How to Bleed a Radiator Properly

A safe, step-by-step method for bleeding radiators, including what to do when a valve is stuck and how to restore pressure afterwards.

Last updated: 2026-10-19

Bleeding a radiator releases trapped air so hot water can fill the whole panel. It is a safe, straightforward job — provided you let the system cool first and know when to stop.

When bleeding helps

Bleed if a radiator is cold at the top but warm at the bottom, or if you can hear gurgling. Bleeding does not fix a radiator that is cold at the bottom — that is usually sludge (see why radiators go cold at the bottom).

What you need

• A radiator key (most common) or a flat-head screwdriver for some modern valves
• A cloth or small towel
• A container if the valve is awkward to reach

The procedure

1. Turn the heating off and wait for the radiators to cool. Bleeding hot water can scald.
2. Start with the radiator furthest from the boiler, then work back towards it. Upstairs first is usually sensible.
3. Locate the bleed valve at the top corner of the radiator.
4. Hold the cloth beneath it. Turn the valve anti-clockwise a quarter turn — no more.
5. Listen for the hiss of escaping air. Keep the valve open until water dribbles out steadily.
6. Close the valve firmly, but do not overtighten.
7. Repeat on each radiator that needs it.
8. Check the boiler pressure. Bleeding lowers system pressure. If it has dropped below about 1 bar cold, top it up — see how to repressurise a boiler.
9. Turn the heating back on and check that each radiator now heats evenly.

If the valve will not turn

Do not force it. Bleed valves are brass and small; excessive force rounds the head or snaps it, which turns a five-minute job into a repair. If it is seized, stop and ask an engineer.

If the valve leaks after closing, it may need tightening slightly or replacing.

If air keeps returning

One bleed is normal. Radiators that repeatedly collect air usually have air being drawn into the system — often a pump, a cold feed issue on older systems, or corrosion producing gas. That needs diagnosis rather than repeated bleeding.

A note on balancing

If some radiators heat quickly and others stay lukewarm after bleeding, the system may be unbalanced rather than airlocked. Balancing adjusts the flow through each radiator so heat is distributed evenly — an engineer job, and worth doing alongside a repair.

When to call

Book a central heating visit if radiators stay cold after bleeding, a valve is seized or leaking, or the problem keeps coming back.