The function of the radiator air valve or air vent is to provide a means for air to be driven out of the radiator as steam enters. A quality radiator air valve will provide quieter and faster heating radiators. Air valves are differentiated by orifice size, and selecting a radiator valve is determined by the radiator's distance from the boiler. A main vent with a large orifice should be installed closest to the boiler. A radiator located on the 2nd floor of a home should have an air valve with a smaller orifice. Air valves are available in straight or angle configurations and can be serviced simply by unscrewing the vent cap.