Thermostatic radiator valve (TRV) stuck open — radiator always hot
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Most likely cause & what to check
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.
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Frequently asked questions
Can a TRV stuck open damage my boiler?
A TRV stuck open does not damage the boiler directly, but it wastes energy by heating a room regardless of temperature and makes the system harder to balance. If all TRVs are open simultaneously, the boiler may short-cycle (fire and cut out repeatedly) if the system is oversized for the current demand.
Should TRVs be replaced regularly?
TRV heads typically last 10–15 years before the wax element that operates the pin degrades. If multiple TRVs on a system are sticking or not responding accurately to temperature, replacing them all at once is more cost-effective than piecemeal replacement.
Can I set all TRVs to the same number?
TRV numbers are not calibrated temperatures — they are relative scales that vary by manufacturer. Setting all TRVs to 3 is a reasonable starting point for most rooms, then adjust: higher numbers for rooms that feel cold, lower for those that overheat. Leave the room with the main thermostat at its maximum so the thermostat (not the TRV) controls the boiler.