What do the numbers on a radiator valve mean?

The numbers on a thermostatic radiator valve are relative room-temperature settings: a lower number asks for a cooler room and a higher number for a warmer one. Energy Saving Trust warns that the numbers do not represent the same exact temperatures on every model.

A TRV senses the temperature around the valve and changes the flow through that radiator. It does not turn the boiler into a different type of appliance and it does not directly set boiler water temperature. Start around the middle and move one step at a time until the room is comfortable.

Keep curtains, furniture and radiator covers from trapping warm air around the valve. Energy Saving Trust says a cover can make the TRV sense a misleading local temperature and shut the radiator sooner than intended.

The main room thermostat and programmer still control the wider heating demand. If one room never warms despite a higher setting, the cause may be a stuck valve, air, balancing or circulation rather than the printed number. Ask a heating professional if basic control checks do not explain it.

A high number does not make the radiator heat the room faster; it asks the valve to keep admitting flow until a warmer local temperature is reached. The frost or star symbol, where fitted, is usually a low protective setting rather than an ordinary comfort setting, but check the valve manual. Avoid turning the head with force. If it is loose, damaged or has no readable markings, identify the model before buying a replacement head because fittings are not universally interchangeable.

Review the setting after a day rather than changing it every few minutes, because the room and building fabric take time to respond.

Sources

• Heating controls — Energy Saving Trust, accessed 14 November 2026.

On Leatmere

Read radiator valves and TRVs and the TRV glossary entry.