Return temperature
The return temperature is the temperature of the heating water flowing back to the heat pump from the radiators or underfloor heating.
- Flat curve, typical for underfloor or wall heating: low flow temperature even in frosty conditions.
- Steep curve, typical for radiators: when it is cold, a significantly higher flow temperature is required.
- The flatter and lower the curve, the more efficiently the heat pump runs.
The heating water goes on a round trip: it leaves the heat pump at the flow temperature, releases heat in the rooms, and returns cooled down. The temperature on the way back is called the return temperature. Experts call the difference between flow and return the temperature differential.
Why this is important for your house
The klimaaktiv Leitfaden zum hydraulischen Abgleich (guide to hydraulic balancing) states: the lower the return temperature in the heating circuit, the higher the efficiency of the entire heating system, and every single degree has a positive effect. For heat pumps and underfloor heating, the guide lists system temperatures with a small temperature spread, for example 35/30 or 40/30 (flow/return).
Typical values
For comparison: classic radiators are often designed for a 75 °C flow and 65 °C return. With a heat pump, both values are significantly lower.
What you can observe
- According to the klimaaktiv Wegweiser (guide), the flow and return temperatures should be readable for each heating circuit. Ask your installer where you can see this.
- If the return is almost as warm as the flow, too much water may be flowing through the heating surfaces. This is an indication to have the hydraulic balancing checked.
- If the temperature spread is reduced when switching from a boiler to a heat pump, the required amount of water increases. According to the guide, this can lead to flow noise if it is not rebalanced.
Related terms
Flow temperatureHydraulic balancingLow-temperature radiatorSurface heating (underfloor and wall heating)
Sources
- klimaaktiv guide to hydraulic balancing (BMK, 2023) (accessed on 4 October 2026)
- klimaaktiv Wegweiser zur guten Installation von Wärmepumpen (klimaaktiv Guide to Good Installation of Heat Pumps, BMK, 2020) (accessed on 4 October 2026)