Heat-Timer VARIVALVE 925006-00 straight 1/8" Steam Vent Radiator Air Adjustable Vent Valve. The venting capacity of radiator air vents and main line air vents used in single pipe steam heating systems has an important effect on the operation of these systems. In order to obtain a better understanding of these effects, The Center for Energy and Environment conducted experiments both in the laboratory and in the field to investigate the properties of single pipe steam air venting. Commercially available radiator and main line air vents were tested and compared. Main line air vents in one building were replaced with open pipe orifices of different sizes, and the resulting fill characteristics of the main steam lines were observed. The purpose of the main line air vent is to rapidly vent the relatively large quantity of air in the main lines. This allows the main lines to fill more quickly and thus reduces the time difference of steam arrival at the different risers. A larger orifice allows the air to be pushed out at a lower back pressure. The results show that the larger the vent, the more quickly you can vent a system, which lowers operating pressure requirements.