<P> The valves are normally installed with the first installation of a system, then remain in place for the life of the system . They are generally reliable and require no maintenance . If they do eventually fail, they are replaced rather than repaired . A leaking valve commonly shows a dribble of water from the air vent, or stains of rust or limescale . </P> <P> 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 . Internally there is a large hollow body, normally filled with water, and containing a float valve . The valve must always be installed vertically, usually at the top of a short vertical pipe . </P> <P> If the valve body is filled with water, the float moves upwards and closes the valve . If air collects in the valve body, the waterline lowers, as does the float, and the valve opens to release it . To give more sealing force on the valve, the float generally works the valve through a lever . A greater movement of the float gives a shorter, but more forceful, movement of the valve . This lever mechanism may be simply, almost crudely, made of a pressed steel strip which acts as a bending hinge . </P> <P> Larger valves are available for larger systems and these may be more sophisticated . They usually have a manual bleed valve or screw . They are also dismantlable for cleaning or repair . The float linkage may have multiple lever arms, in order to give greater mechanical advantage on the valve . </P>

How does a float type air vent work
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