A thermostatic radiator valve (TRV) is made up of two parts. The thermostatic head contains an actuator – a canister filled with gas or wax – that expands and contracts as the temperature rises and falls. The valve body has a sprung plunger inside it that closes and opens the supply of water into the radiator.
When the two are put together the supply of heat into the radiator is automatically controlled with respect to the temperature of the room. When the room reaches the desired temperature, the valve closes and the radiator stops heating. The room then cools, the valve opens and the radiator begins to heat again.
Turning the handle of the thermostatic head adjusts the distance that the actuator must expand by to close the valve. Many TRVs have a 1 to 5 scale – with 1 representing the smallest distance and 5 the greatest. The larger the distance, the warmer the room must be before the valve is closed.
Thermostatic valves are designed to let a small amount of water pass through when the room is cold to protect against frost. If you need to shut the valve off completely, when removing the radiators from the system in order to decorate, for instance, you’ll need to use the decorator’s cap supplied with the valves.