Is your combi boiler burning gas when nobody’s using it?
Published 26 October 2026. Costs at the price cap for 1 October 2026 to 31 December 2026.
Many combi boilers keep themselves hot all day and all night, so that hot water reaches the tap a few seconds sooner. It’s usually called preheat, and it’s often on by default. In one home we measured, it was burning about £490 of gas a year. Turning it off took one button.
What preheat does
A combi heats water as it flows through, so it has no tank of hot water waiting. Without preheat, the first water out of the tap is cold until the boiler fires up. With preheat on, the boiler fires every so often to keep its hot water side warm, ready for a tap. Worcester Bosch explains that its boilers keep themselves “hot in readiness for a demand”, and that turning preheat off saves the energy spent doing that, at the cost of hot water taking “a little longer” to arrive.
The catch is that it runs whether or not anyone needs hot water: through the night, while you’re at work, while you’re away.
What it cost in one home
We looked at one household’s half-hourly gas readings for October 2026. Its combi had preheat on until about 3pm on 23 October, when they pressed the eco button.
- Before: between midnight and 6am, on nights with the heating off, the meter recorded 4.7 to 5.5 kWh every night. The readings never dropped to zero: the boiler was drawing about 0.84 kW on average, in bursts, around the clock.
- After: the first full night with preheat off recorded 0.0 kWh. During the day the quiet half-hours, with no heating or hot water running, fell to almost nothing too.
That’s about 17 kWh of gas a day: about £490 a year at the Great Britain average gas rate of 7.97p a kWh. One home isn’t every home: how much preheat burns depends on the boiler, and some only keep warm for a while after a tap is used.
Check whether yours does it
If you have a smart meter, your supplier’s app or website will let you download your gas readings by the half-hour. Look at the small hours on a night the heating was off. If every half-hour shows some gas, something is burning around the clock.
Our gas smart meter analyser does this for you: it finds the gas your home uses with the heating off and everyone asleep, puts a yearly cost on it, and tells you whether it looks like a boiler keeping warm, which comes in bursts, or a pilot light, which burns steadily. It runs in your browser, so the file never leaves your computer.
How to switch it off
- Worcester Bosch: press the ECO button on the front of the boiler. A green ECO light means economy mode, with preheat off.
- Other makes: look in your boiler’s manual for preheat, comfort mode, warm start, keep-hot or eco. It’s often a button or a setting in the boiler’s menu, and some smart thermostats and boiler apps can change it.
You can always switch it back on. If you want fast hot water only at busy times, such as weekday mornings, Worcester notes that a twin-channel programmer can run preheat on a timer.
While you’re at the boiler
The other free saving on a combi is the flow temperature, the temperature of the water sent to your radiators. Many are left at the factory setting, which is higher than most homes need.
Common questions
What is preheat on a combi boiler?
A setting that keeps the boiler’s hot water side warm so hot water reaches the tap sooner. To do that the boiler fires every so often, day and night, whether or not anyone runs a tap. Worcester Bosch calls it domestic hot water preheat; other makers use names like comfort mode, warm start or keep-hot.
Should I turn eco mode on?
For most households, yes. You still get hot water whenever you turn a tap; it just takes a little longer to arrive, because the boiler starts from cold. If waiting matters at certain times, some controls can run preheat only then.
How much gas does preheat use?
It varies by boiler, and Worcester Bosch says the difference is impossible to estimate in general. In the household we measured it burned about 0.84 kW around the clock, and switching it off saved about 17 kWh a day, roughly £490 a year at the price cap.
Is it safe to turn preheat off?
Yes. It only changes how quickly hot water arrives. A combi heats water as you use it rather than storing it, so turning preheat off doesn’t leave stored water sitting lukewarm.
Gas rate from Ofgem’s price cap, checked 23 October 2026. Preheat explained by Worcester Bosch.