Standard Kinetic Air Vent / Vacuum Relief Valves close once the line is under pressure. These are usually used in the drip zone at the high points, downstream of the zone valve. They will automatically exhaust large volumes of air from the system when it is being filled and also allow air to re-enter the pipe line when being emptied. When the system is filled, the fluid lifts the float until it closes the orifice. The orifice will remain closed until the system is emptied. Air may enter the valve and displace the fluid while the system is in operation, however, internal pressure will continue to hold the valve closed. The valve will not re-open until the system pressure drops to near atmospheric pressure and the float is no longer buoyed.