Steam cannot reach a radiator until the air ahead of it is pushed out, and main vents release that air from the mains before steam gets to the radiators. When steam arrives the vent heats up and closes. Undersized or dead main vents mean uneven heat, long burner runs and radiators far from the boiler staying cold, and adding main venting is often the cheapest fix in an old steam building.
End of the long main has a vent the size of a radiator vent, put two big main vents on an antler and the back rooms will heat.
Fast main venting and slow radiator venting is the classic recipe for even heat in one-pipe steam.
Also heard as
- main air vent
- steam main vent
Related terms
Spitting
Describes a steam radiator vent that throws out water and steam instead of just air.
Steam trap
An automatic valve that drains condensate and air out of steam lines and equipment while holding steam back.
Dead men
Steam heating slang for the long-gone engineers and fitters who designed and piped the steam systems that remain in service.
Wet return
A steam system return line run below the boiler water line, so it always stays full of water.