How to Bleed a Radiator and Fix Cold Spots

If your radiators are cold at the top, slow to warm up, or noisy, trapped air is the most likely culprit, and letting it out is one of the easiest home jobs there is. This guide shows you how to bleed radiators safely, how to top up the pressure on a sealed system, and how to read different cold-spot patterns so you know what to try next. It is written for hot-water central heating with a boiler; electric radiators and storage heaters are different and are not covered. General information, not professional advice: gas boilers must only be opened or repaired by a suitably qualified engineer.

Why Radiators Need Bleeding

Central heating systems pump hot water through pipes and radiators. Over time, small amounts of air and gas collect in the system. Air gets in when the system is drained or refilled, through tiny leaks, and from gas created by slow corrosion inside steel radiators. Because air is lighter than water, it rises to the top of each radiator and stays there.

A pocket of air at the top means hot water cannot fill the whole radiator, so the top stays cold while the bottom gets hot. The system also has to work harder to heat the room, which wastes energy. Bleeding simply means opening a small valve to let that air out until water appears.

• The radiator is cold at the top and warm at the bottom.
• Rooms take much longer to warm up than they used to.
• You hear gurgling, trickling or banging sounds when the heating comes on.
• The heating was recently drained, refilled or worked on.
• It is the start of the heating season and nobody has bled them for a year.

What You Need Before You Start

You need very little equipment. Gather it before you start so you are not hunting for a key with water dripping onto the carpet.

The bleed valve is usually at one of the top corners of the radiator, on the end opposite the pipes or above one of them. It looks like a small round fitting with a square pin or a slot in the middle. Towel rails often have one at the very top.