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You turn the heating on, wait for the room to warm up... and one radiator still feels cold across the top.

That’s often a sign there’s air trapped inside. It stops hot water circulating properly through the whole radiator and can leave you with cold patches, gurgling noises and a room that never quite gets comfortable.

It's a frustrating scenario. But bleeding a radiator is actually one of the simpler heating jobs you can tackle yourself, and once you know what you’re doing, it usually only takes a few minutes.

This guide explains how to bleed a radiator safely, what to check afterwards and when a cold radiator might point to a different problem.

Quick answer: How do you bleed a radiator?

To bleed a radiator:

• Turn the heating off and let the radiator cool

• Put a towel beneath the bleed valve

• Insert your radiator key into the valve

• Turn it slowly anticlockwise until you hear air hissing out

• Close the valve once the hissing stops and water begins to appear

• Check your boiler pressure afterwards and top it up if needed

Never bleed a radiator while the heating is on or the radiator is hot. The water inside can be hot enough to scald.

Why does my radiator need bleeding?

Air can gradually collect inside a central heating system and become trapped at the top of radiators.

Because that space is filled with air rather than hot water, you may notice:

• The radiator is warm at the bottom but cold at the top

• It makes gurgling or bubbling noises

• It takes longer than usual to warm up

• The room feels cooler even though the heating is running

Bleeding lets the trapped air escape so hot water can circulate through the radiator again.

If several radiators heat up at very different speeds even after you’ve bled them, the system may also need balancing. See our guide to how to balance a radiator for the next step.

What do you need to bleed a radiator?

You don’t need much:

• Radiator bleed key

• Old towel or cloth

• Small bowl or container to catch water

Some bleed valves can also be opened with a flat-head screwdriver, but check the valve first rather than forcing a tool that doesn’t fit.

Before you start

There are a couple of things worth checking before you get the radiator key out.

Turn the heating off

Always switch the central heating off and allow the radiators to cool first. Apart from avoiding hot water, this stops the pump moving water and air around the system while you work.

Check the boiler pressure

If you have a sealed heating system, take a quick look at the boiler pressure gauge before you begin.

The correct pressure varies between boilers, so check your manufacturer’s instructions rather than relying on one figure for every system.

Bleeding radiators can lower the system pressure, so it’s useful to know where you started.

How to bleed a radiator in 6 steps

Step 1: Check the radiator

If you haven't already checked where the radiator feels cold, turn the heating on briefly and carefully feel the radiator.

Cold across the top but warm lower down is the classic sign of trapped air.

Once you’ve identified the problem radiator, turn the heating off again and wait until everything has cooled.

Step 2: Protect the surrounding area

Find the bleed valve, usually positioned near the top corner of the radiator.

Place an old towel underneath and hold a small bowl or cloth just beneath the valve.

A little water can come out at the end, and it has an annoying habit of finding the carpet if you’re not ready for it.

Step 3: Insert the radiator key

Fit the key securely over the bleed valve.

Turn it slowly anticlockwise. You normally only need a small turn.

Step 4: Listen for the air escaping

You should hear a hissing sound as the trapped air leaves the radiator.

Keep your hand on the key so you’re ready to close the valve.

Step 5: Close the valve when water appears

Once the hissing stops and a steady trickle of water begins to appear, turn the valve clockwise to close it.

Don’t overtighten it. It only needs to be firmly closed.

Step 6: Check the boiler pressure

Once you’ve finished bleeding the radiators, check the boiler pressure again.

If it has fallen below the recommended level for your boiler, you may need to repressurise the system using the filling loop.

Follow your boiler manufacturer’s instructions carefully. If you’re unsure how to do this, it’s worth asking a heating engineer rather than guessing.

How to bleed a heated towel rail

Heated towel rails can trap air too, and the principle is much the same as bleeding a standard radiator.

You’ll normally need:

• Radiator key or suitable screwdriver

• Towel or cloth

• Small container

Then:

• Turn the heating off and let the towel rail cool

• Locate the bleed valve, usually near the top

• Protect the area underneath with a towel

• Turn the valve slowly anticlockwise until you hear air escaping

• Close it once the hissing stops and water appears

• Check your boiler pressure afterwards

There’s normally no need to adjust the lockshield or thermostatic valves simply to bleed a towel rail unless the manufacturer’s instructions say otherwise.

What if you can’t find the bleed valve?

Check the upper corners and look behind any caps or covers. You may be able to gently remove these with a flat-head screwdriver to see if the bleed valve is hidden underneath.

If the valve is damaged, seized or leaking rather than simply difficult to find, don’t force it.

Our guide to how to change radiator valves explains your options, or a heating engineer can replace it for you.

When bleeding a radiator won’t fix the problem

Bleeding only fixes trapped air.

