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Main and return venting for 2-pipe steam

BM2 Member Posts: 10
We have a large house with 2-pipe steam. A plumber came in over the summer and replaced an old Trane vent on the end of a long main with 5 Hoffman 75s on an antler. He also replaced an old green Hoffman 75 at the top of the dry return in the boiler room with a new single Hoffman 75 (see picture).

What's great is that the side of the house fed by this main with the new 5 Hoffman 75s heats up instantly (it used to heat poorly). What's bad is that the other side of the house heats up more slowly and therefore remains cooler (all rads heat up eventually).

I see a lot of discussion about mains venting on one pipe systems but less about two pipe. Specifically, is the dry return supposed to vent at the same rate as the main? (i.e. as fast as possible based on pipe diameter and length)? Is there a difference between venting the common dry return back at the boiler vs dry returns which drop down to a mud leg?

My practical questions are:

1) Is the single Hoffman 75 at the dry return in the boiler room sufficient? How would I know? While running, it hisses pretty much constantly. The 5 Hoffman 75s on the end of the main are almost silent.

2) The colder side of the house has a main which ends in a crawl space, see photo. The pipe closer in the photo is the main, and the one a few inches behind it the return. Both drop down on the other side of the brick wall into the wet return. The vents installed on both look ancient, should I try and replace them to improve venting? If so, with what?

Thanks in advance for any advice.


  • Jamie Hall
    Jamie Hall Member Posts: 23,313
    In two pipe steam, the vents on the dry returns are actually more important than the ones on the steam mains themselves, and need to be at least as big as the ones on the mains. I doubt that a single Hoffman 75 is adequate for either that steam main or the associated dry return, so yes -- those two in the crawl space need to be replaced with larger vents. I'd suggest a nice pair of Gorton #2s. The one in the boiler room also needs to be one of the big Gortons.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • BM2
    BM2 Member Posts: 10
    Thanks Jamie I'm very grateful for your advice. Much of what I've learned from this forum has been from your postings.

    Regarding the boiler room--it's hard to tell from the photo but the Hoffman vent is quite close to the ceiling, maybe only 0.5" clearance. So I don't think a Gorton #2 will fit. I can either 1) put in an additional Hoffman 75 on the antler or 2) reduce the vertical nipple (currently a 6") by about 1.5" to replace with a Gorton #2.

    In the crawl space--a question about technique. You can see there is a lot of rust below those old vents. I have a hunch my 14" pipe wrench won't budge it. How do you professionals manage to remove old rusty iron pipe? My only intuition is to either invest in a longer pipe wrench, or use WD-40?
    EBEBRATT-Ed Member Posts: 15,532
    @BM2 WD 40 or penetrating oil is questionable but iether one won't hurt anything either.

    A couple of wrenches (use one for a back up) so you don't loosen anything you don't want to and they should come off with some effort.

    Keep the nipples as long as possible (the higher the vent is above the main the better) but do what you have to do to get larger vents installed

    Sometimes a smaller wrench like a 14" woks better with a cheater pipe over the handle than a bigger heavy wrench that becomes awkward due to weight. A scrap piece of copper tubing or electrical conduit EMT is lighter than regular pipe to use for a cheater