How To Bleed A Radiator (And When It's More Than Just Trapped Air)

If a radiator is warm at the top but stone cold at the bottom, or takes forever to heat up, trapped air is usually the culprit. Bleeding it is one of the few plumbing jobs you can safely do yourself, and it only takes a few minutes.

Before you start

Turn your heating on for around 15 minutes so you can feel which radiators are underperforming, then turn the system off completely. Bleeding a radiator while the heating is running can force hot water out along with the air, so it’s worth the short wait.

You’ll need a radiator key (a small, inexpensive tool from any hardware shop) and an old cloth or small container to catch any drips.

Step by step

1. Find the bleed valve, a small square fitting usually at the top corner of the radiator.
2. Hold your cloth underneath it and fit the radiator key onto the valve.
3. Turn it slowly, anti-clockwise, about a quarter turn. You’ll hear a hissing sound as trapped air escapes.
4. As soon as the hissing stops and a steady trickle of water appears, close the valve again, clockwise, until it’s snug.
5. Once you’ve bled all the affected radiators, check your boiler pressure gauge and top it up if it’s dropped below the normal range, following your boiler’s instructions.

When it’s more than just air

If you find yourself bleeding the same radiators every few weeks, or several radiators are cold at the bottom no matter how often you bleed them, that’s usually a sign of sludge and debris building up inside the system rather than simple air. A power flush clears this out properly, restoring even heat across every radiator and helping protect the boiler from working harder than it needs to.

Persistent air can also point to a slow leak somewhere in the system, or a radiator valve that isn’t sealing properly. If bleeding isn’t solving the problem for good, it’s worth having it looked at rather than working around it every winter.

Not sure whether your heating needs a quick bleed or something more involved? Get in touch and we’ll talk you through it.