Automatic bleeding valve

If air is trapped within the boiler this may cause pump cavitation or boiling and overheating within the heat exchanger.

In those cases, individual bleed screws on each radiator, or high pipe run, are opened manually.

The valves are installed at a high point of the system, where trapped air collects.

A leaking valve commonly shows a dribble of water from the air vent, or stains of rust or limescale.

[citation needed] Construction is of the usual cast brass housing for plumbing parts, with a threaded connection to the pipework at the base.

The top has a small outlet, usually hidden beneath a plastic dust cap.

This lever mechanism may be simply, almost crudely, made of a pressed steel strip which acts as a bending hinge.

The float linkage may have multiple lever arms, in order to give greater mechanical advantage on the valve.

A dry valve acts as an air vent until a dribble of water passes through it and swells the cartridge.