I think what Charlie Brown is getting at is that at -20°, an air heat pump is a purely electric heater, i.e. you can also put electric heaters in it. An air heat pump only has a good efficiency at 'warm' temperatures; when you really need it (in winter), the efficiency is not that good.
Yes, I know that myself. But it's not that dramatic. As I could observe in February, the heating element switched on at -10 degrees. Until then, the pump managed on its own. How many days a year do we already have below -10 degrees...
... that at -20° an air heat pump is a purely electric heater, i.e. you can also put electric heaters in it. An air heat pump only has a good efficiency at 'warm' temperatures; when you really need it (in winter) the efficiency is not so good.
A lot of nonsense is told/written about air-to-water heat pumps, but that does not make it an electric heater.
As with everything in life, it is important that a reasonable planning of the system is in place and that the system installed matches the demand. If these conditions are met, the proportion of "supplementary heating" with electricity remains within an acceptable range. By the way, no heating system runs without electricity.
Whereas with gas boilers, electricity costs are almost negligible. They are between 100 and 200 kWh per year for the circulation pump, ignition spark, and valve. Roughly speaking, about 50 € per year.
Yes, I know that myself. But it’s not quite that dramatic. As I observed in February, the heating element switched on at -10 degrees. Until then, the pump managed on its own.
Formally speaking, this bivalence point sounds quite reasonable. Unfortunately, the fear of the dreaded heating element often leads to oversizing. The installer is happy because he has sold a more expensive device than might actually be necessary. The problems (lifespan, seasonal performance factor) usually arise mostly in partial load operation, especially near the heating limit, particularly with ON/OFF devices that cannot modulate.
...How many days per year do we even have below -10 degrees...
The days are irrelevant; what matters are the hours at the respective outside temperature. What seasonal performance factor is actually achieved?
Best regards