Return Temperature

The return temperature refers to the temperature of the heat transfer medium (typically water or a water mixture) as it returns to the heat generator after passing through the heating or process heat system. Along the way, the medium has transferred thermal energy—for example, via heating surfaces, heat exchangers, or process-related heat transfers.

The return temperature thus represents the “cold end” of the heat sink and is a key parameter for controlling, designing, and evaluating the efficiency of heating systems and thermal processes. It is directly related to the supply (flow) temperature—the difference between the two is known as the temperature spread.

Impact on efficiency and system design

Typically, the return temperature is significantly lower than the supply temperature. A large temperature spread results in a comparatively lower mass flow, which allows for smaller pipe diameters, reduces the required pump capacity, and lowers investment costs. Optimizing the return temperature is therefore an important energy efficiency measure. 

Return temperature in high-temperature heat pumps

In industrial high-temperature applications, return temperatures must be considered separately on the heat source and heat sink sides.

On the heat sink side, a large spread between supply and return temperature enables the integration of a subcooler. By further subcooling the condensed refrigerant in the condenser, additional efficiency gains can be achieved, significantly increasing the heat pump’s COP.

On the heat source side, the return temperature is one of the key factors determining the COP of the heat pump. On the refrigerant side, the heat pump must bridge the temperature difference between evaporation and condensation. The critical evaporation temperature is closely linked to the return temperature of the source: the higher the return temperature, the higher the possible evaporation temperature—improving the COP. If the return temperature decreases, the required pumping power is reduced, but at the same time, the COP worsens. In practice, the goal is to strike the right balance between higher return temperatures for improved efficiency and lower return temperatures for reduced pumping effort.

Integration of low-temperature waste heat

When integrating low-temperature waste heat sources, return temperature becomes a decisive factor. Many industrial waste heat streams are available at relatively low temperature levels (30 – 60 °C) and can be used economically if the required temperature lift is not too high. The return temperature should therefore not be more than 5 – 10 K below the supply temperature of the heat source. In this way, a heat pump can operate efficiently even with waste heat sources of limited temperature potential — a key advantage for decarbonizing industrial processes.

Thus, the return temperature is far more than just a technical measurement. It is an essential tool for optimizing efficiency, integrating processes, and managing loads. Purposefully lowering the return temperature can reduce operating costs, stabilize system temperatures, and open access to renewable or previously unused heat sources.