How to bleed a radiator properly

Tools you'll need

• A radiator bleed key, sometimes called a square key, in the standard size that fits most UK radiators
• An 8mm or 10mm clock key for older radiators with a square recessed spindle rather than the common vent
• An old cloth or towel to catch drips, plus a small bowl to hold under the valve
• A flat screwdriver for radiators with a slotted bleed screw instead of a square one
• Your boiler's pressure gauge in view, or a phone torch if it's tucked in a cupboard

Know your valve types before you start

Every radiator has two valves at the bottom, an inlet and a lockshield, and a bleed valve up at the top corner. The inlet valve is the one you turn day to day, often a thermostatic radiator valve, or TRV, with a numbered dial. The lockshield sits at the opposite end under a plastic cap and is set once when the system is balanced, so leave it alone unless you're deliberately rebalancing.

The bleed valve itself comes in two forms. A manual bleed valve needs the key and has to be opened and closed by hand every time. An automatic air vent, more common on modern pressurised systems and always fitted at the highest point of the pipework, releases trapped air on its own via a small float mechanism, so you'll rarely need to touch it, though it's worth checking it isn't weeping.

The correct order to bleed radiators in a whole house

Bleeding radiators in the wrong order just pushes air from one to another. Work downstairs first, starting with the radiator furthest from the boiler and finishing with the one nearest to it, then repeat the same furthest-to-nearest pattern upstairs, finishing on the top floor last. This follows the natural direction air migrates as it rises through the system, so you're always chasing it upwards and out.

On a system with an open-vented cold water tank in the loft rather than a sealed system, this order matters even more, because air can re-enter from the feed and expansion tank while you're working, so it pays to do the whole round in one session rather than one radiator a day.

Bleeding the radiator, step by step

Turn the heating on and let radiators warm up for ten minutes, so you can clearly feel which ones are cold at the top. Then turn the heating off before you actually bleed anything; releasing air from a hot pressurised system can spit scalding water.

Hold your cloth and bowl under the valve, fit the key, and turn it a quarter turn anticlockwise. You'll hear a hiss as air escapes. As soon as that hiss stops and water starts to dribble out steadily, close the valve clockwise, snug but not overtightened. Move to the next radiator in your planned order.

Repressurising afterwards

Bleeding a sealed system loses a small amount of water each time, which drops boiler pressure. Once you've finished the whole round, check the gauge on your boiler. Most domestic boilers want to sit between 1.0 and 1.5 bar when cold, and many boilers show a green band on the dial for reference.

If it's dropped below 1.0 bar, use the filling loop, usually a braided flexible hose with two valves under the boiler, to top it back up slowly until the needle sits in the green zone, then close both valves fully. If your system is open-vented with a loft tank rather than sealed, it will refill itself automatically via the ball valve and there's no filling loop to worry about.

When bleeding a radiator won't fix the problem

• The radiator is cold at the bottom rather than the top, which points to sludge or magnetite settling in the base, not air
• You're bleeding the same radiators every few weeks, which suggests a slow leak or a failing automatic air vent letting air back in
• A whole zone or floor stays cold no matter how much you bleed, which points to a stuck zone valve, failed pump, or thermostat fault rather than trapped air
• The system won't hold pressure after refilling, which means there's a leak somewhere that needs finding, not more bleeding

Frequently asked questions

Is it normal to need to bleed radiators every autumn?

A small amount of air after months of not running the heating is normal, so one round of bleeding when you switch the system back on is nothing to worry about. Needing to repeat it every few weeks through the winter usually points to a leak or a failing automatic air vent.

Can I bleed a radiator with the heating switched on?

You can, but it's safer to turn the heating off first and let the system cool a little. Bleeding a radiator full of hot pressurised water can cause it to spit, so most engineers turn the system off, or at least off at that radiator's TRV, before opening the bleed valve.

What if water won't come out of the bleed valve at all?

That can mean the valve is seized, the radiator is genuinely full with no air pocket, or there's a blockage. Don't force a seized valve with pliers, as that risks snapping the spindle, and it's better to get an engineer to replace it.

Do I need to bleed both TRV and non-TRV radiators the same way?

Yes, the bleed valve itself works the same regardless of whether the inlet is a TRV or a plain valve. The TRV only controls how much hot water flows in based on room temperature, it doesn't affect how you release trapped air at the top.

What if bleeding doesn't stop the noise or the cold patch?

If you've bled the whole system in the correct order and topped the pressure back up, but a radiator is still cold or the system is still noisy, that's a sign of something bleeding won't fix, and it's worth getting a Gas Safe registered engineer to check for sludge, pump issues or a circulation fault.