How does a bleed valve work?

 
     
     
 Shop for Radiator Keys    
     
     
 Brass bleed valve 

Radiator bleed valves have many names, including air release valve, bleeder valve and bleed nipple.

 
     
 Bleed valve on cast iron radiator 

The purpose of the bleed valve is to release air that sometimes gets trapped inside radiators, making them less efficient.

 
     
 Bleed plug and bleed screws 

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 with screwdriver adjustment 

Bleed screws on most modern radiators also have a slot in the head so that they can be loosened and tightened with a screwdriver.

 
     
 External hexagonal bleed plug 

Some plugs have external hexagonal heads that can be turned with an ordinary spanner of the right size, or an adjustable spanner.

 
     
 Machined square end of radiator valve 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.

 
     
 How to use a bleed key 

Turning the bleed screw anti-clockwise with a radiator bleed key releases any air. Turning it clockwise tightens it up again.

 
     
 Bleeding a radiator 

For a step-by-step guide to the bleeding procedure, see: How to bleed a radiator

 
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