What's the difference between Steam and Vapor systems?

Subset

Vapour systems -- of which there were a very large number -- are a subset of two pipe steam systems.  The primary distinguishing feature of them is that they all are designed to operate on very low pressure differentials between the supply mains and the returns -- on the order of ounces (one might provide an arbitrary cutoff at about 8 ounces).  A second feature of most, but not all, of them was that they were balanced at installation so that each radiator got as much steam as it could condense, and no more; that feature, though, has often been lost in subsequent fiddling.



In terms of physical characteristics, well... perhaps the only real distinguishing characteristic is that most vapour systems had somewhat larger pipe sizes for the mains, but that isn't always diagnostic.  Presence or absence of traps, strange outlet gadgets, orifices, calibrated valves, sundry marvelous contraptions and the like are often found in vapour systems as well -- but each designer of vapour systems had their own idiosyncracies!



The idea of using pressures as low as will work will save for any steam system.  The advantage of the vaporstat is that it will control reliably at the low pressures; pressurestats are not really meant to regulate below a pound or two.  Since "regular" steam systems were typically sized to operate on about a pound to two pounds, they're OK there.