The automatic air vent is a pressure safety mechanism in the central heating unit. They work by using floatation to release an accumulation of air. As water flows through the heating/cooling system, a float within the automatic air vent opens or closes depending on the water level. If the system is full of water, the float will be pushed to the top and the vent will be closed until the water level drops.
We recommend that all parts are fitted by a fully qualified gas safe certified plumber.