Thermostatic radiator valve (TRV) stuck open — radiator always hot

A TRV (thermostatic radiator valve) controls the flow of hot water to a radiator by sensing the room temperature. The valve head contains a wax or liquid element that expands to push down a pin, closing the valve as the room warms. If the pin is stuck in the open position, hot water flows freely and the radiator stays permanently hot.

Remove the TRV head — unscrew the large ring nut at the base of the head (anti-clockwise) and lift it off. Underneath you will see the valve pin — a small round metal pin protruding from the valve body.

Try pressing the pin down with your finger and releasing it. It should move freely (approximately 3–5mm travel) and spring back up when released. A pin that does not spring back, or that requires significant force, is stuck.

Apply a small amount of penetrating oil (WD-40 or Plus Gas) around the base of the pin. Wait 10 minutes, then try pressing and releasing again. Work it up and down gently with pliers if necessary — the pin can seize with mineral deposits.

Once moving freely, refit the TRV head and set it to position 3 (approximately 20°C). The radiator should now cool down when the room reaches temperature.

If the pin cannot be freed, or if the valve body is corroding, the full TRV valve and head can be replaced. Valve bodies cost £10–£30 and require the radiator to be isolated and drained to replace.