Radiator completely cold — systematic diagnosis
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Most likely cause & what to check
First check the TRV is open: remove the TRV head and check the pin is not stuck down (see our "TRV stuck closed" guide). This is the most common cause of a single cold radiator.
Check the lockshield valve is open: remove the plastic cap and use a spanner to turn the spindle. It should be open (turned anticlockwise as far as it will go).
Bleed the radiator — even if air doesn't seem likely, it takes 30 seconds and rules out the most common cause of cold radiators. If the radiator is cold throughout and not just at the top, air is less likely but still worth checking.
Feel the pipework leading to the radiator. If the incoming pipe is warm but the radiator is cold, the valve is almost certainly the problem (TRV or lockshield). If the incoming pipe is cold, the issue is further upstream.
Check if this radiator is on its own circuit or at the end of a long run. A blocked section of pipework or a completely closed balance valve further up the system can starve it of flow.
If the radiator is new or has been recently refitted, check the flow direction is correct — some radiators must have flow entering from one specific side. Check the manufacturer's fitting instructions.
If you've checked everything above and the radiator still won't heat, a plumber can use a thermal imaging camera to identify blockages or flow issues in the pipework under floors or behind walls.
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