MoneySavingExpert, the consumer website founded by Martin Lewis, has clarified whether households should leave their hot water boiler running all day or only switch it on when needed. The most cost-effective approach depends on the type of heating system installed.
Timers Work Best for Gas Systems
For homes with a gas, oil or LPG central heating system, it is always cheaper to set the timer so hot water only heats up when required. This avoids paying to heat water that sits unused.
Those with a combi boiler could also save by lowering the hot water temperature, with around 55°C considered sufficient for most households, according to Octopus Energy.
Electric Immersion Heaters and Off-Peak Tariffs
The situation differs for households using an electric immersion heater on an Economy 7 or Economy 10 tariff. These tariffs offer cheaper electricity overnight, making it more cost-effective to heat water during those hours rather than throughout the day.
To prevent the water cooling before use, the hot water tank needs to be adequately insulated. If unsure which tariff applies, MoneySavingExpert recommends checking the latest bill or contacting the energy supplier for clarification.
Social Media Debate Over Always-On Settings
The question was raised on social media by one user with an Ideal Logic Air R32 heat pump, whose Ideal Halo Air settings suggested keeping hot water set to "always on" rather than timed, as this saves the most energy. The user noted the logic of keeping water topped up to 45/50C versus heating from cold each time, but questioned whether heating during cheap hours or midday peak temperatures was more efficient.
Another user responded that scheduling hot water overnight (using cheap electricity) or around 1pm (using excess solar after multiple showers) worked best, adding: "I'm pretty certain that overnight only (with the right tariff) is the cheapest option." They also expressed doubt that heating a full tank from cold uses more power than keeping a tank topped up, explaining that maintaining 50C water is an inefficient temperature to run at, whereas letting the tank empty allows heating at a lower 30C flow temperature, which is more efficient.
