What does a thermostatic radiator valve do, and where should the thermostat go?
They control heat in different ways, and getting the relationship between them wrong is a common cause of a house that is simultaneously too hot and too cold.
A thermostatic radiator valve (TRV) controls one radiator. It contains a wax or liquid capsule that expands as the surrounding air warms, closing the valve and reducing flow. As the room cools, the capsule contracts and flow resumes.
The numbers are not temperatures. They are relative settings, roughly corresponding to a range — typically around 3 giving about 20°C, with each number worth a few degrees. Turning a TRV to 5 does not heat the room faster; it simply keeps the valve open until the room is hotter. Rooms warm at the same rate regardless.
A room thermostat controls the boiler. When the temperature at the thermostat reaches its setting, the boiler stops firing — for the whole house.
How they interact. The thermostat decides whether heating happens at all; TRVs decide which rooms receive it. If the room containing the thermostat gets warm, everything shuts down even if other rooms are cold.
Where the thermostat should go:
A frequently used room, typically a hallway or living room.
On an internal wall, around 1.5m high.
Away from direct sunlight, draughts, doors, lamps, televisions and radiators — anything giving it a false reading.
Not in a room with a TRV that will be turned down, and not in a kitchen, where cooking heat shuts the boiler off while the rest of the house is cold.
The important rule: the radiator in the thermostat's room should have no TRV, or the TRV should be left fully open. Otherwise the two controls fight — the TRV closes the radiator, the room cools, the thermostat calls for heat indefinitely, and the boiler runs while that room never warms.
Bypass provision matters too, so the boiler always has a path for flow.
Smart TRVs allow per-room scheduling.