How to Bleed a Radiator
Remove trapped air from a radiator so it heats evenly and your boiler runs efficiently.
Quick answer
Turn off the heating and let the radiator cool. Place a towel and container under the bleed valve. Insert a radiator key and turn anti-clockwise until air hisses out. Close the valve as soon as water appears. Check your boiler pressure afterwards and top up if needed.
If the top of your radiator stays cold while the bottom is hot, air is probably trapped inside. Bleeding releases that air so hot water can fill the whole radiator again.
What you’ll need
- Radiator key or flat-head screwdriver
- Old towel
- Jug or small container
Step by step
- 1
Turn off the heating and let the system cool
Switch off your central heating and wait at least 30 minutes. Opening the valve while the system is hot can spray scalding water.
- 2
Place a towel and container under the bleed valve
The bleed valve is a small square pin near the top corner of the radiator. Lay an old towel on the floor and slide a jug or container underneath to catch any water.
- 3
Insert the key and turn anti-clockwise
Fit the radiator key into the valve. Turn it slowly anti-clockwise by about a quarter to half a turn. You should hear a hissing sound as the trapped air escapes.
- 4
Close the valve when water appears
Keep the key in place. As soon as a steady stream of water starts to dribble out, turn the valve clockwise to close it. Do not over-tighten—finger-tight plus a small nudge is enough.
- 5
Check your boiler pressure
Bleeding can lower the pressure in your central heating system. Look at the pressure gauge on your boiler. It should read between 1 and 1.5 bar. If it is below 1 bar, top it up using the filling loop according to your boiler manual.
Let the radiator cool completely before you start—hot water can cause serious scalds. If water sprays out violently, the valve may be loose or faulty; close it immediately and call a heating engineer. If you need to re-bleed the same radiator every few weeks, there is probably a leak in the system—get it checked by a professional.
Common mistakes
- Bleeding the radiator while the heating is running
- Forgetting to top up the boiler pressure afterwards
- Over-tightening the bleed valve and damaging the thread
- Opening the valve too far and letting water pour onto the floor
Frequently asked
How often should I bleed my radiators?
Check them once a year before you turn the heating on for winter. Bleed any that have cold spots at the top.
What if no air comes out when I open the valve?
Water should appear almost immediately. If nothing happens, the valve pin may be stuck. Gently tap the valve body with a spanner, or call a heating engineer if you are unsure.
Do I need to turn off the boiler completely?
Yes. Turn off the heating at the programmer or thermostat so the pump does not push more air into the radiator while you work.
Questions about this guide
No questions yet — be the first to ask one and we’ll help you out.
Comments
No comments yet. Start the conversation.
Did this guide help?
Did you try this?
Help others by sharing how it went.
Show your result
Tried this guide? Share a photo of how it turned out.