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

If one or more of your radiators are cold at the top but warm at the bottom, trapped air is usually to blame. Bleeding your radiators releases that air so hot water can circulate properly again — and it’s one of the simplest bits of home heating maintenance you can do yourself. This guide walks you through how to bleed a radiator safely, step by step.

What does bleeding a radiator do?

Over time, air becomes trapped inside your central heating system. Because air rises, it collects at the top of your radiators and stops hot water from filling that section. The result is a radiator that’s warm at the bottom and cold at the top, meaning your heating works harder, your rooms take longer to warm up, and your energy bills creep up. Bleeding simply lets that trapped air escape.

Signs your radiators need bleeding

The radiator is cold at the top but warm at the bottom
Rooms take a long time to heat up
You hear gurgling or rattling noises from the radiators
Some radiators stay cold while others get hot
Damp or mould appearing because rooms aren’t heating evenly

What you’ll need

A radiator bleed key (available cheaply from any DIY shop) or a flat-head screwdriver for newer valves
A cloth or old towel
A small container to catch drips

How to bleed a radiator: step by step

Turn your heating on fully so the system reaches pressure and you can feel which radiators have cold spots.
Turn the heating off and let the radiators cool. Never bleed a hot radiator — you risk scalding yourself with hot water.
Find the bleed valve at the top corner of the radiator. Hold your cloth underneath and fit the bleed key onto the square valve.
Turn the key slowly anti-clockwise, about a quarter to half a turn. You’ll hear a hissing sound as the trapped air escapes.
Wait for the hiss to stop. When a steady trickle of water comes out instead of air, the radiator is fully bled.
Close the valve by turning the key clockwise. Don’t overtighten it — snug is enough.
Repeat for each radiator, working from the ground floor up, then the upstairs radiators.

Check your boiler pressure afterwards

Bleeding radiators releases water as well as air, which can lower the pressure in your system. Once you’ve finished, check the pressure gauge on your boiler — it should sit at around 1 to 1.5 bar when the heating is cold. If it has dropped below 1 bar, you’ll need to top it up using the filling loop. If instead you find your boiler pressure is too high, we cover the causes and fixes in a separate guide.

How often should you bleed your radiators?

For most homes, bleeding your radiators once a year — ideally at the start of autumn before you rely on the heating — is enough. If you notice cold spots returning quickly after bleeding, it can point to a bigger issue such as a build-up of sludge or a circulation problem, which is worth having a professional look at.

When to call a professional

Bleeding radiators is a safe DIY job, but call a heating engineer if radiators stay cold after bleeding, if you keep having to bleed the same radiator, if the water that comes out is very dark or dirty (a sign of sludge), or if you spot leaks around the valves. Our plumbing and heating team can flush the system, balance your radiators, and get your heating running efficiently again.

Frequently asked questions

Should the heating be on or off when bleeding a radiator?

Turn the heating off and let the radiators cool before bleeding them. Bleeding a hot radiator can spray out scalding water.

Do I need to bleed radiators upstairs or downstairs first?

Start with the downstairs radiators, then move upstairs. On each floor, begin with the radiator furthest from the boiler.

Why is my radiator still cold after bleeding?

If a radiator is cold at the bottom rather than the top, the problem is usually sludge build-up rather than air, and the system may need a power flush. If it’s cold all over, check the valves are open. Persistent cold radiators are worth a professional inspection — see our full guide on why a radiator is not heating up.

Need help with your heating in West London?

If your radiators or boiler still aren’t behaving after trying the steps above, our Gas Safe registered engineers can help. We provide fast, reliable plumbing and heating services across West London, from radiator and boiler repairs to full system checks.

Tel: 020 3561 4415