Steam Radiator Air Vents and Air Valves

A radiator in an unfurnished room
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The radiator air vent, or air valve (also called a steam vent or steam valve), is used only in one-pipe steam boiler systems. It is usually located at the end of the radiator opposite the supply pipe, about halfway up or higher towards the top. Many air vents are bullet-shaped, but they can be many different shapes and sizes, or get covered to make them more visually appealing. Don't confuse the air vent with the control valve (supply valve), which is connected to the supply pipe. Unlike the control valve, the air vent does not have a knob because it works completely without human intervention (when it works, that is).  

How a Steam Radiator Air Valve Works

When the steam boiler is not in a heating cycle, the vent is open, allowing ambient air to fill the radiator. During a heating cycle, the air in the radiator must be replaced with steam. As steam flows into the radiator it pushes out the room-temperature air through the vent. As this occurs, the air vent, which is a heat-sensitive valve, heats up and closes, thereby containing the steam in the radiator and preventing it from escaping into the room. This “breathing” is responsible for the characteristic hissing sounds of a one-pipe steam boiler system. You may 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.

Steam Radiator Air Valve Types and Sizes

The radiator air valve comes in different sizes to provide different rates of airflow. This is necessary for the system to be balanced. Balancing the system involves adjusting the amount of steam provided to the radiators, through both the supply valve and venting air valve, so that an individual radiator produces heat appropriate for the room it serves.

Although not at all intuitive, the names given the different sizes of air valves, from smallest to largest, are #4, #5, #6, C, D and #1.

Common applications of the different valve sizes include:

  • #4: Used on radiators in rooms with thermostats and on radiators affecting a thermostat
  • #5: Used on radiators near the boiler and in warm rooms
  • #6: Used on radiators farther from the boiler and in cold rooms (e.g., 2nd floor)
  • C: Used on radiators farthest 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

In general, larger valves are used at the end of long pipe runs (mains) and in colder rooms. Smaller air valves are used nearer to the steam boiler and in rooms that have a thermostat.

Steam Radiator Tips

  • Air vents must point straight up. "Up" is the pointy end or the end opposite the threaded nipple that screws into the radiator. When vents get turned to the side or upside down (got kids, by chance?), they can leak water. 
  • Keep the radiator control valve either all the way open, or all the way closed. Again, the control valve has a knob and is connected to the incoming pipe supplying the radiator, usually near the floor. It's designed as a shutoff valve, not something that you adjust to raise or lower the amount of steam the radiator receives. Unless you want to shut off the radiator completely, you must leave the valve all the way open at all times.