How do you bleed a radiator?

Bleeding a radiator means letting out air that has collected at the top of it, so hot water can fill the whole panel again. If a radiator is warm at the bottom but cold across the top, trapped air is almost always the reason, and it is one of the few heating jobs most people can do themselves in a few minutes with a radiator key and an old towel. Late September is exactly when this crops up, because the heating goes on for the first time in months and any air that has gathered over the summer makes itself known.

Here is how to do it. Turn the heating on and let the radiators warm up fully, so you can feel which ones are cold at the top. Then switch the heating off and give it a little while to cool. Bleeding a hot radiator works, but the water that comes out can be very hot and the pump can pull more air in while it is running, so it is better done with the system off. Find the bleed valve, a small square fitting at one of the top corners of the radiator. Hold a cloth underneath it, fit the radiator key and turn it slowly anticlockwise, no more than a quarter or half turn. You will hear a hiss as the air escapes. As soon as water starts to appear, turn the key back clockwise to close it. Do not force it tight; snug is enough. Work your way round any other cold radiators, starting downstairs and finishing upstairs.

Then check the boiler pressure. This is the step people miss. Every time you bleed a radiator you let a little water out along with the air, and on a sealed system that means the pressure drops. Have a look at the gauge on the front of the boiler or underneath it. If it has fallen below the green zone, top it back up using the filling loop, following the instructions in your boiler manual, and bring it back up to where it normally sits when cold. If you have an older open vented system with a tank in the loft, it will refill itself and you can skip this part. Turn the heating back on, let it run, and check the radiators are now warming evenly from top to bottom.

A few things are worth knowing. If a radiator is cold at the bottom but warm at the top, bleeding will not help. That pattern points to sludge settling in the bottom of the panel, which is a different problem and usually calls for a clean of the system rather than a bleed. If one radiator is completely cold while the others are fine, check the valves at each end are open before anything else, as a decorator or a curious child may have turned one off. And if the bleed valve is seized, snapped or leaking when you close it, stop there rather than forcing it. Old valves in West London houses can be brittle, and a broken one is much easier for us to replace than to deal with water on a carpet.

Air getting into a system once in a while is normal, especially after work has been done on the pipes or when the heating has sat unused all summer. Needing to bleed the same radiators every few weeks is not. Regular air usually means one of a few things: a small leak letting air in as water gets out, a pump problem drawing air through the system, or corrosion inside the radiators producing hydrogen gas. You can sometimes tell which, because gas from corrosion can make the water that comes out look dark or grey. Here in West London the hard water adds to the wear on valves and seals over time, so we see plenty of systems where a slow air problem has been quietly building for years. Left alone, that corrosion becomes the black magnetite sludge that blocks radiators and wears out boilers.

If you have bled your radiators and they are still not heating properly, or you find yourself doing it over and over, that is the point to get someone in. We can check the pressure, the pump and the expansion vessel, look at the condition of the water in your system, and tell you plainly whether it needs a simple fix, a magnetic filter, or a proper clean. We are Gas Safe registered, Which? Trusted Traders and accredited Worcester Bosch and Vaillant installers, and we cover Brentford, Chiswick, Ealing, Hammersmith, Fulham, Acton, Richmond, Twickenham, Kew, Chelsea and the streets around them. If your radiators are not behaving this autumn, get in touch and we will take a look.