Bivalence point

The bivalence point is the outdoor temperature below which the heat pump alone is no longer sufficient and a second heat generator helps out.

The colder it is outside, the more heat your house needs. At the same time, an air-source heat pump in particular delivers less output in cold weather. The bivalence point is the operating point at which the heat pump can just about manage the required output on its own. If it gets colder, a second heat generator helps out: usually an auxiliary electric heating element, sometimes an existing boiler.

Why this is important for your house

The bivalence point is a key factor in determining how much heat is generated via the more expensive route. The Bundesverband Wärmepumpe (German Heat Pump Association) notes: the higher the outdoor temperature at the bivalence point, the more heating capacity and runtime the heating element needs. However, a bivalence point that is as low as possible is not automatically better either. This would require a larger appliance, which would switch on and off more frequently during the transitional periods.

Practical rule of thumb

For air-to-water heat pumps, the association recommends not aiming for purely monovalent operation. At the standard outdoor temperature, the heat pump should cover about 50 to 80% of the standard heating load. The auxiliary electric heater takes care of the rest during the few very cold hours of the year. This must be based on a standard heating load calculation, not an estimate.

Ask in the quote where the bivalence point lies and what share the heating element is expected to cover. A first assessment of your heating load is provided by the heating load calculator.

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