If your radiator still isn’t heating properly afterwards, something else may be going on.

Common possibilities include:

• A stuck thermostatic radiator valve (TRV)

• Sludge inside the radiator or heating system

• Incorrect system balance

• Low boiler pressure

• A faulty valve

• A circulation problem elsewhere in the system

One useful clue is where the radiator is cold.

Cold at the top often points to trapped air. Cold across the bottom can be more consistent with sludge or restricted circulation.

At that stage, repeatedly bleeding the radiator probably isn’t going to get you very far.

How to balance a radiator: step-by-step guide (2026)

Expand your DIY knowledge by learning how to balance a radiator. Follow our step-by-step guide to ensure your home is heated evenly and efficiently.

Learn more

Can you bleed a radiator yourself — and should you?

For most households, bleeding a standard radiator is a manageable DIY task.

It’s usually suitable if:

• The radiator is cold at the top

• The bleed valve turns normally

• You’re comfortable checking the boiler pressure afterwards

• The heating system otherwise works normally

It’s worth calling a heating engineer if:

• The bleed valve is seized or leaking

• Boiler pressure keeps dropping

• The radiator remains cold after bleeding

• Several radiators are affected

• You suspect sludge or a wider circulation problem

• You’re unsure how to repressurise the system

Bleeding the radiator itself is usually the easy bit. Working out why it’s still cold after you’ve done it is where things can become more complicated.

Thinking about hiring a professional?

If you’ve bled your radiator and it’s still refusing to warm up, a heating engineer can check what’s happening elsewhere in the system rather than simply treating the symptom.

They can look for issues such as:

• Stuck or faulty radiator valves

• Poor system balance

• Sludge build-up

• Pressure problems

• Wider circulation issues

That saves you repeatedly reaching for the radiator key when the trapped air wasn’t really the problem in the first place.

Radiator bleeding checklist

• Turn the heating off first

• Allow radiators to cool completely

• Check the boiler pressure before starting

• Protect the floor with a towel

• Open the bleed valve slowly

• Close it when the air stops and water appears

• Check the boiler pressure afterwards

• Test the radiator once the heating is back on

Find checked and reviewed heating engineers near you

If bleeding hasn’t solved the problem, or you’d rather leave your heating system in experienced hands, a local heating engineer can help.

With Checkatrade, tradespeople are:

• Checked - Every Checkatrade member must pass up to 12 checks and commit to upholding the Checkatrade Standard

• Reviewed - Over 6.2 million reviews have been published on Checkatrade about real jobs, on everything from communication, tidiness, timekeeping, and quality of work

• Guaranteed - Jobs booked through Checkatrade are covered up to £1,000 (T&Cs apply)

Whether you need help with a stubborn cold radiator, boiler pressure problems or a heating system that just isn’t warming your home evenly, search for checked and reviewed heating engineers near you and get the problem properly diagnosed.

FAQs

How do I bleed a radiator without a key?

Some radiator bleed valves have a slot that can be turned carefully with a flat-head screwdriver. Don’t use an adjustable spanner or wrench unless the valve is specifically designed for one, as forcing the wrong tool can damage it.

Can I bleed a radiator with a screwdriver?

Yes, if the bleed valve has a suitable slot for a flat-head screwdriver. Turn it slowly anticlockwise until you hear air escaping, then close it once water begins to appear.

How long does it take to bleed a radiator?

Releasing the trapped air often takes less than a minute, although you should allow extra time for the radiator to cool beforehand and to check your boiler pressure once you’ve finished.

Do I need to bleed all my radiators?

Not necessarily. You generally only need to bleed radiators showing signs of trapped air, such as cold patches at the top or gurgling noises. If several are affected, it makes sense to check them all.

Should the heating be on or off when bleeding radiators?

The heating should be switched off, and the radiators should be allowed to cool before you start. This prevents hot water escaping and stops the circulating pump moving air around while you bleed the system.

Do I need a heating engineer to bleed a radiator?

Usually not. Bleeding a radiator is a straightforward DIY job for many people. Call a heating engineer if the valve is damaged, boiler pressure won’t stay stable or the radiator remains cold afterwards.

Why is my radiator still cold after bleeding it?

If the radiator stays cold after bleeding, trapped air may not be the problem. Sludge, a stuck TRV, poor system balance or another circulation issue could be responsible.

How often should you bleed radiators?

There isn’t a fixed schedule that every heating system needs. Check your radiators when you notice cold spots, gurgling noises or before the heating season if they haven’t been used for a while.

Do I need to repressurise my boiler after bleeding a radiator?

Not always, but bleeding can reduce pressure in a sealed heating system. Check the boiler pressure afterwards and follow the manufacturer’s instructions if it needs topping up.

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