How to bleed a radiator: a step-by-step guide

Bleed a radiator in seven steps

The job takes about ten minutes per radiator, most of it waiting for the system to cool. Nothing here involves opening the boiler or touching the gas supply, so it is safe to do yourself.

1. Turn the heating off and let the radiators go cold. Worcester Bosch's instruction is to "turn off your heating system and wait for the radiators to cool". Vaillant puts a time on it and suggests waiting about an hour so every radiator is cold to the touch.


2. Find the bleed valve. It sits at the top of the radiator, at one end or the other. It is a small round fitting with a square peg in the middle, and that peg is what the bleed key grips.


3. Protect the floor and the wall. Put a towel down and hold a mug under the valve, because water follows the air out. Slide a piece of card behind the radiator if there is wallpaper there.


4. Open the valve slowly. Turn the key anti-clockwise. Vaillant's instruction is that "a quarter to half a turn should be sufficient". You should hear a steady hiss as the trapped air escapes.


5. Close it as soon as water appears. Once the hissing stops and a dribble of water comes out, the air is gone. Turn the key clockwise and nip the valve up firmly by hand, without over-tightening it.


6. Move on to the next radiator in the right order, which is explained below.


7. Check the boiler pressure, then turn the heating back on. Bleeding lets water out, so the gauge will have dropped. Worcester Bosch is explicit: "make sure you top up the pressure on your boiler", because pressure is lost when you bleed. You are aiming for 1 to 1.5 bar on a cold system.

Then let the heating run for twenty minutes and feel each radiator you bled. It should now be warm from top to bottom.

How to top the pressure back up

How do I know a radiator needs bleeding?

The classic sign is a radiator that is cold at the top and warm at the bottom. Worcester Bosch names that pattern as the point at which you need to bleed, and Vaillant agrees: hot at the bottom but cold at the top "can also suggest there is air trapped in the system". Two other things point the same way:

• Gurgling, banging or a boiling sound shortly after the heating comes on, which is air being pushed around the pipework.


• Rooms that never quite get warm after an hour of heating, with the boiler firing normally.

A radiator that is cold all over is usually a different problem, and bleeding will not fix it. That one is covered further down.

What you need before you start

That is the whole kit. The square peg on the valve comes in more than one size, so on an older system try the key on one radiator before committing to the whole house. Have the boiler's pressure gauge in mind too, because you read it at the end. It sits on the front panel or just underneath the boiler, and what the reading should say is a guide in itself.

What order should you bleed radiators in?

Order matters, because air moves upwards through the system as you release it. Vaillant's instruction for a house on more than one level is to "bleed all radiators in the ground floor before heading upstairs if you have a multi-story property". So:

• Ground floor first, working outwards from the radiator furthest from the boiler.


• Then the first floor, again furthest from the boiler first.


• Then anything higher, such as a loft conversion or a top-floor bathroom, which is where the last of the air tends to collect.

Work downwards instead and air released upstairs settles back into radiators you have already done.

Does bleeding a radiator reduce boiler pressure?

Yes. Bleeding lets water as well as air out of a sealed system, so the pressure gauge will read lower afterwards. That is normal and expected, not a fault. Worcester Bosch tells homeowners to top the pressure up after bleeding for exactly this reason, and Vaillant notes that releasing air may leave you "below 1 bar" with a low-pressure warning showing.

Check the gauge with the system cold and the heating off. Worcester Bosch's figure for a cool system is "between 1 and 1.5 bar on the pressure gauge". Under that, topping the pressure back up takes a couple of minutes with the filling loop.

Bleeding also works in the opposite direction, as a way of dropping pressure that is too high, but Ideal Heating's own advice is that "while bleeding a radiator does release pressure, it's very slow, so we recommend you try one of the other methods first". If your gauge reads high, use how to reduce boiler pressure.

One sanity check afterwards: Worcester Bosch treats a system needing a top-up "once or twice a year" as normal, and says you should contact your installer if you are repressurising much more often than that. If you are topping up every fortnight, take that advice: something is losing water.

How often should you bleed your radiators?

Once a year is enough for most homes, and the sensible time is early autumn, before the heating goes on properly. Air collects over the summer while the system sits idle, and the first cold week is the worst moment to find the upstairs radiators half working. In September or October, run the heating for half an hour, feel every radiator, bleed the ones with cold tops, then check the pressure. After that, only bleed mid-winter if a radiator underperforms or you hear gurgling. If you find yourself bleeding the same radiator every few weeks, air is getting in from somewhere, which is a system fault rather than a maintenance job.

