How to Bleed a Radiator (UK Step-by-Step)

If a radiator is warm at the bottom but stubbornly cold at the top, air has collected inside it and is stopping hot water from circulating properly. Releasing that air — "bleeding" the radiator — is one of the few central-heating jobs that's genuinely safe for any homeowner to do. It needs no gas work, no tools beyond a cheap key, and about five minutes per radiator.

What you'll need

• A radiator bleed key (a small square key, a couple of pounds from any DIY shop). Many modern radiators take a flat-head screwdriver instead.
• An old cloth or towel, and a small cup or jug to catch drips.
• A few minutes, and ideally a second person to watch the boiler pressure gauge if your system is sealed.

How to tell which radiators need bleeding

With the heating on and the radiators hot, carefully feel each one from bottom to top. The ones that are warm low down but cool or cold across the top have trapped air. Gurgling sounds when the heating fires up are another giveaway. Note which radiators need doing before you start.

Step-by-step: how to bleed a radiator

1. Turn the heating off and let the radiators cool. Never bleed a hot radiator — the water inside can scald. Switch off your heating and give it 20–30 minutes until the radiators are cool to the touch. Cooling also lets the trapped air rise to the top of each radiator where the bleed valve is.
2. Find the bleed valve. It's the small square nipple at the top corner of one end of the radiator, usually recessed in a round housing.
3. Protect the floor and wall. Hold a cloth under the valve and have your cup ready — a little dirty water will come out.
4. Open the valve slowly. Fit the bleed key (or screwdriver) and turn it anticlockwise, no more than a quarter to half a turn. You'll hear a hiss as the trapped air escapes. Don't fully unscrew the valve.
5. Wait for water. Keep the key in place until the hissing stops and a steady trickle of water appears instead of air. That means all the air is out.
6. Close the valve. Turn the key clockwise to close it again, firmly but without overtightening — you don't want to strip the valve. Wipe up any spills.
7. Repeat for each affected radiator. If you have several, work from the radiator nearest the boiler outwards, and on the lowest floor first if your home has more than one level.

Check and top up your boiler pressure afterwards

Bleeding lets water — and therefore pressure — out of a sealed system, so the final step is to check the boiler. Look at the pressure gauge on the front of the boiler. When the system is cold it should read roughly 1 to 1.5 bar (it rises towards about 2 bar once the heating is hot). If bleeding has dropped it below about 1 bar, you'll need to top it back up.

Topping up is also a homeowner-safe job. With the boiler off, open the filling loop — the small braided hose with one or two valves beneath the boiler — slowly, watch the gauge climb to around 1.2–1.5 bar, then close the valves again fully. Our step-by-step guide to repressurising a boiler walks through it, and our low boiler pressure guide covers what to do if the pressure keeps falling.

Once the pressure is back in range, turn the heating on and check that the radiators you bled now heat up evenly all the way across.

How often should you bleed your radiators?

For most homes, once a year is plenty — autumn, just before the heating season starts, is the natural time. Beyond that, only bleed a radiator when you notice cold spots at the top or hear gurgling. There's no benefit to bleeding radiators that are already heating evenly, and doing it needlessly just drops your boiler pressure for no reason.

When bleeding doesn't fix it

If a radiator is cold at the bottom rather than the top, the problem usually isn't air — it's a build-up of sludge (corrosion debris) settling at the base and blocking the flow. Bleeding won't shift that. The usual fix is a power flush or a chemical flush, which should be carried out by a heating engineer, not attempted yourself.

Also call a professional if:

• You bleed a radiator and it's cold again within days — air is getting back in, which can point to a fault that needs investigating.
• The boiler keeps losing pressure even after topping up — there may be a leak somewhere in the system.
• You see any sign of a leak from the boiler itself, or you smell gas. For a suspected gas leak, leave the property and call the National Gas Emergency line on 0800 111 999.

Recurring sludge and breakdown repairs are exactly the kind of thing a boiler cover plan can take the cost and hassle out of. If you're weighing that up, our guide to what boiler cover includes is a good starting point, and you can compare boiler cover plans across our panel.