Radiator Thermostat Not Working? TRV Fixes
When a radiator thermostat (TRV) is not working, the usual cause is a stuck pin under the head: take the head off, check the pin moves in and out freely, and if it is jammed, gently work it loose — if the head itself has failed or the valve body weeps, it needs replacing.
When a radiator thermostat (TRV) is not working, the usual cause is a stuck pin under the head: take the head off, check the pin moves in and out freely, and if it is jammed, gently work it loose — if the head itself has failed or the valve body weeps, it needs replacing. A room that overheats regardless of the number on the valve, or a radiator that never comes on.
What is actually going on
A room that overheats regardless of the number on the valve, or a radiator that never comes on.
The realistic causes, in order
A seized pin under the head, the most common cause by a wide margin A failed wax capsule inside the head that no longer expands with temperature A valve fitted on the wrong side or in a position where it senses radiator heat rather than room air
What we test on site
Pin travel and free movement Head response when warmed and cooled by hand Whether at least one radiator is left without a thermostatic valve as a bypass, as good practice requires
Telling a stuck pin from a failed head
With the head removed, a pin that springs back when pressed means the valve body is fine and the head is the suspect. A pin that stays down, or will not move at all, points to the valve. If the radiator heats fully with the head off but never shuts down with it on, the head's capsule has usually stopped responding to room temperature.
Get a thermostatic radiator valve that won't control diagnosed properly
Heads and inserts are inexpensive; replacing them across the house is a genuinely worthwhile efficiency job.
Questions we get asked
Is a thermostatic radiator valve that won't control a controls fault or a boiler fault?
A seized pin under the head, the most common cause by a wide margin
What normally puts a thermostatic radiator valve that won't control right?
Heads and inserts are inexpensive; replacing them across the house is a genuinely worthwhile efficiency job.
Can I carry on as normal while living with a thermostatic radiator valve that won't control?
At least one radiator, usually in the same room as the room thermostat, should be left uncontrolled so the system always has a heat path.
How do TRVs work?
The head contains a wax or liquid capsule that expands as the room warms, pressing a pin that gradually closes the valve. When the room cools the capsule contracts, the pin springs back and hot water flows again.
What temperature is 3 on a TRV?
The numbers are relative, not degrees. Many manufacturers place 3 at roughly 20°C, but it varies between brands, so check the maker's leaflet for the heads fitted to your radiators.
Should the radiator heat up with the TRV head removed?
Yes. Without the head nothing presses the pin, so the valve sits fully open. If the radiator still stays cold with the head off, the problem is elsewhere — air, balancing or a closed lockshield.