One radiator stays cold while the others heat up? Usually a stuck thermostatic valve pin, a closed valve or trapped air. Safe checks to try and when to call us.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
If one radiator stays completely cold while the rest of the house heats up, the most common cause is a stuck pin in the thermostatic radiator valve (TRV). Other causes are a valve that has been closed, or air trapped in the radiator. You can usually check all three yourself without draining anything.
Quick guide: cold at the top means air (bleed it). Cold at the bottom means sludge (needs flushing). Cold all over while others are hot usually means a stuck valve.
What it could be and what to do
Stuck thermostatic valve pin You may be able to fix this
Likely cause: The small pin under the TRV head can seize, especially after the summer. Stelrad says a stuck pin usually leaves the whole radiator cold after about 20 minutes of heating, while turning the head does nothing.
What to do: Turn the TRV to its highest setting and unscrew the head. Gently work the pin up and down a few millimetres with pliers until it springs back on its own, then refit the head. Do not force it.
Valve closed You may be able to fix this
Likely cause: The TRV may be turned to off, or the lockshield valve at the other end (under a plastic cap) may have been closed, for example after decorating.
What to do: Turn the TRV fully on. Remove the lockshield cap and check it is open, turning it anticlockwise a little if it is fully closed. Note how far you turn it.
Trapped air You may be able to fix this
Likely cause: If the radiator is cold at the top but warm lower down, air is stopping the hot water reaching it.
What to do: Turn the heating off, let it cool and bleed the radiator, then check the boiler pressure.
Pin will not move, or the valve leaks Engineer needed
Likely cause: Stelrad advises calling an engineer if the pin will not move without heavy force, sticks again within weeks, or the valve leaks after refitting. Forcing it can break the pin or crack the valve.
What to do: Stop and book an engineer. The valve may need replacing, which means isolating or draining the radiator.
Safe radiator checks you can do
These are the checks we suggest without an engineer. Never open the boiler casing.
Heating off first: turn the heating off and let radiators cool before you bleed them or touch a valve. The water can scald.
Feel the radiator: with the heating on, note whether it is cold at the top, the bottom or all over. That tells you which page above to use.
Valves open: check the thermostatic valve is turned up and the lockshield at the other end is not closed.
Bleed: open the bleed valve slightly with a radiator key, catch the water and close it when water flows steadily.
Pressure: after bleeding, check the boiler gauge shows about 1 to 1.5 bar when cold, and top up as your manual shows. Low boiler pressure guide →
Radiator still cold or leaking? We can sort it.
Our callout is from £75 in standard hours (out of hours £100–£180) for attendance and diagnosis. Repair labour and any parts are agreed with you before work starts.
Our emergency line is open 24 hours a day, 7 days a week.
Every heating system is different. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why is one radiator cold when the others are hot?
Usually a stuck pin in the thermostatic radiator valve, a closed valve or trapped air. Check the TRV pin, make sure both valves are open and bleed the radiator. If it is still cold, call a heating engineer.
How do I free a stuck radiator valve?
Turn the thermostatic head to maximum and unscrew it. Gently work the pin underneath up and down with pliers until it springs back, then refit the head. If it will not move without force, call an engineer rather than risk breaking it.
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