BoilerFaultCodesHow to bleed a radiator
Two minutes per radiator, and the only tool you need costs about a pound. If your radiator is cold at the top and warm at the bottom, trapped air is the cause and this is the fix.
The short version
Turn the heating off and let it cool. Hold a cloth under the bleed valve at the top of the radiator. Turn the key anticlockwise a quarter turn until it hisses. When water dribbles out instead of air, close it. Then check the boiler pressure and top it up if it has dropped below 1 bar.
First, check it is actually air
Put your hand on the radiator with the heating on. Where it is cold tells you what is wrong, and bleeding only fixes one of these.
Bleeding is the fix. Carry on down this page.
What you need
A radiator bleed key, a cloth, and something to catch the water. Bleed keys are sold in every hardware shop and most supermarkets for around a pound. Buy two, because they are small and everyone loses them.
No bleed key? Three things that work
- A flat-head screwdriver. Many newer radiators have a slotted bleed screw rather than a square nipple. Look closely before assuming you need a key.
- Small pliers or an adjustable spanner. These will grip a square nipple. Go gently, because it is soft brass and rounding the corners off makes the job harder next time.
- A flat-blade key from an old radiator. Bleed keys are near enough universal in the UK, so one from any radiator in the house will fit the rest.
The steps
Turn the heating off and let it cool
Bleeding a hot radiator sprays scalding water. Turn the heating off and give it 20 to 30 minutes until the radiators are cool to touch. It also lets the trapped air gather at the top where you want it.
Find the bleed valve
It is the small square nipple set into a round fitting at the top of one end of the radiator. Some newer radiators have a slotted screw instead, which takes a flat screwdriver.
Hold a cloth and a container underneath
What comes out is usually dirty and will stain carpet. A tea towel and an old mug is enough.
Turn the key anticlockwise, a quarter turn only
You will hear a hiss. That is the air escaping. Do not open it further than a quarter turn, and never take the valve all the way out.
Wait for the hiss to stop and water to appear
When a steady dribble of water replaces the hissing, all the air is out. It usually takes 20 to 30 seconds.
Close it firmly, but do not overtighten
Turn clockwise until it stops. Snug is enough. Forcing it can damage the seat and leave you with a weeping valve.
Check the boiler pressure
This is the step people miss. Letting air and water out drops the system pressure. Look at the boiler gauge and top up with the filling loop if it has fallen below about 1 bar.
Do not skip the pressure check
Manufacturers treat bleeding and repressurising as one job, not two. Worcester's remedy for trapped air is to "bleed radiators and recharge heating system". Ideal's is to "bleed radiators, refill system". If you bleed several radiators and leave the pressure low, the boiler may lock out and you will have swapped one fault for another.
How to repressurise with the filling loop →Which order to do the house in
If you are doing the whole house, start downstairs with the radiator furthest from the boiler and work back towards it. Then go upstairs and do the same. Air rises, so the upstairs radiators usually hold the most, and doing them last means any air pushed along by the earlier bleeds ends up somewhere you are about to get to.
What the water tells you
| What comes out | What it means |
|---|---|
| Clear water | Healthy system. Nothing more to do. |
| Black or brown water | Sludge. Worth a flush and an inhibitor before it reaches the boiler. |
| Nothing at all | No air and no water means no flow. Check the valves are open. |
| Air, again and again | Air is getting in from somewhere. See below. |
When it does not work
Still cold after bleeding
If no air came out, air was never the problem. A radiator cold all over is usually a stuck valve pin or a flow problem rather than anything you can bleed away.
Work through the cold radiator checks →It needs bleeding every few weeks
Repeatedly refilling with fresh water is bad for the system, because fresh water brings oxygen and oxygen causes corrosion. Persistent air usually means a small leak drawing air in, a failing expansion vessel, or an automatic air vent that has stuck open. Worth getting looked at rather than living with.
The valve is dripping now
It has probably not seated properly. Try closing it a fraction more. If it still weeps, the valve is worn and needs replacing, which is a cheap part but means draining down that radiator.
The pressure keeps dropping after bleeding
Bleeding drops pressure once, and topping up should hold it. If it falls again over the following days, you have a leak somewhere rather than a bleeding problem.
Why boilers lose pressure →How often should you do it?
Once a year is plenty for most homes, and the sensible time is early autumn before you rely on the heating. Beyond that, do it when a radiator tells you it needs it. There is no benefit to bleeding radiators that are heating evenly, and every bleed brings a little fresh oxygenated water into the system.
Safety
Only bleed a cool radiator. Hot systems are pressurised and will spray scalding water. Bleeding and topping up the pressure are both safe to do yourself. Anything inside the boiler casing, on the gas supply or on the flue is for a Gas Safe registered engineer. If you smell gas, leave the house and ring 0800 111 999.
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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