Automatically vents air from system whilst filling.
AAV’s have to have their caps left unscrewed for them to be able to let air out.
They are used at high points in pipework where air could collect. Sometimes, for example in or near boilers, they MUST be left permanently open.
When they get old they are inclined to scale up and leak water out, in which case they should be replaced.
Note that when the cap is left undone they will let air in if the water is drained from the connected pipe, which can be useful for draining a system.
Courtesy of ChrisR
