When one radiator is not working but the rest heat up fine, the problem is almost always that radiator, not your boiler. A stuck thermostatic valve pin is the most common cause, followed by a closed lockshield valve, trapped air and sludge. Most of these you can check yourself in ten minutes.
We’re Clever Energy Boilers, Gas Safe registered (no. 554138) and accredited Worcester Bosch installers based in Bingley, covering Yorkshire, the North West and the Midlands. We’re asked about single cold radiators all the time, and the fix is usually simpler than people expect.
Are the pipes to the radiator hot or cold?
Feel the two pipes going into the radiator with the heating on. This one check tells you where to look next.
| Pipes | Radiator | Most likely cause |
|---|---|---|
| Both hot | Cold all over | Stuck valve pin or very heavy sludge |
| One hot, one cold | Cold | Valve closed or stuck on one side |
| Both cold | Cold | Valves closed, airlock in the pipework, or system needs balancing |
| Both hot | Cold at the top only | Trapped air |
| Both hot | Cold at the bottom only | Sludge |
Why is one radiator not working?
Is the thermostatic radiator valve stuck?
This is the number one cause. Thermostatic radiator valves (TRVs) have a small metal pin under the head that moves up and down to open and close the valve. Over summer, when the heating’s off, the pin can stick in the closed position.
To check it, unscrew or unclip the TRV head. You’ll see the pin sticking up. Press it gently with your thumb. It should spring back up. If it’s stuck down, tap it lightly with something like the handle of a screwdriver, or wiggle it with pliers until it moves freely. Don’t force it. Then refit the head and turn it to maximum.
Is the lockshield valve closed?
The lockshield valve is on the other end of the radiator, usually under a plain plastic cap. It sets how much water flows through. If someone has decorated or replaced a radiator, it may have been closed and never reopened. Take the cap off and turn the spindle anticlockwise a couple of turns with a spanner or adjustable grip.
Is there air trapped inside?
A radiator that’s cold at the top and warm at the bottom needs bleeding. Our radiator bleeding guide shows you how. Check your boiler pressure afterwards.
Is the radiator full of sludge?
Cold at the bottom, warm at the top, and black water when you bleed it all point to sludge. Our guide to a radiator cold at the bottom explains your options.
Does the system need balancing?
If the cold radiator is the one furthest from the boiler, the radiators nearer the boiler may be taking all the hot water. Turning down their lockshield valves slightly pushes more flow to the end of the run.
Why is my radiator still not working after bleeding?
If bleeding didn’t help, air wasn’t the problem. Check the TRV pin and lockshield valve next. If the radiator’s cold at the bottom, it’s sludge. And if bleeding let a lot of water out, top up your boiler pressure, as low pressure can stop radiators heating. Our guide on how to repressurise a boiler shows you how.
What if more than one radiator is cold?
Then it’s probably not a radiator problem. Several cold radiators usually point to low pressure, sludge through the system, a pump fault or a diverter valve issue. Our guide to radiators not heating up covers the system-wide causes.
When should I get an engineer in?
Call an engineer if a valve won’t move, a valve is leaking, or the radiator needs taking off the wall and flushing and you’d rather not do it. Replacing a TRV or lockshield means draining part of the system, which is best left to a professional if you’ve not done it before.
Is it time to upgrade your radiators or boiler?
Old radiators with valves that keep sticking are easy to replace, and new ones heat up faster. Our central heating team fits replacement radiators, and our post on designer radiators has some ideas if you fancy something different.
If the cold radiator is one of several heating problems and your boiler is over 8 years old, it might be the boiler that’s struggling. We offer boiler replacement with fixed pricing, plus the option of a long, market-leading warranty through Clever Energy for peace of mind.
One radiator not working FAQs
How do I know if my radiator thermostat (TRV) is not working?
If the radiator stays cold with the TRV turned to maximum but heats up when you take the head off, the head is faulty. If it stays cold with the head off, the pin underneath is stuck.
Why is the radiator not working in just one room?
Usually a stuck TRV pin or closed lockshield valve on that radiator. If a room has two radiators and both are cold, check whether a zone valve or room thermostat controls that area.
How do I unstick a radiator valve?
Remove the TRV head, then press and gently tap the pin until it moves up and down freely. Don’t use force, as the pin can snap.
Should my TRV be set to 5?
Not all the time. Most rooms are comfortable on 3, which is roughly 20 degrees. Setting it to 5 while you test a cold radiator makes sure the valve is fully open.
Why is my radiator only warm on one side?
Usually a valve that’s only partly open, or sludge building up at one end. Open both valves fully and check again.
Why is one radiator not working but the pipes are cold?
Both valves may be closed, or there’s an airlock in the pipework feeding it. Open the valves fully, bleed the radiator, and if the pipes stay cold, call an engineer.
