Steam Radiator Air Valves Used in a Steam Boiler System
The radiator air vent or air valve (also called a steam vent or steam valve) is uniquely used in one-pipe steam boiler systems and is found on the opposite side of the supply valve on a radiator.
When the steam boiler is not in a heating cycle, the vent opens the radiator to room temperature air. During a heating cycle the air in the radiator must be replaced with steam, so the heated steam flows into the radiator and pushes the contained air out. As this occurs, the air vent (actually a heat sensitive valve) will heat up and close, thereby containing the steam in the radiator and preventing it from escaping into the room. This “breathing” provides the characteristic pipe banging and hissing sounds of a one-pipe steam boiler system. You will also find a variation of this valve in the supply lines of one-pipe systems and the supply and return lines of two-pipe steam boiler systems.
The radiator air valve comes in different sizes to allow different amounts of air to escape from at different rates. This is necessary for the system to be balanced. Balancing the system involves adjusting the rate of steam flow provided to the radiators both with the supply valve and venting air valve, so that an individual radiator produces heat appropriate for the room it is heating.
Although not at all intuitive, the names given the different sizes of air valves, from smallest to largest, are as follows: #4, #5, #6, C, D and #1.
Common applications of these different valve sizes include:
- #4: Used on radiators in room with thermostat and on radiators affecting the thermostat;
- #5: Used on radiators near the boiler & in warm rooms;
- #6: Used on radiators further from the boiler & in cold rooms (e.g., 2nd floor);
- C: Used on radiators furthest from the boiler (e.g., 3rd floor);
- D: Used on radiators needing a lot of venting (e.g., long branches or extra large radiators);
- #1: Used at the end of steam piping mains.