Air, or other gas, may collect within plumbing. For water delivery systems to taps and basins, this air is flushed through with the water flow and does not cause a problem. In a closed heating system though, it has no other means of escape and builds up. An air bubble trapped within a radiator means that no hot water circulates in the upper part and so the heating power of the radiator is reduced. If air is trapped within the boiler this may cause pump cavitation or boiling and overheating within the heat exchanger.
Automatic valves are used to release trapped air as it collects. They are not a substitute for bleeding a system manually when it is first filled during commissioning, nor for remedial bleeding if the system becomes choked with trapped air. 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. They may be installed at the highest point in a system, but in a system with pumped circulation this is rarely necessary. Instead they are placed at some local high point, such as the highest point within a utility cupboard or boiler room, even though this is in a basement. European domestic gas boilers often include such a valve within the boiler casing itself.
A working system should not generate further trapped gas. Air may be drawn in if there is a small leak, or dissolved air in make-up water may come out of solution, but this generally indicates a system leak if new water is needing to be added. The most likely cause of continual gas bleeding is hydrogen, rather than air. If there is corrosion or rusting of internal metal components, this releases hydrogen gas. The appropriate cure here though is the use of a chemical inhibitor, rather than continual bleeding.