It is one of the most stubborn heating debates in the country. One camp swears that keeping the boiler ticking over all day is cheaper than letting the house cool and reheating it. The other camp says that is nonsense, and you should only heat when you are home. Both sides usually quote a heating engineer they once spoke to, and both rarely have any numbers. So here are the numbers, using current UK gas prices, plus the one important exception that makes the question more interesting than a simple yes or no.
The short answer. For almost every home with a gas boiler, it is cheaper to heat only when you need it rather than leave the heating on all day. A house loses heat fastest when it is warmest, so keeping it warm around the clock means paying to replace heat you are not using. A timer or smart thermostat that warms the house for the hours you are in it will normally cost less than a constant low background heat.
The short answer, and the physics behind it
The cheaper-to-leave-it-on idea sounds reasonable, so it is worth seeing exactly where it falls down. A heated house is always leaking warmth to the colder outside, through walls, windows, the loft and gaps around doors. The bigger the gap between your indoor temperature and the outdoor temperature, the faster that heat escapes.
That single fact decides the whole argument. If you keep your home at a steady 20C all day, you are sitting at the maximum temperature gap for the maximum number of hours, so you leak heat at the fastest rate for the longest time. Every unit of heat that escapes is a unit your boiler has to buy back. If instead you let the house drift down to, say, 14C while you are out or asleep, the gap shrinks, the leak slows, and you simply use less gas during those hours.
Yes, reheating a cooled house takes a burst of energy. But that burst is smaller than the steady trickle you would have spent holding the temperature the whole time. The total energy you put in over a day is what you pay for, and heating on a timer puts in less of it.
The trade-off is comfort. Heating on demand means a cool house for the first stretch after the timer kicks in. A good smart thermostat hides this by learning how long your home takes to warm up and firing the boiler early, so the house is at temperature exactly when you want it, without paying to hold it there all day.
What it costs: leave-on versus timed
Let us put rough figures on a typical three-bed semi. A house like this might need around 1.5 kW of average heat input to hold temperature on a cold day, more in a draughty home and less in a well-insulated one. Numbers vary hugely between houses, so treat these as illustrative rather than a quote for your home.
The table compares leaving the heating on to hold 20C for 16 waking hours against timing it to run only when needed. Heating on demand still uses gas, just less of it overall, because the house spends part of the day cooler and leaks less heat.
| Approach | Rough gas used per day | Daily cost | Cost per year (Oct to Mar) |
|---|---|---|---|
| On all day, holding 20C | ~32 kWh | ~£2.35 | ~£428 |
| Timed: warm mornings and evenings only | ~24 kWh | ~£1.76 | ~£321 |
| Saving from timing it | ~8 kWh | ~£0.59 | ~£107 |
At 7.33p per kWh, Ofgem gas cap July to September 2026. Figures assume a roughly 182-day heating season and a typical, moderately insulated three-bed home. Your own usage will differ.
The standing charge does not change between the two approaches, because you pay the daily standing charge whether the boiler runs or not. So the whole difference comes from the units of gas burned, and the timed approach burns fewer of them.
Why the myth survives
If the maths is this clear, why do so many people, including some tradespeople, still say to leave it on? A few reasons keep the idea alive.
- Condensation and damp. Homes that go stone cold and then get blasted with heat can suffer condensation on cold surfaces, which over time feeds mould. A constant low heat avoids that. The fix here is not to heat all day but to deal with the damp directly, with ventilation and, where needed, insulation. See our notes on cheap fixes for condensation and mould.
- Boiler efficiency confusion. People sometimes assume firing a boiler from cold is wasteful, like a car doing lots of cold starts. A modern condensing boiler does not work like that. What matters far more is your boiler flow temperature, and running it lower so the boiler condenses properly saves more than any on-versus-off choice.
- Slow-to-warm homes. A poorly insulated house can take so long to warm that timed heating feels like it never gets comfortable, which makes constant heat seem necessary. The real answer is insulation, not more gas.
None of these make leaving the heating on all day cheaper. They are reasons people reach for, after deciding to do it for comfort.
The exception: heat pumps and very heavy homes
The advice flips for some homes, so it is worth being honest about that. The leave-it-on logic gets closer to true in two cases.
First, heat pumps. A heat pump is most efficient when it runs gently and steadily at a low flow temperature, rather than blasting hard to recover a cooled house. With a heat pump you generally do leave it running most of the time, set to a steady comfortable temperature, and let it tick along. That is not the same as the gas-boiler myth, because the reason is the appliance, not the building. If you are weighing a heat pump, our guide on heat pump running cost versus a gas boiler goes into how the running pattern differs.
Second, homes with enormous thermal mass, such as thick solid-stone walls, warm up and cool down so slowly that letting them go cold barely saves anything and reheating takes hours. For these, a steady gentle background heat can be the practical choice. But this is a small minority of UK homes, and even then the saving from going cold is small rather than negative.
For the standard British home with cavity walls, a gas combi or system boiler and a timer, heating on demand wins.
How to get the saving without the cold mornings
The reason people leave the heating on is comfort, not a belief in thermodynamics. So the goal is to keep the comfort and drop the cost. A few moves do most of the work.
- Use the timer or a learning thermostat. Set the heating to come on shortly before you wake and before you get home, and to drop back when you are out or asleep. A smart thermostat that learns your home's warm-up time removes the cold-start wait.
- Set a sensible target. Most people are comfortable around 18 to 20C. Each degree you turn down trims a meaningful slice off the bill. Our guide to thermostat settings covers how to find your number.
- Set back rather than fully off overnight. A modest overnight setback, say to 15 to 16C, keeps the house from going fully cold so the morning reheat is quick, while still using far less than holding 20C all night.
- Slow the heat loss. The less heat your home leaks, the less the on-versus-off question even matters. Draught-proofing and topping up loft insulation cut the rate at which warmth escapes, so a timed schedule holds comfort for longer.
- Zone your home. Turning down radiators in rooms you are not using, with thermostatic valves, means you are not paying to heat empty space. See radiator valves and zoning.
The bottom line
For the typical UK home with a gas boiler, leaving the heating on all day is not cheaper. A warm house loses heat fastest, so holding it warm around the clock means paying to replace heat you are not even using. Heating only when you need it, on a timer or a learning thermostat, uses fewer units of gas and costs less, on the order of £100 a year for a typical three-bed in our worked example.
The real exceptions are heat pumps, which prefer to run steadily for their own efficiency reasons, and a small number of very heavy solid-walled homes that barely cool down anyway. If neither applies to you, set a sensible target temperature, use the timer with a gentle overnight setback, and spend any saved money on draught-proofing and insulation so the question matters even less next winter.