Vapor Steam Heating Systems
A vapor steam heating system is one that commonly uses steam at approximately atmospheric pressure or slightly more, and that operates under a vacuum condition without the aid of a vacuum pump (see Vacuum Steam Heating Systems and Figures 8-16 through 8-18).
The steam pressure at the boiler necessary to operate a vapor system is generally very low (often less than 1 lb), being no more
than is required to overcome the frictional resistance of the piping system. Under most operating conditions, the pressure at the vent will be zero or atmospheric.
Vapor steam heating systems may consist of various combinations of closed or open, upfeed or downfeed, and one-pipe or two-pipe arrangements, depending on the requirements of the installation.
Any of these combinations will have certain advantages and disadvantages.