Inverters and power modulation
A modulating heat pump continuously adjusts its output to demand, usually via an inverter. This is more efficient than simply switching on and off.
- A device without output control constantly switches fully on and fully off. This is called cycling.
- A modulating device (inverter) adjusts its output and runs smoothly and for longer.
Older or simple heat pumps only have two states: fully on or off. If it is mild outside, they deliver too much heat, switch off and then switch back on shortly afterwards. A modulating heat pump, on the other hand, regulates the speed of its compressor and thus its heating output according to demand. The technology for this is usually a frequency converter, the so-called inverter.
Why this is important for your house
According to the klimaaktiv Wegweiser (guide), modulating heat pumps are significantly more efficient in operation than cycling units with a constant speed. Variable-speed heat pumps are therefore preferred. In most cases, this even makes a separate heating buffer tank unnecessary. The Bundesverband Wärmepumpe (Federal Heat Pump Association) describes a device as output-controlled if it modulates in at least two stages or continuously.
What you should look out for
A modulating heat pump also has a minimum output. If the heat demand falls below this, it will still cycle. The Bundesverband (Federal Association) calls the outdoor temperature at which this happens the cycling point. Ideally, this should be at +5 °C or above. A significantly oversized unit will reach this point sooner. It is therefore important that the heating load and the modulation range match.
Ask in the quote whether the device is power-controlled and to what minimum output it can modulate down. You can find out more about switching on and off under Cycling.
Sources
- klimaaktiv Wegweiser zur guten Installation von Wärmepumpen (klimaaktiv Guide to Good Installation of Heat Pumps, BMK, 2020) (accessed on 4 October 2026)
- Bundesverband Wärmepumpe (BWP): Leitfaden Wärmepumpendimensionierung (German Heat Pump Association: Heat Pump Dimensioning Guideline) (accessed on 4 October 2026)