Returløb

Returløb is a term used in Danish to describe the return leg of a closed hydraulic loop in heating and other fluid-circulation systems. It carries the fluid that has given up its heat at points of use—such as radiators or floor-heating circuits—back to the heat source, where it is reheated and recirculated. In most buildings with central heating or district heating, the returløb runs alongside the fremløb (supply line) and connects to the boiler, heat pump, or heat exchanger. The arrangement enables continuous circulation and allows sensors and controls to regulate temperature, pressure, and flow across the loop.

The temperature of the return water is central to system performance. Lower returløb temperatures generally improve the efficiency of modern boilers, particularly condensing models, because cooler water increases the potential for condensation in the boiler and higher overall efficiency. Typical return temperatures vary with design: low-temperature systems for underfloor heating might see return water in the 30–40°C range, while conventional radiator-based circuits may return in the 40–60°C range; older high-temperature systems can be higher.

To function reliably, returløb includes components such as pumps, valves for balancing, air elimination, and filtration. Some installations also use devices to limit return temperature, protect equipment from thermal shock, or ensure even heat distribution. In district heating networks, the returløb represents the portion of the network where water, after transferring heat, returns to the central plant for reheating and reuse.

See also: central heating, district heating, hydronic heating, underfloor heating, radiators.