Selection Criteria for Air Vent Valves（https://www.huibo-valve.com/product/air-vent-valve/） Choosing the right Air Vent Valve involves several considerations. Material, size, shape and the right positioning in the system are all important factors. Stainless steel, brass and plastic are all common materials, with specific choice often depending on the nature of the system in which the valve will be placed. An Air Vent Valve should be installed wherever an air bubble is likely to form. In a heating system, this tends to be at the highest point or any point where the pipe size increases suddenly. In terms of size, this again will depend on the size of the system. Larger systems will likely require a larger valve. Importantly, the valve must be capable of withstanding the maximum pressure the system can exert.