot outside), heat in your home. This can be measured in watts, or usually kilo-watts [KW], times a factor (specific heat capacity), and the mass of air (and to some extent, other things, furniture, walls etc.) inside your home. Further, there are some losses owning to the insulation in your home, which depend on the temperature difference between the inside and outside of your home.

Recall from our previous post the coefficient of performance reduces as the temperature outside gets colder. Thus, the amount of heat we get from the heat-pump as it gets colder, as shown in the graph below for my Daikin heat-pump, thus, my heat-pumps ability to warm up the house is reduced as it gets colder. This is also the main argument for having a backup-heat system in Toronto’s climate, as we do have a few days every year where it gets to -20 C (see this post).

Alas, this is the main reason why the “set-it-and-forget-it” might make some sense. If we do not have to increase the temperature of the house, we can get by with a smaller heat-pump, or use the one we got, a bit further than our current -10 C balance point (see this post).

Economics is the main argument for the schedule philosophy, the price of electricity varies greatly during the day. As shown in the figure below (also from this post). During the night, our heat-pump is cheaper than gas, and it gets close when its warmer on the weekend rate.

From the equation above, we also note that when its colder outside, the heat-pump has to work harder, hence even if you went with “set-it-and-forget-it” philosophy, your overnight usage would probably be higher as it tends to be colder outside at night.

I do suspect you should still schedule it in such a way that we avoid the relatively high peak rate, if you are on the ultra-low overnight rate. Then why not have it work a little harder in the morning and bump the set point?

Time	Temperature setting
5 am to 4 pm	19 C
4 pm to 9 pm	18 C
9 pm to 5 am	17 C