Flow temperature

The flow temperature is the temperature of the heating water on its way to the heating surfaces. The lower it is, the more efficiently the heat pump operates.

The flow temperature is the temperature at which the heating water leaves the heat pump and flows to the radiators or underfloor heating. It is the most important lever for efficiency: the greater the difference between the heat source and the flow temperature, the more electricity the heat pump requires.

Which values are good

• According to the Energieinstitut Vorarlberg (Energy Institute Vorarlberg), the ideal flow temperatures for a heat pump are below 35 °C.
• The klimaaktiv Wegweiser (guide) states that low flow temperatures below 40 °C are a prerequisite for a high seasonal performance factor. If it is higher, it advises seeking independent energy advice.
• klimaaktiv notes: if the planned flow temperature does not exceed 55 °C on the coldest day, a heat pump is suitable for meeting the heating demand.

This always refers to the temperature on the coldest day. During the transitional seasons, it is significantly lower.

What the public subsidy requires

For the federal Kesseltausch, the flow temperature of the heat distribution system may not exceed 55 °C, as stated in the info sheet for the 2026 relaunch. Some federal states require less: Tyrol for air heat pumps maximum 50 °C, Vienna maximum 40 °C or proof of efficiency. What applies in your federal state can be found under Public subsidies.

How you can lower it

• Have the heating curve set correctly.
• Carry out hydraulic balancing.
• Replace individual radiators that are too small, see low-temperature radiators.
• Insulate: less heat demand allows for lower temperatures.

klimaaktiv writes that even small measures, such as targeted radiator replacement or an adjusted heating curve, can lower the flow temperature by a few degrees and extend the lifespan of the heat pump. This is explained in detail in the flow temperature guide.

All terms · Heat pump knowledge