The right temperatures: rooms, boiler flow and hot water

There are three separate temperatures in your heating system, and mixing them up costs money — or worse. Here's what each one does and where to set it.

Room temperature: the thermostat

Most households are comfortable with living spaces at 18–21°C and bedrooms cooler. Every degree lower saves a noticeable slice of the heating bill over a winter, so the cheapest upgrade in any home is nudging the dial down one degree and giving it a week to see if anyone actually notices.

Boiler flow temperature: the efficiency dial

This is the temperature of the water your boiler sends to the radiators, and it's the setting almost nobody touches. Condensing boilers only achieve their headline efficiency when the returning water is cool enough to condense the flue gases — which means a lower flow temperature, around 60–65°C for many homes, keeps the boiler in its efficient mode. The radiators feel warm rather than scorching and take slightly longer to heat the room, but the gas saving is real. On most combis it's a dial or menu setting marked with a radiator symbol.

Hot water: where lower is not better

If you have a hot water cylinder, it must store water at 60°C — that's the temperature that keeps legionella bacteria in check, and it's not a place to economise. Combi boilers are different: they heat water fresh on demand rather than storing it, so setting their tap temperature to a comfortable 45–50°C is safe and avoids scalding taps.