Glossary
Vorlauftemperatur
Vorlauftemperatur: The Vorlauftemperatur (German for flow temperature) is the temperature at which heating water flows from the heating system to the radiators or underfloor heating. The lower it can be, the more efficiently a heat pump runs.
What is the Vorlauftemperatur?
In a heating circuit, warm water flows from the heating system to the radiators (Vorlauf, flow) and returns cooler (Rücklauf, return). The flow temperature is the temperature on the way out. The smaller the radiators and the more heat the house loses, the higher it has to be.
Why it matters for heat pumps
A heat pump has to lift heat from the air or ground to the flow temperature. The bigger that gap, the more electricity it needs. That is why the Jahresarbeitszahl depends directly on the flow temperature.
How to lower the flow temperature
- replace undersized radiators with larger or low-temperature models
- have the system hydraulically balanced
- insulate the roof, top floor ceiling or windows
You can test how the flow temperature changes the 20-year cost in the Heating Cost Comparison Calculator.
This page is for information only and does not replace an energy consultation.
Frequently asked questions
Vorlauftemperatur means flow temperature: the temperature of the water leaving the heating system on its way to the radiators. The water coming back is the Rücklauf (return).
Yes, but less efficiently. At 55 °C an air-source heat pump reaches a seasonal performance factor of about 3.0, at 35 °C about 4.0. Electricity use at 55 °C is therefore about a third higher.
Under § 559f BGB, landlords generally need proof of a seasonal performance factor of at least 2.5 to pass the full cost of a heat pump on to tenants. Buildings that can be heated with a flow temperature of 55 °C or less are among the exceptions.