Flow temperature is the temperature of the heating water leaving the heat generator toward the emitters; return temperature is what comes back. Underfloor circuits run at 30–40 °C, modern radiator systems at 45–55 °C, legacy radiators at 70 °C or more. Every 1 °C of flow temperature reduction improves heat pump efficiency by roughly 2–3%, making it the single strongest efficiency lever.
What is flow temperature?
Flow temperature (Vorlauftemperatur) is the temperature of the water a boiler or heat pump sends into the heating circuit; the water arriving back is the return temperature. Systems are designed as temperature pairs — 35/28 °C for underfloor, 55/45 °C for modern radiators, 75/65 °C in old boiler practice.
The spread between flow and return also differs by technology: heat pumps are designed for a narrow 5–8 K spread with generous flow rates, while boilers traditionally ran 20 K. This is why a heat pump cannot simply be piped into an unchanged boiler system without checking flows.
Why does flow temperature decide heat pump efficiency?
A compressor's work grows with the temperature lift between heat source and heating water. Each 1 °C shaved off the flow temperature cuts that lift and improves efficiency by roughly 2–3%; moving a system from 55 °C to 40 °C therefore saves on the order of a third of the electricity. Datasheets show the same physics: the identical unit typically posts a 20–30% higher rating at W35 than at W55, which is why the headline Solimpeks Varm Up figure — COP up to 4.92 — is stated at A7/W35.
Over a season this is exactly what separates a mediocre JAZ from an excellent one on the same hardware.
What flow temperature does each emitter need?
| Emitter | Design flow temperature |
|---|---|
| Underfloor heating | 30–40 °C |
| Fan-coil convectors | 35–45 °C |
| Generously sized (oversized) radiators | 45–55 °C |
| Standard radiators, older design | 55–70 °C |
| Legacy high-temperature radiator systems | 70–80 °C |
Radiator physics helps the retrofit case: output scales with about the 1.3rd power of the temperature difference to the room, so a radiator running at a 30 K difference delivers roughly half its 50 K nameplate output. If a room's radiator was oversized to begin with — common in older buildings — a 45–50 °C flow often suffices as it stands.
How do you lower the design flow temperature?
Four measures, usually in this order of cost-effectiveness:
- Activate weather compensation so the flow temperature tracks the actual outdoor conditions instead of sitting at the design maximum all winter.
- Hydraulically balance the circuits, letting every emitter receive its intended flow — unbalanced systems are driven hot to feed the worst room.
- Exchange the few limiting radiators for larger or fan-assisted models rather than replacing all of them.
- Improve the fabric: every insulation step lowers the heat load and with it the temperature the same radiators need.
Running longer at lower temperature beats running briefly at high temperature — continuous low-temperature operation is the design philosophy of every efficient air-to-water heat pump system.
What about domestic hot water?
Hot water is the exception that must stay hot: cylinders are charged at 50–60 °C for comfort and legionella control regardless of how low the space heating runs. Efficient systems therefore treat domestic hot water as separate, short charging cycles at elevated temperature while the heating circuit continues at its own low level — never by raising the whole system to hot water temperature.
Frequently asked questions
What is the best flow temperature for a heat pump?
As low as the emitters allow while still heating the rooms: 30–40 °C with underfloor heating, 45–50 °C as a realistic radiator target. Every degree saved is worth roughly 2–3% efficiency, so design decisions that shave 10 °C pay back through the system's whole life.
Can normal radiators work at 45 °C flow temperature?
Often yes. Radiator output at a 30 K room difference is about half the 50 K nameplate value, and many older buildings have generously oversized radiators. A room-by-room check identifies the few radiators that actually need exchanging.
What is the difference between flow and return temperature?
Flow is the water leaving the generator; return is the water coming back after releasing heat. The spread between them, multiplied by the flow rate, is the delivered power — heat pumps run a narrow 5–8 K spread, boilers traditionally 20 K.
Does lowering the flow temperature reduce comfort?
No — it changes the rhythm, not the result. Low-temperature systems run longer and more steadily, which most occupants perceive as more comfortable than the hot-cold swings of high-temperature on/off operation. Only undersized emitters make a low flow temperature a comfort problem.