At the school gate, a neighbour told me the outdoor temperature at which his furnace took over. I wrote it down as though he had given me the postal code for winter. Our houses were different, but for a moment I forgot that our heating systems might be different too.
At home, I found three phrases in the paperwork: balance point, changeover and lockout. I needed them explained before touching the thermostat's installer menu.
Capacity and cost are separate questions
Source: Natural Resources Canada: heating and cooling with a heat pump
A thermal balance point concerns the outdoor condition where the heat pump's available output meets the home's heating requirement. An economic comparison concerns operating cost, which depends on performance and energy prices. A controller's changeover or lockout setting is a configured instruction. Those concepts can be related without being identical.
NRCan's heat-pump guide discusses balance points and supplemental heating in the context of the actual building and equipment. The question is not simply how cold the machine can run, but how much heat it supplies under those conditions and what the house needs.
The useful handover question
I asked the installer why our thresholds had been selected and what would happen if the heat pump could not maintain temperature. I also asked which settings were intended for us to adjust and which belonged to the system design.
If energy prices or the equipment arrangement change, the control strategy may merit review. That review should use our system's data and compatible controls. It is not a reason to copy a number heard over the school fence.
I kept the neighbour's note, but crossed out the implication that it was advice for our house. It became a reminder of a good question to ask, which was more valuable than the number itself.
