Posted by jtown on February 15, 2000 at 08:29:48:
In Reply to: vacuum relief valve posted by Rod on February 15, 2000 at 06:44:53:
If you have one pipe connected to the radiators you have a one-pipe steam and the thing is a "main air vent". The main goes out and then back as a dry return (water runs back in it when steaming). The steam pushes air out at each cycle and when the vent gets hot by the steam it should shut. When the system cools it opens allowing air back in, breaking vacuum. If the system stayed sealed, air would leak in and then steam wouldn't get to where the air is. Fresh water is very bad for the boiler, so replace or open and clean the vents so they work and not lose steam/water. On the radiators also.