How does a bleed valve work?
Radiator bleed valves have many names, including air release valve, bleeder valve and bleed nipple.
The purpose of the bleed valve is to release air that sometimes gets trapped inside radiators, making them less efficient.
The valve comprises a plug that screws into a radiator tapping at the top of the radiator, and an adjustable bleed screw, with a 5mm square head, in its centre.
The plug, which usually has a half-inch British Standard Pipe (BSP) thread, is screwed into one of the two top tappings - internally threaded holes at each corner of the radiator.
Bleed screws on most modern radiators also have a slot in the head so that they can be loosened and tightened with a screwdriver.
Some plugs have external hexagonal heads that can be turned with an ordinary spanner of the right size, or an adjustable spanner.
Others have a square recess, also known as a square section, which is fitted or removed with the square end of some multi-purpose radiator keys.
Turning the bleed screw anti-clockwise with a radiator bleed key releases any air. Turning it clockwise tightens it up again.
For a step-by-step guide to the bleeding procedure, see: