How to Bleed a Radiator
Last updated: 7 September 2026
Turn off the heating, use a radiator key to open the bleed valve, release air until water appears, then close the valve.
Overview
A radiator that is cold at the top but warm at the bottom has trapped air. This guide covers bleeding the radiator to restore full heat.
Step-by-Step Fix
Before beginning any repair, ensure you have isolated power or water supplies. Clear your workspace, gather standard hand tools, and wear appropriate personal protective equipment (PPE).
- Turn off your central heating and allow the radiators to cool completely. This is important to prevent burns and to ensure the system is not under pressure while you work.
- Place a towel or small container underneath the bleed valve on the radiator to catch any water that may escape. The bleed valve is usually located at the top corner of the radiator.
- Insert a radiator key into the square slot on the bleed valve. Turn the key anti-clockwise slowly. You will hear a hissing sound as air escapes from the radiator.
- Continue to let the air escape until water begins to trickle out of the valve. The water will initially splutter, but once a steady stream appears, all the air has been released.
- Turn the radiator key clockwise to close the valve firmly. Do not overtighten, as this could damage the valve. Wipe away any water that has escaped.
- After bleeding all the radiators in your home, check the boiler pressure. The pressure may have dropped and will need to be topped up using the filling loop.
- If the radiator is still cold after bleeding, check the valve is fully open. The thermostatic valve should be set to the highest setting, and the lockshield valve should be fully open.
- If the radiator remains cold at the bottom after bleeding, there may be a sludge buildup. This may require a system flush to remove the debris.
Common Symptoms & Diagnostic Resolutions
Review the common error codes and operating symptoms below to quickly identify the root cause of the fault before ordering replacement parts.
| Symptom / Code | Likely Root Cause | Recommended Resolution |
|---|---|---|
| No water comes out | Valve blocked or boiler pressure low | Check the boiler pressure and repressurise if needed. If the valve is blocked, it may need to be cleaned or replaced. |
| Water splutters | Air still trapped | Continue bleeding until a steady stream of water appears. If the water continues to splutter, there may be a persistent airlock. |
| Radiator still cold at top | Not all air removed | Repeat the bleeding process. If the radiator is still cold, there may be a blockage or a valve issue. |
| Pressure drops after bleeding | Water loss from system | Repressurise the boiler using the filling loop. If the pressure continues to drop, check for leaks. |
Frequently Asked Questions
Do I need to turn the heating off before bleeding?
Yes, you should turn the heating off and allow the radiators to cool before bleeding. This prevents burns and ensures the system is not under pressure, which could cause water to spray out.
What if I do not have a radiator key?
A flat-head screwdriver can fit some bleed valves, but a radiator key is recommended for a better fit. Radiator keys are available at most DIY stores for a few pounds.
How often should I bleed my radiators?
You should bleed your radiators when you hear gurgling noises or notice that the top of a radiator is cold. This typically happens once or twice a year, often before winter.
Why does my radiator need bleeding?
Air can become trapped in the radiator over time, preventing hot water from circulating fully. This causes the top of the radiator to be cold while the bottom remains warm.