Dry returns vs. steam mains vs. whatever.
This is an area of fuzzy terminology which often leads to problems -- so I am a bit of a fanatic on my interpretation of the words! Which makes me a nuisance, I suppose...
As I use the terms:
A steam main (perhaps better "line") is any pipe directly connected to the boiler header. Clearly it must be vented either with main vents or with a crossover trap to a dry return. It may, or may not, continue past the last runout.
A dry return is any pipe used to conduct air and condensate, but not steam. In a properly operating system it should never have steam in it, but it must be vented. It has no direct connection to a steam main.
A wet return is any pipe located below the system (usually boiler) water line which is used to conduct condensate from various drips from steam mains or dry returns (on some occasions individual radiator returns or risers) back to the boiler.
The problem is that many times the section of the steam main between the last radiator runout and the boiler is referred to as a dry return in parallel flow systems. While it does fit part of my definition of a dry return -- it conducts condensate -- it's confusing. For instance, two or more of this type of "dry return" must not be joined together, while two or more true dry returns not only can be joined together, but sometimes must be and quite commonly are.
Or look at it another way. If I drill a hole in the side of steam main with the boiler running I expect to get steam. If I drill a hole in the side of a dry return, I'd better not get steam. Or this way. If I have a one pipe steam system, I don't have dry returns -- although I may have a nice length of steam main after the last radiator runout back to the boiler. If I have a two pipe system, I'd better have a dry return or my radiators won't be vented.
But I have been accused of being pedantic and fussy and admit that that is sometimes true...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch