How Steam Radiator Air Vents Work

A radiator in an unfurnished room

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The radiator air vent (also known as the air valve, steam vent, or steam valve), is found only on radiators one-pipe steam boiler systems. It is usually located at the end of the radiator opposite the supply pipe, about halfway up or towards the top. Many air vents are bullet-shaped, but they can be many different shapes and sizes. They can also be concealed by a decorative radiator cover. Don't confuse the air vent with the control valve (supply valve), which is connected to the supply pipe and is generally located near the bottom of the radiator. 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 Vent Works

When the steam boiler is not in a heating cycle, the radiator air vent is open, allowing ambient air to fill the radiator. As a heating cycle begins, steam flows up through the supply pipe to the radiator. As the radiator fills with steam, it pushes the air out through the open vent. This replacement of cool air with hot steam heats up the vent, which is a heat-sensitive valve, until the vent closes, trapping the steam in the radiator to capture its heat.

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 in the supply and return lines of two-pipe steam boiler systems.)

Steam Radiator Air Vent Types and Sizes

Radiator air valves come in a variety of sizes that provide different rates of airflow. By adjusting the airflow to an individual radiator, you can affect how quickly the radiator heats up. This fine-tuning allows you to balance the heating system so that radiators get the right amount of heat for the rooms they serve. More airflow means more heat; less airflow means less heat.

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 (such as rooms on upper floors)
  • C: Used on radiators farthest from the boiler (e.g., third 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?), 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 in a one-pipe steam heating system.