Bleeding new radiators, or after work on the system

Any time the system has been drained and refilled, air goes in with the fresh water. A new radiator, a swapped valve or a repaired leak will almost always need bleeding twice: once when the system is first refilled, then again a day or two later, after the remaining air has worked its way up to the high points.

What does bleeding a radiator actually mean?

Bleeding a radiator removes the air pockets that get in the way of hot water circulating around your central heating system. A sealed system should hold water and nothing else. Air works its way in over time and, being lighter than water, floats to the highest point it can reach, which in a radiator is the top few inches. Worcester Bosch puts it plainly: air building up in a radiator affects how well it works, leaving some parts of it colder than others.

When bleeding does not fix it

If the radiator is still cold after bleeding, the problem is not air. The usual causes:

• Cold at the bottom, warm at the top. That is the opposite pattern and usually means sludge: corroded metal settled as a black deposit in the bottom of the radiator, blocking the flow. Vaillant says that if radiators still feel cold after bleeding, the system may need flushing, and that "this must be done by a heating engineer".


• The whole radiator is cold. Check the valves at both ends are open. A thermostatic valve can seize with the pin stuck down, especially one left on its lowest setting all summer.


• Every radiator is cold. That points at the boiler, the pump or the controls rather than one radiator. Start with no heating.


• Nothing comes out when you open the valve. No hiss and no water means there was no air to release, or the valve is blocked. Close it and look elsewhere.

Safety, and when to call an engineer

Bleeding a radiator is a homeowner job. Draining, flushing, moving pipework or anything involving the boiler itself is not. Only a Gas Safe registered engineer may work on a gas appliance, and that includes the boiler your radiators are connected to.

Stop and get help if the valve or radiator body leaks, if the water comes out gritty, if the boiler locks out with a fault code afterwards, or if the pressure will not hold once topped up.

If you ever smell gas, do not touch any switch and do not try to diagnose it yourself. Call the National Gas Emergency Service on 0800 111 999 straight away.

If the repair is bigger than the radiator

Bleeding costs nothing, but the fault sitting behind a cold radiator sometimes needs an engineer. Smart Plan's boiler and central heating cover is a service plan, not insurance. If the Boiler & Central Heating module is on your cover sheet, it covers breakdowns of the boiler and central heating equipment listed on that sheet, with parts and labour included up to the cover limit: £500 per 12-month period if your boiler is under seven years old, £200 if it is over seven. Repair or replacement of the radiators themselves, and work caused by sludge or scale deposits, including a system flush, are not covered (cover plan terms, section 2 exclusion (e) and general exclusion 22). The work is done by a national network of thousands of vetted engineers and tradespeople, including Gas Safe registered engineers for work on the boiler itself. Call-out hours are Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days. A £95 call-out fee applies.

If a radiator or the system behind it is playing up today, tell us what it is doing and we will come back to you, or call 0333 772 6247.

Bleeding radiators: FAQs

Should the heating be on or off when I bleed a radiator?

Off, and cold. Worcester Bosch says to turn the heating system off and wait for the radiators to cool. Vaillant suggests roughly an hour so that every radiator is genuinely cold. Bleeding a hot, pressurised radiator risks scalding water coming out of the valve rather than a dribble.

How long should I hold the bleed valve open?

Until the hissing stops and water appears, usually a matter of seconds. Leaving it open longer only loses pressure you then have to put back.

Which radiator do I bleed first?

The ground floor before upstairs, and within each floor the radiator furthest from the boiler first. Vaillant gives the ground-floor-first rule directly.

Why does the water come out brown or black?

That is magnetite, the black deposit that forms as metal inside the system corrodes. A little discolouration is common in an older system. Thick, gritty black water suggests the system needs flushing, which Vaillant says must be done by a heating engineer.

Do I have to bleed every radiator in the house?

No, only the ones with symptoms. Feel each radiator after twenty minutes of heating and bleed any that are cold at the top.

The pressure dropped after I bled the radiators. Have I broken something?

No, that is what is supposed to happen. You let water out of a sealed system. Top it back up to 1 to 1.5 bar with the boiler cold and the heating off.

Related guides and sources

• How to increase boiler pressure, for after you have bled the system.


• How to reduce boiler pressure, if the gauge reads too high.


• What should boiler pressure be? The manufacturer figures.


• No heating, for when it is all of them.


• Boiler and central heating cover from Smart Plan.

Sources used on this page: Worcester Bosch on how to bleed a radiator, Worcester Bosch on topping up heating system pressure, Vaillant on how to bleed a radiator, Vaillant on cold radiators and Ideal Heating on high boiler pressure.