Run the lowest flow temperature that still holds every room at target on the coldest design day: about 35 °C for underfloor heating, 40–45 °C for upgraded radiators, and no more than 50 °C for a retrofit on existing panels. Each degree of flow temperature costs roughly 2–2.5% of COP, so moving from 55 °C to 45 °C cuts running cost by about a fifth.
What is the right flow temperature for a heat pump?
The lowest one that works. A heat pump's efficiency is governed by the lift between outdoor air and the water it produces, so every degree you do not have to make is money you do not spend. The right number is a property of your emitters and your building, not of the heat pump.
| Design flow temperature | Emitters it requires | Efficiency vs 55 °C |
|---|---|---|
| 35 °C | Underfloor heating sized to 35 °C | about +40% |
| 40 °C | Underfloor, or radiators uplifted ~3.9 × | about +30% |
| 45 °C | Radiators uplifted ~2.8 × | about +20% |
| 50 °C | Radiators uplifted ~2.2 × | about +10% |
| 55 °C | Original boiler-era radiators | baseline |
| 60 °C+ | High-temperature R290 or EVI unit | about −15% |
The figures come from the well-established 2–2.5% COP change per degree of flow temperature. They are also why emitter upgrades pay: the radiator you fit once buys the efficiency every winter afterwards. See do I need bigger radiators for a heat pump?
What is the "20 degree rule"?
It is the shorthand circulating in heat pump circles that a well-designed system should need a design flow temperature about 20 °C above the room set point — roughly 40 °C for a 20 °C room. It is a design target, not a standard, and it is achievable in an insulated house with properly sized emitters. Treat it as an ambition to design towards, not a setting to type into a controller.
Why does weather compensation beat a fixed setting?
Because the design flow temperature is only needed on the coldest day of the year. Weather compensation slides the flow temperature down as outdoor air warms, following a curve set by two points: the design outdoor temperature with its design flow temperature, and a mild day with the lowest flow the emitters still work at. A fixed 50 °C setting delivers that maximum for the entire season and wastes efficiency on every mild day — which is most days.
Tune it like this: pick a cold week, drop the curve 3 °C, and give each setting several days. When a room stops reaching target, go back one step. Open the reference room's valves fully first, otherwise you are tuning a throttled circuit.
Should hot water use a different flow temperature?
Yes, and it does automatically. Space heating runs the weather curve; hot water needs a fixed, higher charge temperature. The goal is to charge the cylinder at the lowest temperature that still gives a usable store — typically 48–52 °C — because DHW COP is usually a full point below heating COP. That depends entirely on the cylinder's coil heat exchanger being large enough to transfer heat at a small temperature difference; an undersized boiler-era coil forces the heat pump to 60 °C to do the same job. Solimpeks builds heat pump cylinders with correspondingly enlarged coil surfaces for exactly this reason. Legionella control is handled by a scheduled weekly pasteurisation rather than by storing hot all the time — see what temperature kills legionella in a heat pump system?
What if my house genuinely needs 55 °C?
Then run 55 °C and accept the penalty, or change the emitters. Do not run a low flow temperature that fails to heat the house: rooms below target trigger longer run times, backup heaters and eventually the occupant turning everything up. High-temperature heat pumps using R290 propane or EVI compressors reach 70–75 °C and make boiler-swap retrofits possible, at roughly 30–40% higher running cost than a 45 °C system. That is a legitimate engineering choice for a listed building; it is a poor default for a house where three radiators could have been enlarged.
Frequently asked questions
What is the ideal flow temperature for an air source heat pump?
35 °C with underfloor heating and 45 °C with correctly sized radiators are the realistic targets. Both assume weather compensation, so the system only reaches those figures on the coldest days and runs lower for most of the season.
How much does lowering the flow temperature save?
Roughly 2–2.5% of COP per degree. Dropping from 55 °C to 45 °C therefore saves about 20% of heating electricity, and dropping to 35 °C saves about 40%, provided the emitters can still deliver the room's heat load.
Is a higher flow temperature better for hot water?
Only up to the point the cylinder needs. Charge at 48–52 °C through a large coil and run a weekly 60 °C pasteurisation cycle; charging to 60 °C every day costs efficiency continuously and gains nothing in stored energy you actually use.
Should I use a room thermostat or weather compensation?
Weather compensation as the primary control, with a room thermostat only as a high-limit or reference sensor. A thermostat alone forces the heat pump to run at full flow temperature and then stop, which causes cycling and higher bills.