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Water Line in Converted Two-pipe Vapor/Vacuum System

I have a question about the water line of a converted (two pipe) vapor/vacuum heating system that I've taken stewardship of. There's no steam traps at the end of mains before they dump (along with the dry returns) into the wet return and eventually back to the boiler. At the end of the dry return, slightly above the wet return running along the floor, there appears to be a check valve. I'm assuming the dead men did this to keep the boiler pressure from running into the dry return, and it seems to be working, kinda. The dry stays relatively cool, but the main and wet return on the floor get quite warm. I've done a bunch of measurements, and it looks like the guys who installed the new boiler left very little margin - I think the wet return sometimes is dry, and that check valve always dry. Do I need to find a way to raise the water line up so it covers the check valve, or is a little bit of steam in this part of the wet return (it drops a few inches as it runs toward the boiler) nothing to worry about?

I've attached a picture...that insulated pipe is the end of the main, and the bare pipe with the check valve goes up to the dry return, capped by a vent. My system probably best (at a basic level) resembles Figure 7, Page 232 of LAOST, although the wets are more complicated and the check valves are a couple inches higher. There's also no Hoffman Differential Control.

Also, I can post this later: but any advice on the main vents? It's a 450,000 btu two pipe venting entirely thru a couple of Hoffman 75 knockoffs. I'm thinking that's not enough, but that's another discussion....

Thank you all very much! Lots of smart people on this forum..


  • Jamie Hall
    Jamie Hall Member Posts: 22,973
    I'm sorry to say it, but the water line needs to be raised so that when the system is off it's at least above that bit of horizontal pipe with the check valve in it. The check valve is an optimistic attempt to keep any steam which might get into the area from backing into the dry return and pressurizing it, which will hurt (if not stop) your venting into the dry return, which the radiators have to do.

    As soon as the boiler starts to build pressure, the water line in the drip from the steam main will hold where it is, and the water will rise in response to the pressure in the drip from the dry return, and maintain the seal -- but if there is no seal, the pressure in the steam line will come right around the corner and up into the dry return.

    As you say, as it is it may work. Kinda. But you are depending on that check valve to really seal, which in my experience is rather optimistic...

    If you have crossover traps between the steam mains and the dry returns, you can get away with, and might just as well get away with, just really good venting at the boiler, where the dry returns drop. Otherwise, you need vents on the steam mains at their ends, as well as vents on the dry returns.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England