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

If your central heating isn’t working well, you may need to bleed your radiators.

It’s a relatively straightforward DIY task that you can do yourself.

But first, you need to establish whether you need to.

And it helps to understand why you’re doing it, too!

Why bleed your radiators?

Bleeding your radiators helps to remove air which is trapped in your central heating system.

When air is present, the system is less efficient because there is less space for hot water.

So, it will cost more for you to heat your home and it’s less environmentally-friendly.

Common signs that your radiator needs bleeding

infographic by YourRepair titled 'Signs You May Need to Bleed Your Radiator'. Under the heading 'What to look out for', it lists five signs: 1) Cold top, warm bottom — if the top of your radiator feels noticeably cooler than the bottom, trapped air is likely preventing hot water from filling the whole radiator. 2) Entire radiator is cold — check the lockshield valve is fully open; if the radiator is still cold with pipes that should be warm, it may need bleeding. 3) Gurgling or rattling noises — gurgling, gulping or rattling sounds from your boiler or radiators can indicate that air is trapped somewhere in the system. 4) Damp patches or mildew — uneven heat around your home can cause condensation; small damp patches or mildew on walls may indicate a room isn't heating properly. 5) Higher heating bills — when air reduces the space available for hot water, your system works harder to heat your home, pushing up your energy costs. A callout at the bottom reads: 'It could be something else… Whilst these signs might indicate you need to bleed your radiators, they could also be signs of other issues. Contact a professional if you are unsure.'

1. The top of the radiator is cold

Turn your central heating on and wait for it to warm up. Then check your radiators.

Does the temperature at the top of your radiators feel the same as the temperature at the bottom of them?

If it’s cold (or just slightly warm) at the top of the radiator, it likely needs to be bled.

2. The entire radiator is cold

First, check that the lockshield valve is open by turning it as far as you can anti-clockwise.

Next, check the pipes going into the radiator by placing your hand on them.

If this has resolved the issue, one of the pipes should start to feel warm.

3. Check thermostat valve

If this hasn’t helped, check the thermostatic valve is moving properly.

Take the cap off the thermostatic valve and push the pin down. This should be moving freely.

If it’s stuck and not moving freely you may need a replacement thermostatic valve.

4. Mildew on the walls

Unbalanced heat around the home can cause condensation.

So, check your home for small damp patches or mildew on the walls.

It could be an indication that the room where they appear in your home is too cold.

(However, note: mildew might have other causes, too.)

5. Strange noises from your heating

Does your boiler make noises similar to gurgling, gulping and rattling?

This can be a sign that air is trapped in the system although it could also be an indication of various other issues too.

Checking your radiators to make sure the problem is not more serious.

How to bleed a radiator system

infographic by YourRepair titled 'How to Bleed a Radiator'. Under the heading 'Step-by-step guide', it lists the equipment needed — a radiator key, cloth or rag, old towel, and small container or bucket — followed by eight steps: 1) Turn the heating on — start your heating system and turn thermostatic valves to their highest setting; wait until the whole system is warm. 2) Check each radiator — feel each radiator for cold spots, uneven temperature, or gurgling noises, and note which ones need bleeding. 3) Turn the heating off — let the radiators cool until you can comfortably place your hand on them, to prevent scalding. 4) Locate the bleed valve — starting with the radiator closest to the boiler, find the bleed valve, a round hole with a square metal bar inside, always at the top of one end. 5) Protect the area — place an old towel on the floor directly beneath the valve end of the radiator to catch any water drips. 6) Open the bleed valve — holding the key with a cloth, turn the valve anti-clockwise one quarter turn only; leave open until the hissing stops and water begins to trickle, around 20–30 seconds. 7) Retighten and repeat — close the valve firmly, mop up any water, and move to the next radiator. 8) Restart and check pressure — restart your heating, check all radiators are reaching full heat, then check your boiler pressure gauge, which should read between 1.2 and 1.5 bar when cold. A callout at the bottom reads: 'Still not heating properly after bleeding? If radiators are cold at the bottom, sludge may be the issue. Consider a powerflush — this requires a professional heating engineer.'

Step 1: Gather your equipment

To bleed your radiators, you will need;

A radiator key. The bleed valve keys – usually with a square hole - are supplied with the radiator, but you can buy them at a DIY shop. Some modern radiators have a new style of valve that can be turned with a flathead screwdriver.
A cloth to hold against the open valve
An old towel for protecting the floor
A container (to catch any sizeable water leak).

Step 2: Turn the system on

Start up your heating system. On each radiator, turn the integral thermostatic valves to their highest setting.

Wait until the whole system has had time to warm up and the water is flowing through it.

Step 3: Touch the radiators

You need the system to be warm to be able to identify which radiators need bleeding.

Bear in mind that they may be hot to the touch. Go around and feel the condition of each of your radiators.

Check if any of your radiators are:

Cool to the touch
Have a top section that is significantly cooler than the bottom part
Making gurgling noises.

Step 4: Turn the heating off

Turn the heating off.

Allow your radiators to cool to the point where you can place your hand on them without any discomfort at all.

This is to avoid scalding when you open the valve.

Step 5: Locate the bleed valve

Starting with a radiator closest to the boiler, locate the first bleed screw.

(This is because air tends to accumulate further from the boiler, so working outward ensures even bleeding.)

This is always at the top of a radiator and located at one end. It looks like a round hole with a square metal bar inside it.

The square section is used to drive a threaded section that forms the valve.

Step 6: Protect the area

By undoing the bleed valve, air – and possibly some water – will be released.

So, to protect your carpets and floors, place an old towel on the floor so that it covers the area directly under the valve end of the radiator.

Step 7: Loosen the valve

Holding the radiator key with the cloth for protection (against hot water), place the square bit fully over the bleed valve.

(Or, if you’re using a flathead screwdriver, put the blade into the groove.) Turn it anti-clockwise one-quarter of a turn only.

This should be sufficient to let trapped air get out but without a huge flow of water.

The air should make a whistling sound as it comes out. Any water should be caught by the cloth and will drip onto the old towel.

Leave the valve open with the key still in place until the whistling stops. It may take 20 to 30 seconds.

Retighten the valve and mop-up any water on the radiator or floor. Move on to the next radiator.

Step 8: Restart the system

With each radiator checked in the same way, recheck that all bleed screws are fully tight.

Once that is done, restart your heating system and allow it to warm up.

Once warm, go around each radiator again and check that they are reaching full heat.

When you are happy with the heat, turn down the thermostatic valves as appropriate.

If some radiators are still heating unevenly, the system may need balancing.

This is an adjustment of the flow of water to each radiator (unlike bleeding, which is the removal of trapped air).

Step 9: Check boiler pressure

You also need to check that your boiler is operating at the right pressure.

By bleeding your radiators and releasing excess air, you may have lowered the overall pressure of your system.

So look at the gauge on your boiler (if you have one) to see if it’s between 1.2 and 1.5 bars of pressure when it's cold.

If it’s below 1.2, you need re-pressureise your boiler. This will likely require a heating engineer.

Consider a powerflush

If bleeding doesn’t resolve your radiator’s issue, you should consider a powerflush for it.

This is a more extensive pipe-clearing process that focuses on clearing sludge rather than air.

It requires the help of a professional.

Home care cover from YourRepair

Homeowners can bleed their own radiators.

However, central heating systems are complex and with many parts that go wrong.

For peace of mind and to avoid any unwanted engineer fees, take out one of our home cover plans.