Turning down your combi boiler's heating flow temperature is one of the few genuinely free ways to cut your gas use — and it keeps your home just as warm. Here's what flow temperature is, the setting most homes can use, and the one number you must not turn down.
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The "flow temperature" is how hot your boiler heats the water it sends out to your radiators. It is not the same as the temperature of the air in your rooms (that's set by your thermostat) and, importantly, it is not the same as the temperature of the hot water that comes out of your taps. On most modern combi boilers all three are controlled separately — and turning the heating flow temperature down is a setting many UK households can change in under a minute, with no tools and no engineer.
Almost every boiler installed in the UK in the last 15 years or so is a condensing boiler. The clever part is in the name: when the water coming back from your radiators is cool enough, the boiler can pull extra heat out of its own exhaust gases — heat that would otherwise vanish up the flue — by condensing the water vapour in them. That recovered heat is essentially free, and it's what makes a condensing boiler more efficient than the old non-condensing ones it replaced.
The catch is that condensing only happens properly when the return water is roughly below 55°C. If you run a high flow temperature (many boilers leave the factory set around 70–80°C), the water comes back too hot to condense, and a lot of that efficiency is wasted. Lowering the flow temperature lets the return water cool enough for the boiler to spend more of its time in efficient condensing mode — so you burn less gas for the same warmth.
For a typical, reasonably insulated home, a heating flow temperature of around 55–60°C is a sensible target. Your radiators run a little cooler than they used to, but they stay warm for longer and the room reaches the same temperature — your thermostat still decides when the heating switches off. You don't lose comfort; the heat just arrives more gently.
Independent UK testing and energy-efficiency campaigns have found that turning a combi's flow temperature down (without touching the room thermostat) can reduce gas use for heating by a meaningful single-digit-to-low-double-digit percentage in many homes. The exact saving depends on your insulation, radiators and habits, so treat it as a "worth trying, won't cost you anything" change rather than a guaranteed figure. It costs nothing to try, and you can turn it back up if any room struggles.
On most combis there are two separate dials or digital settings on the front panel — one symbol for central heating (usually a radiator icon) and one for hot water / taps (usually a tap icon). You only want to lower the heating (radiator) one.
Older homes with smaller radiators, or rooms that were always a bit cool, may need a slightly higher setting — there's no single "correct" number, just the lowest one that still keeps you comfortable. If you can't find separate controls, or your boiler only has one temperature setting, check the manual before assuming; some basic models genuinely do share a control, in which case prioritise comfortable hot water.
| Setting | What it controls | Sensible target |
|---|---|---|
| Heating flow temperature (radiator icon) | How hot water is sent to your radiators | ~55–60°C for many homes |
| Hot water / DHW (tap icon, combi) | Temperature of water at your taps and shower | A comfortable level — leave it set |
| Stored hot water (cylinder, system/heat-only) | Temperature of water in your tank | 60°C or above for Legionella safety |
| Room thermostat | When the heating switches on and off | Your comfortable room temperature |
No — quite the opposite. Condensing boilers are designed to run with cooler return water; that's the condition they're most efficient in. You're not making the boiler work harder or shortening its life by turning the flow temperature down. The only thing to watch is comfort: if a particular room can't reach temperature on the coldest days, that points to undersized radiators or poor insulation rather than a problem with the boiler itself.
Adjusting the flow temperature is an efficiency tweak, not a repair. If your real issue is that radiators are cold in patches, the boiler keeps cutting out, pressure won't hold, or you're getting fault codes, that's a separate fault to investigate. Anything involving the gas valve, pipework, flue, sealed circuit, pressure relief valve or the boiler casing is strictly a job for a Gas Safe registered engineer — never a DIY task. And if you ever smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.
Trimming your flow temperature lowers your running costs; a good cover plan limits your repair costs when something does go wrong. If you're weighing that up, our guide to what boiler cover includes and our look at whether boiler cover is worth it are good starting points — and an annual service (part of many plans, as our boiler service cost guide explains) helps your boiler keep running efficiently in the first place.
For many UK homes, around 55–60°C for the heating (radiator) flow temperature is a good target. It keeps a condensing combi in efficient mode while still warming your rooms. Start at 60°C, and only go lower if every room stays comfortable.
It shouldn't. Radiators run a little cooler but stay warm for longer, and your thermostat still controls the room temperature. The room reaches the same warmth — it just gets there more gently. If one room struggles, nudge the flow temperature up by 5°C.
No — on a combi the heating flow temperature and the hot-water (tap) temperature are set separately. Turn down only the radiator/heating control and leave the tap control where it is, so your showers stay comfortable.
Keep stored water in a cylinder at 60°C or above to control Legionella bacteria. That rule is about stored water, so it doesn't apply to a combi's instantaneous hot water. With a cylinder system you can still lower the radiator flow temperature; just don't drop the cylinder/tank temperature.
No. Condensing boilers are designed to run with cool return water and are most efficient that way. You're not overworking the boiler — if anything, you're letting it do exactly what it was built to do.
A lower flow temperature trims your gas bill for free. Boiler cover does the same for surprise repair bills, turning them into a predictable monthly cost. Compare indicative prices and cover levels across our selected panel.
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