Why radiators need bleeding
A radiator should feel evenly hot from top to bottom once the heating has been running for a while. Air and other gases that get into a heating system rise to the highest point they can reach, which is usually the top of a radiator. The gas pocket stops hot water filling the radiator, so the top stays cool while the bottom gets warm. Bleeding lets that gas out through a small valve so water can take its place.
Air gets in when a system is drained and refilled, when a new radiator or boiler is fitted, and in small amounts from fresh water topping up the system over time. Corrosion inside steel radiators also produces gas, mostly hydrogen, which collects in the same way. A little air after the summer is normal. Air that comes back every week or two is not, and we cover that further down.
If a radiator is warm at the top but cold at the bottom, that is not air. It usually points to sludge, and bleeding will not help. Our guide to radiators cold at the bottom explains the fix.
What you need to bleed a radiator
- A radiator key or a flat-head screwdriver. Most older radiators have a square pin in the centre of the bleed valve that needs a standard radiator key, sold at any DIY shop for a couple of pounds. Many newer radiators have a slotted valve that takes a flat-head screwdriver, and British Gas notes some can be opened with an Allen key.
- A cloth or old towel. Hold it under the valve to catch drips. Water from inside a heating system is often dark and can stain carpets and walls.
- A small container. Handy if the valve is awkward to reach.
- Access to the boiler pressure gauge, and the filling loop or filling key, if you have a combi or system boiler. You will need them at the end.
The bleed valve is at one of the top corners of the radiator, usually on the opposite side to the pipe that feeds it. On towel rails it is normally on the top of one of the upright tubes. If you cannot find it, look for a small round fitting with a square pin or slot, and check it has not been painted over.
Which order to bleed radiators in
Start with the radiator furthest from the boiler and work back towards it. Uswitch's guide to bleeding radiators gives the same advice. In a house with more than one floor, many engineers bleed the downstairs radiators first and finish upstairs, because air that is disturbed lower down tends to move up through the pipes and collect in the upstairs radiators.
You do not have to bleed every radiator every time. Run the heating first, then feel each radiator carefully and note which ones are cold at the top. Those are the ones to bleed. If you have recently drained the system or had work done, bleed all of them, following the same order.
Bleeding a radiator in eight steps
- Find the cold spots. Turn the heating on and let the radiators warm up. Worcester Bosch's homeowner advice is to check all the radiators at this point so you know which ones need bleeding.
- Turn the heating off. Switch the heating off at the programmer or thermostat, and wait until the radiators are cool enough to touch.
- Get ready. Put the cloth under the bleed valve and have the container to hand.
- Open the valve slowly. Fit the key or screwdriver and turn it anti-clockwise, about a quarter to half a turn. Do not unscrew it any further. You should hear a hiss as the air escapes.
- Wait for water. Keep the valve open while the air hisses out. When the hissing stops and water starts to come out in a steady trickle, the radiator is full.
- Close the valve. Turn it clockwise until it is snug. It only needs to be firm, not forced, or you can damage the seat.
- Repeat on the next radiator. Work through the list in the order above.
- Check the pressure and turn the heating back on. Look at the boiler gauge, top it up if needed (see below), then run the heating and check the bled radiators are now hot all over.
Never open the bleed valve on a hot radiator. The water inside a running system can be hot enough to scald, and it comes out under pressure on a sealed system.
Can you bleed a radiator when the heating is on?
No, you should not. Turn the heating off and let the radiators cool first. British Gas calls this very important, because the water and air inside a running radiator become hot enough to scald you when they are released. There is a practical reason too: with the pump off, the water is still and the air settles at the top of the radiator, right by the bleed valve, so more of it comes out.
Run the heating before you start, as in step one, to see which radiators have cold tops, then turn it off and let them cool before you touch the valves.
How long it takes to bleed a radiator
Each radiator usually takes less than a minute once the valve is open, and often only 20 to 30 seconds. Most of the time goes on letting the radiators cool. For a typical three bedroom house with eight to ten radiators, allow around an hour in total, including the cooling time and checking the pressure at the end.
A radiator that hisses for much longer than a minute has a lot of air in it. That is common after a drain-down or when a radiator has not been bled for years, but it is also worth noting which radiator it was. If the same one fills with air again soon, see the section on air that keeps coming back.
Topping up boiler pressure after bleeding radiators
Yes, on most modern systems you need to check and often top up the pressure after bleeding. Worcester Bosch says it plainly in its advice: boiler pressure can be lost when bleeding radiators, so top it up once you have finished. Whether you need to depends on the type of system you have.
Sealed systems (combi and system boilers)
If your boiler has a pressure gauge on the front or a pressure reading on the display, the heating runs as a sealed system. Every bit of air you let out leaves a small gap, and the pressure drops. Bleeding one or two radiators may only lower it slightly. Bleeding a whole house can drop it enough for the boiler to lock out with a low pressure code, such as Vaillant F22 or Worcester Bosch CE 207.
With the system cold, the gauge should read around 1 to 1.5 bar on most boilers. Worcester Bosch and Ideal both give 1 to 1.5 bar cold as the normal range for their boilers, while Baxi gives 1 to 2 bar. If it is lower, top it up with the filling loop or filling key. Our guide to repressurising a boiler takes you through it.
Open-vent systems with a loft tank
Older regular boilers with a feed and expansion tank in the loft do not have a pressure gauge to top up. The small tank refills the system automatically through a ball valve, the same way a toilet cistern refills. After bleeding, it is still worth checking the loft tank has water in it and that the ball valve is not stuck.
If air keeps coming back
Needing to bleed radiators once or twice a year is normal. Needing to bleed the same radiator every week or two means something is letting air in or making gas inside the system. The common causes are:
- Corrosion. Rust inside steel radiators produces gas and black sludge. Dark, smelly water when you bleed is a strong sign. The system needs cleaning and a fresh dose of corrosion inhibitor. Our guide to powerflush costs covers the cleaning options.
- A small leak. On a sealed system, a leak lets water out, and when it is topped up again, fresh water brings more dissolved air in. If you are also topping up the pressure often, read our guide to a boiler leaking water.
- A pump or pipework problem. A pump set too high or fitted in the wrong position can pull air in through the vent on some open-vent systems, or push water up the vent pipe into the loft tank. This needs a heating engineer to look at the layout.
- An air vent that has stopped working. Some systems have automatic air vents on the pipework or on the boiler. If one is stuck, air builds up elsewhere.
If only one radiator is cold and bleeding makes no difference, the fault is more likely a stuck valve than air. See our guide to one radiator not heating.
When to call a heating engineer
Bleeding radiators and topping up the pressure are jobs most people can do. Call a heating engineer if the bleed valve is seized or snaps, if water keeps coming out after the valve is closed, if the pressure drops again within days of topping it up, or if the boiler still shows a fault after the pressure is right. Anything inside the boiler casing, including the expansion vessel, pump and pressure relief valve, is a job for a Gas Safe registered engineer.
Never remove the boiler casing yourself. If you smell gas at any point, open the windows, turn off the gas at the meter if you can, and call the National Gas Emergency Service on 0800 111 999.