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Mystery valve

Hey all. Recently I've been trying to tune up one of my boilers and checking on steam venting and I came across this vent that I don't understand.

I have a 1 pipe steam heat system in a 14 unit apartment building. The wet return comes back from some line somewhere along the floor behind the boiler. On it, there is a tee with a long pipe going to the ceiling that had a very old valve on there with the markings "1949 USAV".

Looks like a straight steam valve but it makes no sense to me that there would be a steam valve off a wet return. It didn't seem to be operating in any way and I replaced it with a maid o mist main valve to see what would happen.. it immediately spewed water like crazy during the heating cycle. I pulled it and plugged it.

Attached are some pics: the old valve as it sat, the top of the now plugged line, and the bottom of the line where it tees into the wet return.

Any ideas what this is?


  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846
    I don't see any reason for it or the pipe it's mounted on to be there. It's not possible that any steam would ever reach it, so it would never close. I'm also incredulous that water would come out of a vent mounted up there.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • FortyTwo
    FortyTwo Member Posts: 46
    The boiler has been set to cycle very high so perhaps the high pressures forced water out? it was set to cut out at 5psi and in at 3.5. I've lowered it not to .5-1.5 It reaches 3psi in 15 minutes so I assume I must have some major venting to do but that may be for another thread. There are no main vents and I have to access the top tenants apartments to check the risers but they don't easily let me in. There is one main vent but it is in what i think, a strange place. But hitting one mystery at a time, I'm curious what the heck I plugged lol.
  • Fred
    Fred Member Posts: 8,542
    @FortyTwo You plugged a vent that was doing absolutely nothing. With pressures as high as you were running, I'm not surprised it pushed water out of the wet return. Put some vents on the end of the Main(s) after the last radiator run-out and you may need a vent on the risers if this is a multi-floor building.
    EBEBRATT-Ed Member Posts: 15,512
    I am guessing that at one time that was a return from a piece of radiation that was removed and the vent vented that radiation. They probably left the riser so they wouldn't have to drain the boiler to remove it. There is a coupling halfway up with teflon on the joint. Perhaps there was a tee at one time
  • FortyTwo
    FortyTwo Member Posts: 46
    An old return is a possibility, but them sticking a vent on top is strange. Also, the line going up from the tee on the floor is 1/2" or 3/4" (i forget) with no bushing - strange size for a return line in this building. The teflon at the coupling in the middle is my doing. I couldn't get the valve or bushing off at the ceiling, so I undid it at the coupling in the center and put it back together. So far unless someone comes up with anything otherwise, the old return line from removed radiation seems likely.

    It's a 5 story building. I'm thinking it is hitting high pressures fast because the riser vents are small or clogged. But the top floor tenants are all difficult so it will take me time to get in there and change them.

    Are well vented risers sufficient for good venting or do mains always need to be vented as well? I have more buildings that don't experience this fast high pressure issue but have minimal-to-no main venting.

    Okay, one more question while I'm posting.. haha.. does cycling at high pressure cause wet steam? Should my reduction of the pressuretrol cycling to .5-1.5 create drier steam?

    Thanks all for your responses and thoughts! I'll get some pics together of what I think is a very strangely placed main vent at that boiler and post them
  • Fred
    Fred Member Posts: 8,542
    As counter intuitive as it may seem, The higher the pressure, the slower steam moves through the system. If I recall correctly pressure will raise the water level in your vertical returns about 28 inches per PSI so you can see how running at 5PSI would raise the water in that vertical by more than 10ft. and cause water to spew out of it. The Empire State Building runs on 2 or 3PSI, max. No reason to run your system at 5PSI.