How to Fix a Radiator That Won't Heat Up
Diagnose why a radiator stays cold and fix the most common causes yourself before calling a heating engineer.
Quick answer
Check that the thermostat and programmer are on. Feel the radiator: cold at the top means trapped air—bleed it. Cold at the bottom means sludge—this needs a power flush by a professional. If the valve pin is stuck, free it carefully with an adjustable spanner.
A cold radiator when the heating is on wastes energy and leaves rooms chilly. Most problems fall into three categories: trapped air, a stuck valve, or sludge buildup. You can sort the first two yourself.
What you’ll need
- Radiator key or flat-head screwdriver
- Adjustable spanner
- Old towel
Step by step
- 1
Check the basics first
Make sure the room thermostat is set above the current temperature and the heating programmer is set to 'on'. Check that the thermostatic radiator valve (TRV) is turned up to at least number three.
- 2
Feel the radiator to find the cold spot
Run the heating for 10 minutes, then switch it off. Feel the radiator carefully. Cold at the top and warm at the bottom means trapped air. Cold at the bottom and warm at the top means sludge. Completely cold means a valve or balancing issue.
- 3
Bleed the radiator if it is cold at the top
Place a towel under the bleed valve. Insert the radiator key and turn anti-clockwise until air hisses out. Close the valve as soon as water dribbles out. Check the boiler pressure and top up if it is below 1 bar.
- 4
Free a stuck valve pin
If the TRV head has been turned off for a long time, the pin inside can seize. Remove the plastic TRV head, grip the metal pin beneath with an adjustable spanner, and press it down gently a few times. It should spring back up freely. Replace the head.
- 5
Check the lockshield valve is open
The lockshield valve is on the opposite side of the TRV. Remove the plastic cap and use an adjustable spanner to open it by turning anti-clockwise two full turns. If the radiator now heats up but others go cold, the system needs balancing—call a heating engineer.
Let the system cool before you work on it. If the radiator is cold at the bottom, sludge is blocking the flow. Do not try to flush the system yourself—call a Gas Safe registered engineer. If more than one radiator is cold, or the boiler pressure drops repeatedly, get professional help.
Common mistakes
- Bleeding the radiator while the heating is running
- Forcing a seized valve pin and snapping it
- Closing the lockshield valve fully instead of opening it
- Ignoring the boiler pressure after bleeding
Frequently asked
Why is my radiator cold at the bottom but hot at the top?
Sludge—mostly rust and dirt—has settled at the bottom and is blocking the flow of hot water. This needs a power flush by a professional heating engineer.
Can I fix a completely cold radiator myself?
Sometimes. Check the TRV and lockshield valves are both open, and bleed it if needed. If it is still cold after these checks, there may be a pump fault or a blockage deep in the pipework—call an engineer.
Do I need to drain the system to free a valve pin?
No. You only need to remove the plastic TRV head and press the pin. Do not unscrew any pipework.
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