Bivalence point (Bivalenzpunkt)

Definition

The bivalence point (Bivalenzpunkt) is a central design parameter for heat pump systems. It describes the outdoor temperature at which the heating output of the heat pump is just sufficient to cover the heat demand of the building. If the outdoor temperature falls below the bivalence point, the heat pump's output is no longer sufficient and a second heat generator — usually an electric heating element, less commonly a gas or oil boiler — is added. Three operating modes are distinguished: in monovalent operation the heat pump covers the entire heat demand alone and there is no bivalence point in the strict sense. In bivalent-parallel operation the heat pump and second generator run together below the bivalence point. In bivalent-alternative operation the heat pump is fully switched off below the bivalence point and the second generator takes over alone. The choice of bivalence point significantly influences the annual coefficient of performance (JAZ), the investment cost and the electricity cost of the system.

When is the term used?

The bivalence point is particularly relevant when planning air-to-water heat pumps, since their heating output decreases as the outdoor temperature falls. For brine-to-water and water-to-water heat pumps the source temperature is almost constant, so they are usually designed for monovalent operation. In older buildings with high flow temperatures and a poor envelope, the bivalence point is set higher (e.g. -3 °C); in renovated existing stock or new builds it is set lower (-7 to -10 °C). The setting is determined as part of the heating load calculation according to DIN EN 12831.

Example