How to Bleed a Radiator
Remove trapped air from your radiator to restore full heating in 10 minutes
⚡Quick Answer
Turn off your heating and wait for radiators to cool completely before you start. Use a radiator key to open the bleed valve until air escapes, then close it once water flows steadily.
Step-by-Step Guide
Turn Off Heating and Let Cool
Switch off your heating system and wait 30–60 minutes for radiators to cool completely before touching the valve. This is essential safety — a pressurised hot radiator can spray scalding water when the bleed valve is opened.
Place a Cloth Below the Valve
Hold an old cloth or small bowl under the bleed valve (the small square nipple at the top corner of the radiator) to catch drips. Protecting your floor and skirting boards from rust-stained water takes seconds and saves a cleaning headache.
Open the Valve with a Radiator Key
Insert the radiator key into the valve and turn it counter-clockwise (about ¼ turn) — you'll hear air hissing out as the trapped pocket escapes. Keep the key in the valve the whole time so you can close it instantly once the hissing stops.
Close When Water Flows Steadily
Once water flows steadily without sputtering, turn the key clockwise to close the valve firmly — the air is fully expelled and the radiator should now heat evenly from top to bottom. Avoid over-tightening, as the brass valve seat can be damaged by excessive force.
Frequently Asked Questions
How do I know if my radiator needs bleeding?
If the top of the radiator is cold while the bottom is warm, **trapped air** has risen and is blocking hot water from filling the panel. You may also notice the radiator taking much longer to heat up than usual, or hear gurgling sounds as water tries to push past the air pocket.
How often should I bleed radiators?
Once a year at the **start of heating season** is usually sufficient for most homes. However, if you notice cold spots mid-season or hear persistent gurgling, bleed the affected radiators immediately rather than waiting — it only takes a few minutes.
What if no air or water comes out?
The **bleed valve** may be stuck or seized from years of inactivity — try applying very gentle pressure with the key rather than forcing it. If the valve still won't turn or you suspect a more serious blockage, stop and call a qualified plumber to avoid damaging the valve seat.
Will bleeding radiators affect my boiler pressure?
Yes — releasing air and water from the system slightly lowers **boiler pressure**, which you can read on the pressure gauge on the front of your boiler. After bleeding all radiators, check the gauge and use the **filling loop** (usually a small braided hose beneath the boiler) to top the pressure back up to between 1 and 1.5 bar.