How to Bleed a Radiator
Last updated: 4 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).
- Firstly, switch off your central heating system at the boiler or thermostat and allow ample time for all radiators to cool down completely. This is crucial to prevent hot water from scalding you and to ensure the system pressure is stabilised.
- Before you begin, protect your flooring by placing an old towel or a small, shallow container directly underneath the radiator bleed valve. This will catch any drips of water that escape when you open the valve.
- Carefully insert a radiator bleed key into the square slot on the bleed valve, which is usually located at the top corner of the radiator. Gently turn the key anti-clockwise, making sure not to force it, until you feel it open slightly.
- As the valve opens, you should hear a distinct hissing sound, indicating that trapped air is escaping from the radiator. Keep the towel or container in place to catch any initial spray of air and water.
- Continue to let the air escape until the hissing sound stops and a steady trickle of water begins to flow from the valve. This indicates that all the trapped air has been released and the radiator is now full of water.
- Once you see a steady stream of water, immediately turn the radiator bleed key clockwise to firmly close the valve. Ensure it is snug but do not overtighten it, as this could damage the valve.
- After bleeding all necessary radiators, check the pressure gauge on your central heating boiler. If the pressure has dropped below the recommended range (typically between 1 and 1.5 bar for a cold system), you will need to top it up according to your boiler's instructions.
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 pressure and repressurise if needed. |
| Water splutters | Air still trapped | Continue bleeding until a steady stream of water appears. |
Frequently Asked Questions
Do I need to turn the heating off before bleeding?
Yes, to prevent burns and ensure the system is not under excessive pressure.
What if I don't have a radiator key?
A flat-head screwdriver can fit some bleed valves, but a key is recommended.