How to bleed a radiator

A radiator that stays cold at the top while the bottom is warm is one of the most common heating problems in a British winter, and usually one of the cheapest to fix. The cause is air trapped inside, and letting it out, called bleeding the radiator, takes a radiator key, a cloth and a few minutes. This guide shows how to do it safely, what to check on the boiler afterwards, and how to tell when a cold radiator is something you should hand to a heating engineer.

It is written for homes in the UK with a gas, oil or LPG boiler and ordinary radiators. Electric storage heaters and most heat pump systems work differently, so check your installer's advice for those.

How do you know a radiator needs bleeding?

Put the heating on and give it twenty minutes or so. Then feel each radiator carefully, from the top to the bottom, without leaving your hand on a hot one. The signs of trapped air are:

• A cold or cooler band across the top while the lower part is warm. Air is lighter than water, so it collects at the top and stops hot water filling the radiator.
• Gurgling, bubbling or ticking from the radiators or the pipes when the heating starts.
• A room that takes much longer to warm up than it used to, with nothing else changed.

Air gets into a heating system in small amounts over time, and more of it shows up after the system has been drained, had work done, or sat unused all summer. That is why many people check their radiators in the autumn, when the heating first goes on.

What you need

• A radiator key. The bleed valve is a small valve at one of the top corners of the radiator, with a square peg in the middle. A radiator key fits that peg and is sold in any DIY shop or hardware store. Some newer valves have a slot and open with a flat head screwdriver.
• An old cloth or towel, and a small tub or cup if you want to catch the water. The first drips can be dark, and they stain carpets.
• Your boiler manual, or the manufacturer's website, so you can find the pressure gauge and the filling loop before you start.

How to bleed a radiator, step by step

1. Find the radiators that need it. With the heating on, check each one as above and make a note of any with cold tops.
2. Turn the heating off and let it cool. Switch off at the thermostat or programmer and wait until the radiators are cool to touch. The water can be very hot, and if the pump is running a valve can draw air in instead of letting it out.
3. Hold the cloth under the bleed valve. Fold it so it catches drips from the valve and covers anything below.
4. Open the valve about a quarter turn. Fit the key and turn it slowly anticlockwise. You should hear a hiss as the air escapes. You do not need to open it any further, and you should never unscrew it fully.
5. Close it as soon as water appears. When the hiss stops and a steady trickle of water comes out, turn the key clockwise to close the valve. Firm is enough, as overtightening can damage the seal. Wipe the valve dry so you can spot a drip later.
6. Repeat on the other radiators. A simple order is downstairs first, starting with the radiator furthest from the boiler, then upstairs. Air rises, so if an upstairs radiator still has a cold top at the end, bleed it again.
7. Check the boiler pressure. This is the step people miss, and it is covered below.
8. Turn the heating back on. After half an hour, feel the radiators again. They should now be warm from top to bottom.

Check the boiler pressure afterwards

Most homes with a combi or system boiler have a sealed heating system, with a pressure gauge on the front of the boiler or underneath it. When you bleed a radiator, a little water leaves the system with the air, and the pressure drops. If it drops too far, the boiler stops working to protect itself, which can look like a breakdown.

Worcester Bosch says the gauge should read between 1 and 1.5 bar when the system is cold, usually shown as a green band, and that below 0.5 bar water has been lost and must be replaced. Other makers give similar figures, but your own boiler manual has the exact range for your model.

To top it up, use the filling loop. This is usually a braided silver hose between two valves under the boiler, or on some boilers a built in key or lever. Open the valves slowly, watch the needle rise, and close them when it reaches about 1 to 1.5 bar. Then reset the boiler if it has stopped. Worcester Bosch says topping up once or twice a year is normal. If you need to do it every week, there is a leak or a fault and an engineer should look.

If you have an older open vented system, with a small feed and expansion tank in the loft, the system tops itself up and there is usually no gauge to check.

A note on the boiler itself

Bleeding radiators and using the filling loop are jobs a homeowner can do. Anything that means opening the boiler casing, the flue or any gas pipework is not. The Health and Safety Executive explains that gas work must be done by an engineer on the Gas Safe Register. If you ever smell gas, leave the boiler alone, open the windows and call the gas emergency line on 0800 111 999.

