Is it cheaper to leave the heating on all day? For most homes with a boiler, turning the heating down or off while nobody is in uses less energy than holding a steady temperature. The reason is simple physics, and it also explains why the answer is different for some heat pump homes. This guide uses a small model to show the effect, then lists the cases where it does not hold.
In this guide
Why a lower temperature loses less heat
A home loses heat through the walls, roof, windows and ventilation. The rate of loss is roughly proportional to the difference between the inside and outside temperatures.
Take a home with a heat loss coefficient of 0.15 kW per °C, which means it loses 150 W for every degree of difference. On a day when it is 5°C outside:
| Inside temperature | Temperature difference | Heat lost per hour |
|---|---|---|
| 20°C | 15°C | 2.25 kWh |
| 17°C | 12°C | 1.80 kWh |
| 14°C | 9°C | 1.35 kWh |
Holding the home at 17 instead of 20 for eight hours saves 3.6 kWh of heat. If the home is allowed to cool further, the saving is larger, though more heat is needed later to bring it back up. The coefficient here is an example. A well-insulated home has a lower one and a draughty one has a higher one.
What this means in money
At 3.6 kWh of heat saved, a gas boiler at 90 percent efficiency and 0.08 per kWh saves about 0.32 per day. Over a heating season of 150 days, that is about 48. A home that sets back by more degrees or for longer saves more, and a home with higher gas prices saves more. The heat pump vs gas boiler calculator shows how the price per kWh of heat changes with the fuel.
The saving is real but modest, so do not give up comfort for a number this small. A lower setting while you sleep and while you are out is a comfortable way to get most of it.
When the answer changes
Heat pumps are different. A heat pump is most efficient when it runs at a low flow temperature for long periods. Letting the house cool and then reheating it asks the pump to run hard at a higher temperature, which lowers efficiency. For a heat pump home, a steady schedule with a modest setback, say one or two degrees at night, usually works better than a deep setback. Your installer can recommend a schedule for your system. The guide to SCOP explains why the flow temperature matters.
A home with underfloor heating has high thermal mass and responds slowly. A deep setback may leave it cold for hours. The same is true of a very well insulated home, which loses little heat anyway, so the saving from setback is small.
Damp is another factor. A home that is kept cold for long periods, especially in humid weather, can develop condensation and mould. Keep ventilation working, and do not let rooms fall below about 15 to 16°C for long.
Practical settings
- Use a programmable thermostat or smart controls to lower the temperature when the home is empty and at night.
- Start heating shortly before you return, not hours before. Many controls learn how long your home takes to warm up.
- Use thermostatic radiator valves to lower the temperature in rooms you rarely use.
- Keep the boiler flow temperature as low as comfort allows. This helps a condensing boiler recover more heat, as the guide to boiler efficiency shows.
- For a heat pump, follow the installer's advice and keep setbacks small.
What to ignore
A common claim is that it costs more to reheat a home than to keep it warm. The reheating needs extra energy, but not as much as the heat lost during the hours you saved. The common exception is a very short absence. If you are out for an hour, the saving is tiny, and there is no point in changing anything.
Frequently asked questions
Should I leave the heating on low all day?
For a boiler, a lower temperature during the day saves more than a steady medium one. For a heat pump, a steady low setting often works well. Check with your installer.
Does a thermostat setting of 18°C cost more than 17°C?
Yes, a little. Each degree adds heat loss in proportion to the temperature difference, so a degree lower saves a few percent of heating use for most homes.
Is it worth turning heating off for a short trip?
For a trip of a day or more, yes, with a frost setting around 8 to 10°C to protect the pipes.