As described in Steam Radiator Air Valves, the air valve or air vent on a radiator allows air in the cooled radiator to be evacuated by incoming steam at the start of a heating cycle. The hissing sound of the air and steam venting from the valve is a sign of normal operation but should stop once the radiator comes up to temperature and the valve closes, thereby retaining steam in the radiator.
One-pipe systems require pressurized steam and gravity flowing condensate (cooled steam returning from the radiator) share the same pipes. That means the supply valve must always be fully open and the radiator pitched toward the supply valve so the condensate can drain back. A correct pitch is approximately 1" for every 10 feet toward the supply pipe / supply valve. If these conditions are not met, problems can arise. In addition, the radiator air valve can be a little fussy, so let's look at some potential problems you can experience and how to repair them.