What a cold radiator is telling you

Where a radiator is cold says a lot about the cause. This is a quick way to decide whether it is a job for you or for an engineer.

Cold at the bottom: sludge and powerflushing

Over the years, the water in a heating system picks up rust and debris from the inside of the radiators and pipes. That forms a black sludge, which is heavier than water and settles at the bottom of radiators, where it blocks the flow. Bleeding will not help, and the water from the bleed valve is often dark.

Which? Trusted Traders lists the signs that a system may need a powerflush: cold radiators with only a hot strip at the top, black sludgy water, poorer circulation than the system used to have, and a magnet sticking to copper pipes. It describes a powerflush as a machine pushing water through the system at high speed, followed by a neutraliser and an inhibitor to slow new sludge forming, and says the whole job can take from several hours to a full day. It also notes that a new boiler installation must include a thorough flush of the system, though not always a full powerflush.

The price depends on how many radiators you have and their condition, so ask for a written quote that says what is included. For smaller jobs, such as freeing a stuck valve, a June 2026 survey by Logic4training put the average UK plumber at about £58 an hour. Our guide to what a plumber charges per hour has more on rates and call out charges.

Cold all over: check the valves

A radiator usually has a valve at each bottom corner. One is the control valve, often a thermostatic radiator valve (TRV) with numbers on it. The other, the lockshield, has a plain plastic cap. If the TRV is turned to 0 or the snowflake, the radiator stays cold, so turn it up. A TRV that has sat closed all summer can stick: take the head off, following the maker's instructions, and gently press the small pin underneath a few times. If it will not move, or the lockshield has been closed, a heating engineer can free it and balance the system so every radiator warms evenly.

If the air keeps coming back

Needing to bleed one radiator every week or two is not normal. Either air is getting in, through a small leak or a fault in the system, or gas is forming inside it through corrosion. Both get worse over time and wear the boiler and pump. Combined with a pressure gauge that keeps dropping, it is a clear reason to book a heating engineer rather than keep bleeding.

Regular boiler servicing catches many of these problems early. See how much a boiler service costs, and if the boiler itself has stopped, what to do when your boiler breaks down.

Getting more from your radiators this winter

A radiator full of water, rather than air or sludge, heats the room faster and lets the boiler run for less time. A few other low cost steps help too, and the figures below are from the Energy Saving Trust.

• Use your heating controls. A boiler system should have at least a programmer, a room thermostat and TRVs. The Energy Saving Trust says installing and using heating controls well could save around £100 a year in Great Britain and £110 in Northern Ireland. If you already have a programmer and thermostat, it puts adding TRVs to every radiator at around £370.
• Radiator reflector panels. On radiators fixed to uninsulated outside walls, purpose made reflector panels can save about £18 a year, according to the Energy Saving Trust, from a roll costing around £25. Kitchen foil does not work.
• Keep radiators clear. Sofas and long curtains in front of a radiator trap the heat behind them.

Gas costs 7.97p per kWh on average on the Ofgem price cap from 1 October to 31 December 2026, so small savings add up over a winter. If you are weighing up a bigger change, our guide on whether to get a heat pump or a new boiler compares the costs.

When to call a heating engineer

• A radiator is still cold at the bottom or in the middle after bleeding.
• The air or the pressure drop keeps coming back.
• A bleed valve or radiator valve leaks, or will not open or close.
• Several radiators are cold, or the whole system is slow to heat.
• The boiler shows a fault code you cannot clear by topping up the pressure and resetting it.

A leak that will not stop, from a valve or a pipe, is a different matter: turn off the water and see what counts as a plumbing emergency. And if a carbon monoxide alarm sounds, or people in the house have headaches, dizziness or nausea that ease when they go out, stop using the boiler, open the windows, go outside and follow the HSE carbon monoxide advice.

Before anyone starts work, it is worth a two minute check that they are who they say they are. Our guide on how to check a tradesperson is genuine shows how to look up a Gas Safe registration.

Find a heating engineer near you

Post your heating job free, saying which radiators are cold, where on the radiator, how old the system is and what you have tried, and hear back from a local heating engineer with a price. You pay the business directly, and nothing is taken online.

Or browse local businesses first: heating engineers in Birmingham, Wolverhampton, Coventry, Dudley or Walsall, or plumbers in Birmingham for a leaking valve. For more guides on heating, repairs and costs, see all our home advice.