Room thermostat

1 What it does

Vaillant says a traditional room thermostat senses the temperature of the room and tells the boiler to turn on, then instructs it to work at 100% until the set temperature is reached⁷. Newer controls can do more. Vaillant says load compensation, common on modern thermostats, trims the boiler's water temperature as the room nears its set point, and that your control's manual will say if it has this⁵. See also weather compensation, which responds to outside temperature.

2 What to set it to

UKHSA advises at least 18 °C in rooms you spend a lot of time in, and the NHS gives similar advice, which matters most with a health condition²³. Baxi gives 18 to 21 °C as generally the best level for comfort and efficiency¹. Baxi's 600 Heat manual suggests lowering the thermostat to about 20 °C to cut costs⁸.

Lowering the setting saves energy, but the size of the saving is not settled. Worcester Bosch's Greenstar 4000 manual says up to 10% per 1 °C⁹, and Viessmann's Vitodens 050-W manual says up to 6% per degree⁴. No official figure was found, and both are makers' claims.

3 Where it should be

Vaillant says thermostats are typically mounted in a living space such as the hallway or living room⁵. Viessmann says a thermostat is better located away from radiators, in a hall or similar space¹⁰. In England, Approved Document L treats a thermostat in one room plus TRVs elsewhere as one way to meet the rules on room control⁶. Moving one is a job for a heating engineer. Electrical rules are outlined under Part P.

4 When the heating will not come on

Several user manuals list the thermostat among the first checks. Baxi's Platinum guide asks whether it is set high enough and whether the timer is on¹¹. Worcester Bosch's Greenstar 4000 manual asks whether it is set too low¹². If these are fine, see the programmer and timer entry, then the boiler's own display.

5 Frost and being away

For an empty house in cold weather, some manuals give a low setting to leave on, and they differ. Greenstar 4000 says 12 °C, while Greenstar i ErP says 15 °C¹²¹³. See frost protection.