An Automatic Bottle Vent is designed to discharge air from and admit air into a water pipe system. They are commonly used on central heating systems to overcome the problem of air lock. They should be at the top of a short section of pipe at any high point in the pipework of the system.
The automatic air vent consists of a float connected to a valve. As the air exceeds a given level inside the float chamber the float drops opening the valve and releasing the air trapped within the chamber. When the air is released it is replaced by water which in turn moves the float back into the up position sealing the valve.