There is no single correct number on a thermostat, but there is a sensible range, and the difference between the bottom and the top of it shows up clearly on your bill. Most UK households are comfortable somewhere between 18 and 21C, and every degree you can knock off without feeling cold trims roughly 1% off your annual heating cost. That sounds small until you remember heating is the biggest single line on most gas bills. This guide gives you a starting number, shows what each degree actually costs in pounds, and clears up the common myths that quietly waste money, like cranking the dial higher to warm a room faster.
The short answer. Set your thermostat to the lowest temperature you are genuinely comfortable at, which for most UK homes is between 18C and 21C. The World Health Organization suggests 18C as a healthy minimum for living areas. Turning the thermostat down by 1C cuts roughly 1% off your heating bill, so start around 18C and nudge up only if you need to.
A sensible starting point: 18 to 21C
The commonly quoted comfort band for UK living rooms is 18 to 21C, and there is good reason for it. The World Health Organization gives 18C as a safe minimum indoor temperature for healthy adults, with a little more recommended for households containing very young children, older people, or anyone with a long-term illness. Below about 16C the air starts to feel cold and damp, and condensation and mould become more likely.
At the other end, much above 21C and you are usually paying to heat a room warmer than you need. The trick is to start low and only go up if you genuinely feel cold. Set the dial to 18C, live with it for a couple of days in normal indoor clothing, and add a degree at a time until you stop noticing the cold. Most people settle somewhere around 19 or 20C and never need the higher end of the band.
Your boiler does not work harder or heat the house faster when you set a higher number. The thermostat is a target, not an accelerator. We cover that point in full under does turning the thermostat up heat faster, but the short version is no, it does not.
What each degree actually costs
The widely used rule of thumb from the Energy Saving Trust is that turning your room thermostat down by 1C cuts your space-heating cost by roughly 1%. The exact figure depends on your home, your insulation, and how cold the winter is, but it is a reliable planning number.
To turn that into pounds you need your annual heating spend. A typical UK home using gas central heating burns somewhere around 11,000 to 12,000 kWh of gas a year, and most of that is heating rather than hot water. The table below shows what trimming the thermostat is worth against a heating cost of about £800 a year, which is a fair middle estimate for a gas-heated home.
| Change | Approx. saving on heating | Per year (on ~£800 heating) |
|---|---|---|
| Down 1C (e.g. 21 to 20C) | ~1% | ~£8 |
| Down 2C (e.g. 21 to 19C) | ~2% | ~£16 |
| Down 3C (e.g. 21 to 18C) | ~3% | ~£24 |
Saving estimate uses the Energy Saving Trust 1%-per-degree rule applied to a typical £800/yr gas heating cost. Gas at 7.33p per kWh, Ofgem cap July to September 2026.
That looks modest per degree, but it stacks with everything else. If you also drop your boiler flow temperature on a condensing boiler, draught-proof, and avoid heating empty rooms, the combined effect on the heating portion of your bill is far larger than any single change. Electrically heated homes pay much more for the same heat, since electricity runs at 26.11p per kWh against gas at 7.33p, so the same percentage saving is worth more in cash.
Use the timer, not just the dial
The temperature you choose matters, but when the heating runs matters just as much. There is little point heating the house to 20C while everyone is asleep or out at work. Set your programmer or smart thermostat so the heating comes on shortly before you need it and goes off before you leave or go to bed. The house holds residual warmth for a while after the boiler stops.
A common pattern that works for many households:
- Living areas at 19 to 20C during the times you are up and at home.
- A setback of 2 to 3C overnight and when the house is empty, rather than switching off completely in a poorly insulated home, to avoid the place getting cold and damp.
- Bedrooms cooler, often 16 to 18C, since most people sleep better in a cooler room.
People often ask whether it is cheaper to leave the heating on low all the time. For most homes the answer is no, timed heating wins, and we explain why under is it cheaper to leave the heating on all day. The exception is a very well insulated home where the boiler barely fires to hold temperature.
Different rooms, different numbers
A single whole-house thermostat is a compromise, because not every room needs the same warmth. The thermostat usually lives in a hallway or living room and controls the boiler for the whole house, while individual radiators are trimmed using their thermostatic radiator valves (TRVs).
The valve on each radiator lets you set a lower target in rooms you barely use and a higher one where you sit. Turn the valve in a spare room down low so its radiator stops heating once that room reaches, say, 16C, while the living room stays at 20C. This is zoning on a budget, and it is one of the cheapest ways to stop paying to heat space nobody is in. See radiator valves and zoning and the best temperature for each room for the detail.
One caution: do not turn the radiator down in the room where the main thermostat is mounted. If you starve that radiator, the thermostat never reaches its target and the boiler keeps firing to heat the rest of the house past the point you wanted.
The myths that waste money
A few persistent beliefs cost people money every winter:
- Turning the dial to 25C to warm up faster. The room heats at the same rate whatever target you set. You just overshoot and either overheat or have to remember to turn it back down. Set your target and leave it.
- Leaving the heating on constant to save money. Heating an empty house wastes gas. Timed heating with a sensible setback almost always wins outside very well insulated homes.
- Closing every radiator and heating one room with a plug-in heater. Electric portable heaters cost about 3.5 times as much per unit of heat as gas central heating, so this only saves money in narrow cases. We compare them under portable heater vs central heating cost.
- Ignoring the boiler flow temperature. A condensing boiler running with a flow temperature too high never condenses properly and wastes gas regardless of your thermostat setting.
If you want the wider set, our energy saving myths page tackles the rest.
The bottom line
Set the thermostat to the lowest temperature you are genuinely comfortable at, start around 18C and nudge up only if you need to, and most people land between 19 and 20C. Each degree you can shed is worth roughly 1% off your heating cost, and that compounds with timing the heating well, using radiator valves to keep unused rooms cool, and getting your boiler flow temperature right. The dial is a target, not a throttle, so resist the urge to crank it. For the bigger picture on cutting your winter heating bill, start with our heating guide and the thermostat settings hub.