Heat pump: how it heats and when it pays off

In short: A heat pump takes heat from air, ground or groundwater and raises it to heating temperature using electricity. From 1 kWh of electricity this yields about 3 to 4 kWh of heat.

In more detail: The principle is like a fridge in reverse. A refrigerant evaporates at low temperature, a compressor raises pressure and temperature, and in the heat exchanger it gives the heat to the heating water.

In Depth

• Types: Air-to-water (most common, cheap, outdoor unit needed), brine-to-water (ground probes or collectors, more efficient, costlier to develop), water-to-water (groundwater, permit required).
• Seasonal performance factor (SPF): Ratio of heat output to electricity input over a year. Air heat pumps are often at 3 to 4, ground-source higher.
• Flow temperature: The lower the required heating-water temperature, the more efficient. Underfloor heating or large radiators (35 to 45 °C) are ideal. In old buildings larger radiators and better insulation help.
• Prerequisites: Heat-load calculation, hydraulic balancing, a suitably sized unit. Oversized units often cycle and are inefficient.
• Costs: Purchase and installation clearly higher than a gas boiler. There is state funding, whose amount and conditions change and must be checked currently.
• Operation: You reduce electricity cost with PV power and a heat-pump tariff. Controllable consumption devices can be throttled temporarily by the grid operator, in return for lower grid fees.
• Noise: Outdoor units need distance from neighbours and bedroom windows. Observe distance requirements.

See also: photovoltaics basics, reducing heating costs