How to bleed a radiator

Air collects at the top of radiators over time, especially after the heating has been drained and refilled. Air doesn't carry heat, so the radiator warms up at the bottom but stays cool at the top, and you may hear gurgling or trickling in the pipes.

Bleeding lets that air out. It takes a few minutes per radiator, and it's one of the few heating jobs most people can do themselves.

• A radiator key, sold at most hardware shops and DIY stores. Some newer bleed valves take a flat-head screwdriver instead.
• An old towel or cloth to catch drips, as the water can be dirty enough to stain carpets
• A small container to catch any water that runs out

1. 1Turn the heating off and wait until the radiators are cool. The water inside can be very hot.
2. 2Find the bleed valve: a small square spindle inside a round fitting at one of the top corners of the radiator.
3. 3Hold the cloth under the valve, fit the key and turn it slowly anticlockwise, about a quarter of a turn. You should hear air hissing out. Don't open it further than you need to, as a valve that's unscrewed too far is hard to close while water is coming out.
4. 4When the hissing stops and water starts to come out steadily, turn the key clockwise to close the valve. Firm is enough; overtightening can damage it.
5. 5Wipe up any drips and move on to the next radiator that needs it.
6. 6Check the boiler pressure (see below), then turn the heating back on. Once it has warmed up, the radiator should be evenly hot from top to bottom.

Every radiator you bleed lets a little water out of the system. If you have a combi or system boiler, check the pressure gauge when you've finished. If it has dropped below about 1 bar with the system cold, top it up through the filling loop: our guide to topping up boiler pressure explains how.

If you have a regular boiler with a small feed and expansion tank in the loft, the system normally refills itself from that tank, so there's nothing to top up.

• Cold at the bottom or in the middle: usually sludge, a build-up of rust and debris that settles in radiators over the years. Black or dark brown water from the bleed valve is another sign. A sludge clean or power flush can clear it.
• One radiator stays cold all over: the valve may be stuck or closed. Thermostatic radiator valves can stick shut, particularly after the summer, and the lockshield valve at the other end may have been turned off.
• Some rooms heat up much faster than others: the system probably needs balancing, which means adjusting each radiator's lockshield valve so the hot water is shared out evenly. We can bleed and balance your radiators in one visit.
• Radiators need bleeding again every few weeks: air or gas is getting into the system, which can point to a pump problem, a leak or corrosion inside the radiators.
• Every radiator is cold: the problem is more likely to be the boiler, pump, controls or a valve. See no heating for what the engineer checks.