Problem
The backup heater is meant for the coldest days and emergencies. If it runs in mild weather, the heat pump effectively becomes an electric heater.
In very cold weather (typically below −15 °C for an air-to-water unit), during the legionella cycle, and to assist defrost. In other cases, it's worth finding the cause.
The backup heat enable temperature threshold, a steep rise in the heating curve, hot water priority, and hydraulic flow rate. Even a single excessive demand (e.g. 55 °C hot water) can trigger the heater regularly.
We don't recommend turning it off completely — it's also a protection function. Adjusting the enable threshold and the curve is more sensible.
Most units have a backup heat running-hours counter. A sudden jump in hourly electricity consumption is also a sign.
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