How to Bleed a Radiator Properly
Last updated: 30 September 2026
Turn off the heating, open the bleed valve with a radiator key until water trickles out, then close it and check the boiler pressure.
Overview
Bleeding a radiator is the most searched DIY question in the UK. This guide covers the correct method to remove trapped air and restore full heat to your radiators.
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 system at the thermostat or programmer and allow all radiators to cool completely. This is essential to prevent burns from hot water and to ensure the system is not under pressure while you work. Allow at least thirty minutes for the radiators to cool.
- Locate the radiators that need bleeding. A radiator with trapped air will be cold at the top but warm at the bottom. Start with the radiator furthest from the boiler and work your way towards the boiler as you bleed each one.
- Place a towel or small container underneath the bleed valve on the radiator to catch any water that escapes. The bleed valve is located at the top corner of the radiator and has a small square slot for a radiator key.
- Insert a radiator bleed key into the square slot on the bleed valve. Turn the key anti clockwise slowly. You will hear a hissing sound as trapped air escapes from the radiator. Keep the towel or container in place to catch any water spray.
- Continue to let the air escape until water begins to trickle out steadily from the valve. The water may splutter at first but once a steady stream appears all the air has been released. This process usually takes between thirty seconds and two minutes per radiator.
- Turn the radiator key clockwise to close the valve firmly once water appears. Do not overtighten as this can damage the valve seat. Wipe away any water that has escaped onto the radiator or floor.
- After bleeding all the radiators in your home check the boiler pressure gauge. The pressure may have dropped during the bleeding process. If the pressure is below one bar repressurise the system using the filling loop to between one and one point five bar.
- Turn the heating back on and allow the system to run for fifteen to twenty minutes. Check that all radiators are now heating evenly from top to bottom. If any radiator is still cold at the top repeat the bleeding process.
- If a radiator remains cold after bleeding there may be a sludge buildup or a stuck valve pin. Check the thermostatic valve by removing the head and pressing the pin to ensure it moves freely. If the radiator is cold at the bottom there may be sludge that requires a system flush.
- If you have bled all radiators and the heating is still not working correctly call a Gas Safe registered engineer. There may be a circulation problem or a fault with the boiler that requires professional diagnosis.
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 of bleed valve | Valve blocked or boiler pressure too low | Check the boiler pressure and repressurise if needed. If the valve is blocked it may need to be replaced. |
| Water splutters continuously | Air still trapped in the radiator | Continue bleeding until a steady stream of water appears without spluttering. This may take several attempts. |
| Radiator still cold at top after bleeding | Air not fully removed or valve not working | Repeat the bleeding process. If the radiator remains cold the bleed valve may be faulty and need replacing. |
| Boiler pressure drops after bleeding | Water loss from the system during bleeding | Repressurise the boiler using the filling loop to between one and one point five bar. |
Frequently Asked Questions
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. Regular bleeding keeps your heating system running efficiently.
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.
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.
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. Bleeding releases the trapped air.