There is a setting on the front of most gas boilers that can cut your gas use by a useful margin, costs nothing to change, takes a couple of minutes, and that the overwhelming majority of households have never knowingly touched. It is the central-heating flow temperature, and the factory default it usually sits on is far higher than your boiler needs to keep you warm.
What the flow temperature actually is
When your heating is running, the boiler heats water and pumps it round the radiators; that water then returns to the boiler a little cooler, having given up its warmth to the rooms. The flow temperature is simply how hot the boiler heats that water before sending it out. Set high, it sends scalding water to the radiators; set lower, it sends warm rather than scalding water. Crucially, this is a separate control from how hot it heats the water for your taps and shower, and separate again from the thermostat on the wall.
Why a cooler flow burns less gas
Almost every boiler fitted in the last fifteen years or so is a condensing boiler, and the whole point of that design is a trick it can only pull off when the returning water is cool enough. Burning gas produces hot exhaust containing water vapour, and that vapour holds a surprising amount of recoverable heat. A condensing boiler has an extra-large heat exchanger that, if the returning water is cold enough to chill the exhaust below about fifty-five degrees, condenses that vapour back to liquid and harvests its hidden heat into your heating rather than losing it up the flue.
Run the boiler too hot and the water coming back from the radiators stays hot, the exhaust never gets chilled enough to condense, and all that recoverable heat is thrown away outside. In that state an expensive condensing boiler performs no better than a crude old one from decades ago. Bring the flow temperature down and the return runs cooler, the boiler spends far more of its time actually condensing, and you get noticeably more heat from every unit of gas you pay for. The fuller explanation is on the how a condensing boiler works guide, but that is the heart of it.
This is not the room thermostat, and not the hot water
This trips people up, so it is worth being plain. The room thermostat decides how warm your rooms get and switches the heating on and off when the air reaches the temperature you set. The flow temperature decides how hot the water in the radiators is while the heating is running. Turning the flow temperature down does not make your house cooler; the thermostat still brings every room up to the same temperature as before. What changes is that the radiators run warm rather than searing, give their heat out more gently, and the boiler sips less gas to keep the house at the temperature you have always had.
On a combi boiler there is usually a second, separate setting for domestic hot water, the water that comes out of your taps and shower. Leave that one hot, comfortably above sixty degrees, both so your showers are not feeble and for safety against bacteria in the system. The saving here comes only from the heating flow temperature, not the tap water.
Finding the setting on your boiler
The control varies by make, but it is almost always on the front panel and almost always marked with a radiator symbol to distinguish it from the tap symbol. On older boilers it is a physical dial; on newer ones it is a button-driven menu with a small display.
On many Worcester Bosch models there is a knob or a menu item with a radiator icon; turn or set it down to the low fifties. On Vaillant boilers the heating flow target is usually set with the heating-temperature button, again shown as a radiator. Ideal, Baxi, Glow-worm, Viessmann and the rest each have their own layout, but all separate the heating temperature from the hot-water temperature, and all let you lower the heating one. If you have the manual, look up central-heating flow or radiator temperature; if you do not, the model number searched online brings it up in seconds. When in doubt, the dial or menu item next to the radiator symbol is the one you want, and the one next to the tap symbol is the one to leave alone.
What number to aim for
A good starting target for the heating flow is somewhere around fifty to fifty-five degrees, down from the seventy, seventy-five or even higher that many boilers ship on. That range keeps the return water cool enough for the boiler to condense reliably while still warming the house. It is a starting point, not a fixed rule: a very well insulated home can often go lower still, while a draughty house with small radiators may need it a touch higher to keep up. The way to settle it is to try the low fifties for a week or two and only nudge it up if the house genuinely struggles to get warm in the cold.
The trade-off, and how to handle a cold snap
Cooler radiators give out heat more gently, so two things change. Rooms warm up a little more slowly from cold, and on the very coldest days a low setting may not quite keep pace. Neither is a real problem once you adjust the approach. Instead of heating the house in short hot blasts, set the heating to come on earlier and run for longer at the lower, thriftier temperature, so it eases the house up to warmth rather than blasting it. In a genuine cold snap you can nudge the flow up temporarily and bring it back down when the weather turns, which is a small manual version of the weather-compensation feature some boilers and controls offer automatically.
What you might save, and the catch
Independent trials have put the gas saving from this single change into the region of several per cent to as much as a tenth of a heating bill, for nothing more than turning a dial, which makes it one of the best-value adjustments in the whole house. The only real catch is that the benefit depends on the boiler being able to condense, so it works best alongside the rest of the basics: bled radiators that fill fully, a system free of sludge, and radiators left clear of furniture so the water gives up its heat and returns cool. Get those right, drop the flow to the low fifties, leave the hot water hot, and you have a meaningful, permanent saving that cost you two minutes and not a penny.