Heating · Flow temperature
One dial, six to fifteen per cent off your winter bill.
Flow temperature is the number your boiler or heat pump aims for when it heats the water going out to the radiators. Most UK boilers ship set to 75°C or higher, which prevents them from ever condensing. Most UK heat pumps are designed at 45 to 55°C, which is why they run efficiently. The physics is the same for both machines; the setting is what changes.
January morning, combi commissioned at 75°C
The house at the factory setting; radiators too hot to touch, flue gases well above the dew point, and none of the condensing efficiency you were sold.
What the number actually means.
Flow temperature is the temperature of the water leaving the heat source and entering the radiators or underfloor loops. Return temperature is what comes back after that water has given up some of its heat to the room. The gap between them is how much heat the emitters are delivering.
The higher the flow, the more heat per minute a given radiator throws off, and the smaller the radiator needs to be. This is why boilers grew up running hot; small radiators, small pipework, quick warm-up. It is also why they were so inefficient at the standard commissioning setting.
Same morning, boiler turned down to 60°C, weather compensation on
Same house, same morning, boiler dialled down; a slightly slower warm-up in exchange for six to eight per cent off the annual gas bill, at zero capital cost.
Why boilers should run cooler than they usually do.
A condensing boiler only earns its name when the return water stays below roughly 55°C. Beneath that threshold, water vapour in the flue gases condenses on the heat exchanger and releases its latent heat back into the system; typically a 6 to 12 per cent efficiency uplift over the same boiler running hot.
Almost every UK combi is installed at 75 to 80°C flow. At that setting the return sits well above 55°C, no condensation happens, and the AA-rated boiler quietly performs closer to a B. Dropping the flow to 60°C typically saves 6 to 8 per cent of annual gas use with no felt penalty in a reasonably insulated home; the price is a slightly longer morning warm-up, which the timer fixes.
Why heat pumps insist on it.
A heat pump moves heat rather than making it. The larger the gap between the outside air temperature and the flow temperature it has to hit, the harder the compressor works and the lower the COP; every 5°C reduction in flow lifts seasonal COP by roughly 8 to 12 per cent.
This is why MIS 3005 requires the whole system to be designed together; the fabric decides the heat loss, the heat loss decides the flow temperature the pump can hit at design conditions, and the flow temperature decides how big the radiators need to be. You cannot buy a heat pump and expect to run it at 65°C; the machine will do it, but the running cost matches a boiler and the point of installing it evaporates.
Where the free wins sit.
Two settings and one commissioning conversation account for the bulk of the achievable saving on a modern boiler.
Turn the boiler flow down to 60°C
- Strength
- Zero capital cost; typically 6 to 8 per cent off annual gas at no comfort penalty in a reasonably insulated home.
- Trade-off
- Slightly longer morning warm-up; a small schedule shift usually absorbs it.
- Best for
- Any modern condensing combi still commissioned at 75 to 80°C, which is nearly all of them.
Enable weather compensation
- Strength
- Ties flow temperature to outside temperature; a further 6 to 15 per cent saving and steadier room temperatures.
- Trade-off
- Needs an outdoor sensor and correct commissioning; almost never turned on out of the box.
- Best for
- Any boiler or heat pump manufactured since 2018.
Design the heat pump at the lowest workable flow
- Strength
- Every 5°C off the design flow lifts seasonal COP by 8 to 12 per cent for the life of the system.
- Trade-off
- Requires radiators sized for the lower flow, which can mean visible upgrades in some rooms.
- Best for
- Homes with the fabric and radiator budget to accept a 45°C design instead of 55°C.
Verdict
Flow temperature, honestly assessed.
Turn the boiler dial down to 60°C tonight and ask your next service engineer to enable weather compensation; if a heat pump is on the horizon, insist the quote is priced at both 45°C and 55°C so you can see what the extra COP costs upfront.
A single dial change is the highest-return five minutes you will spend on the heating system this decade. Weather compensation finishes the job; both settings sit within the reach of any competent service engineer at no equipment cost beyond a possible outdoor sensor.
For heat pumps the design flow is not a preference; it is the number that fixes the running cost for the next fifteen to twenty years. Any installer who refuses to price both design flows is telling you which version they would rather you did not see.
What it gives you
- Zero or near-zero capital cost on the boiler side; a real six to eight per cent recurring saving.
- Weather compensation adds steadier room temperatures on top of the efficiency win.
- Sets the ceiling on how efficiently a future heat pump can run in this house.
What it costs you
- Slightly longer warm-up on the coldest mornings; a schedule adjustment usually solves it.
- Lower design flow on a heat pump can require radiator upgrades in the coldest rooms.
- Poorly commissioned weather compensation can under-heat the house; ask for the curve settings in writing.
Why we think thisOpenClose
Reasoning
The 55°C condensing threshold and the associated 6 to 12 per cent uplift come from BEIS's Money Saving Boiler Challenge evidence base and Heating and Hot Water Industry Council modelling. The heat-pump COP uplift per 5°C reduction is drawn from the SAP heat pump lookup tables and manufacturer performance curves for R32 monoblocs.
Weather compensation savings of 6 to 15 per cent reflect field trials by the Energy Saving Trust and multiple published DECC/BEIS heat metering studies; the wide range reflects how thoroughly the curve is commissioned to the specific house.
Assumptions
- The boiler is a modern condensing model; pre-2005 non-condensing boilers cannot recover the same saving.
- The heat pump comparison assumes an air-source monobloc sized to MIS 3005; ground-source and different refrigerants shift the numbers modestly.
- Weather compensation savings assume the outdoor sensor is correctly located and the curve is genuinely commissioned to the house.
Sources
- The Money Saving Boiler Challenge — Nesta / BEIS
- MIS 3005 Heat pump installation requirements — MCS
- Heat pump field trial data — Energy Saving Trust
If this were our house
If this were our house, the boiler would go on 60°C tonight and the annual service in autumn would end with weather compensation enabled and calibrated.
- 1Turn the central heating flow dial on the front of the combi to 60°C and leave the hot water dial alone.
- 2At the next annual service, ask the engineer to enable and commission weather compensation; add an outdoor sensor if one is not already fitted.
- 3When pricing a heat pump, insist on quotes at both 45°C and 55°C design flow, with the radiator schedule that each design requires.
This order rests on the current HouseState above; if the house is genuinely draughty and cold at 60°C, the fabric rung upstream needs attention first, not the boiler.