How to Bleed Your Radiators

If your radiators have cold spots — particularly across the top — trapped air is the most likely cause. Bleeding releases the air and allows hot water to fill the full radiator panel. This is a routine homeowner task that requires no specialist tools.

What you'll need

A radiator bleed key (available from any hardware shop) or a flat-head screwdriver if the bleed valve has a slotted head
A cloth or small towel
A small container (bowl or cup) to catch any water

Steps

Turn the heating on and identify which radiators need bleeding. Let the system run for 15 minutes, then carefully feel each radiator. A radiator that is hot at the bottom but cool or cold across the top has trapped air. Make a note of which radiators are affected.
Turn the heating off and let the system cool for 10 minutes. Bleeding radiators while the system is hot and pressurised can result in scalding water spraying out. Let the water cool to a safe temperature before proceeding.
Place a cloth or towel under the bleed valve. The bleed valve is a small square-headed fitting located at the top corner of the radiator (usually the side opposite the TRV). Position your cloth to catch any drips, and hold the container ready.
Insert the bleed key and turn anti-clockwise a quarter turn. Do not remove the valve — just open it slightly. You will hear air hissing out of the valve.
Hold the key in position until water appears. The hissing will stop and a steady trickle of water will begin to flow. This means all the air has been released from that radiator.
Close the valve immediately. Turn the bleed key clockwise until the valve is snug. Do not overtighten — the valve only needs to be finger-tight with the key. Overtightening can damage the valve seat.
Repeat for all affected radiators. Work through your list, starting from the radiator furthest from the boiler and finishing with the nearest. This order helps ensure air moves out of the system efficiently.
Check the boiler pressure. Bleeding releases water as well as air, so the system pressure may have dropped. Check the gauge on the boiler — if it has fallen below 1.0 bar, repressurise the system using the filling loop. See our guide: How to Repressurise Your Heating System.

Note: If the water that comes out of the bleed valve is dark brown or black, this is a sign of sludge and magnetite in the system. Bleeding will not resolve this — a professional powerflush is required. Contact a heating engineer.

How often should you bleed radiators?

There is no fixed schedule, but it is good practice to check your radiators at the start of each heating season (typically October). If you notice cold spots developing during winter, bleed the affected radiators promptly — trapped air reduces efficiency and makes the boiler work harder than necessary.