Is it cheaper to leave the heating on all day? The maths says no

Quick answer: No. A home that’s warm 24 hours a day loses more heat than one that’s warm when you need it, so it burns more gas. Our modelled January example puts always-on about 25% higher, roughly £30 extra for the month at the 7.97p/kWh cap rate. The exception is heat pumps, which are built for steady running.

This argument survives every winter because both sides sound plausible. One camp says the boiler “works harder” reheating a cold house, so keeping it warm must be cheaper. The other says heating an empty house is obviously waste. The physics settles it, and it isn’t close.

The physics in one paragraph

Your house leaks heat at a rate set by how warm it is inside compared to outside. Bigger gap, faster leak. Every kWh that leaks out is a kWh you paid to put in. Keep the house at 20°C all day and the gap stays wide for 24 hours. Let it cool while you’re out and asleep, and for those hours the gap narrows, the leak slows, and the total heat you buy falls. The boiler does work harder during the reheat, but for a much shorter time, and it never claws back what the cool-down saved.

The maths at cap rates

Here’s our modelled estimate for a three-bed semi in January; the method behind it is on our costs page. Assume an average outdoor temperature of 5°C. With timed heating (mornings and evenings, 20°C when on), the average indoor temperature across the whole 24 hours works out around 17°C once you include the warm periods and the slow overnight drift. Always-on holds 20°C throughout.

Heat demand tracks the indoor-outdoor gap: 12 degrees averaged over the day for timed, 15 for always-on. That’s 25% more heat through the same walls. If the timed house burns 1,500 kWh of gas on heating in January, always-on burns about 1,875 kWh. At the current 7.97p/kWh gas rate, that’s an extra £30 or so for one month, and the gap widens in a cold snap.

Those are modelled figures for one illustrative house, not a promise about yours. But the direction of the result doesn’t change with the house: more hours warm means more heat lost. For scale, the typical dual-fuel household pays about £1,723 a year at the current cap, with most of the gas burned in the heating months, so a 25% January difference is real money. Our price cap explainer covers where the 7.97p comes from.

Where the myth gets its grain of truth

In a very well-insulated house, the cool-down is slow and the two regimes converge. The saving from timed heating shrinks toward nothing, and the comfort argument for steady warmth starts to win. The myth also borrows credibility from a real problem: cold homes and damp. If timed heating means a house that sits at 13°C all day growing mould, low background heat can be the right call for reasons that have nothing to do with the gas bill. We’ve covered that trade-off in our guide to keeping a home warm without condensation.

And there’s a genuine exception.

The heat pump exception

Heat pumps flip the logic. Their efficiency depends on the temperature of the water they send to your radiators, and gentle, continuous running at low flow temperatures is exactly what they’re designed for. Forcing a heat pump to reheat a cold house from scratch pushes flow temperatures up and efficiency down. So “leave it on all day” is roughly correct advice for a heat pump home, with a modest overnight setback at most. The field data in our cold weather heat pump article shows what steady running achieves in real northern winters.

What to actually do

• Gas or oil boiler: use the timer. Heat for the hours you’re home and awake, with the thermostat somewhere sensible. Our thermostat settings guide has room-by-room numbers.
• Well-insulated newer home: still use the timer, but expect the saving to be modest.
• Heat pump: steady target temperature, small setback if any.
• Damp-prone or hard-to-heat home: consider low background heat, and treat it as a damp decision rather than a savings trick.

Test it in your own house

You don’t have to take our model’s word for it. Run a fortnight’s experiment: one week timed, one week always-on, thermostat at the same temperature, and compare daily gas use in your smart meter app or on the meter itself. Pick consecutive weeks with similar weather, because a mild week will flatter whichever regime it lands on. If your house is anything like our modelled one, the timed week should win by something near 25%. If it doesn’t, you’ve learned something real about your insulation, and that’s useful too.

FAQs

Does the boiler use more gas reheating a cold house?

Per hour, yes. In total, no. The reheat burns hard for a short period, but the hours the house spent cooler leaked far less heat, and that saving is bigger.

Is it cheaper to leave the heating on low all day instead?

Lower and longer still loses to timed for most gas-heated homes, though by less. The house is warmer than it needs to be for more hours, so it leaks more. The genuine case for constant low heat is damp control, not cost.

What about leaving the hot water on all day?

Different question, same principle. A cylinder kept hot around the clock loses heat constantly. Heat it once or twice a day, timed before you use it, unless your household draws hot water constantly.