How to Bleed a Radiator: Step-by-Step Guide

If your radiators feel warm at the bottom but cold towards the top, they need bleeding. Air accumulates in central heating systems over time and prevents hot water from circulating properly through the full radiator. Bleeding releases that trapped air and is one of the simplest home maintenance jobs anyone can do.

What You Need

A radiator bleed key — these cost £1–2 from any hardware shop, B&Q, or Amazon. You will also want an old cloth or a small bowl to catch the water that drips out when you open the valve. That is genuinely everything required.

Step-by-Step Instructions

Step 1 — run the heating. Turn your central heating on and let it run for 15–20 minutes until all radiators are hot. This builds up the system pressure you need and helps identify which radiators have cold spots.

Step 2 — turn the heating off. Switch the heating off and wait 10–15 minutes. The radiators will still be warm but not scalding, which is safer when water starts to drip.

Step 3 — locate the bleed valve. The bleed valve is a small square nut sitting in a round housing at the top corner of the radiator — usually the end furthest from the pipes. Place your cloth or bowl directly underneath it.

Step 4 — open the valve. Insert the bleed key and turn it anti-clockwise about half a turn. You will hear a hissing sound as trapped air escapes. Hold the cloth ready.

Step 5 — close when water appears. Once water starts to drip steadily from the valve — and the hissing stops — close the valve by turning clockwise. Do not overtighten; firm is enough.

Check Boiler Pressure Afterwards

Bleeding releases water from the system which can drop boiler pressure. Check the gauge on your boiler — it should read between 1 and 1.5 bar. If it has dropped below 1, top up the system using the filling loop, which is usually a silver flexible hose underneath the boiler. Your boiler manual will show exactly how.

Bleed your radiators once a year before winter starts to keep your heating running at full efficiency. If bleeding does not resolve cold spots, or the system loses pressure repeatedly, the system may need a power flush — that is a job for a Gas Safe registered engineer.