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Can manual valves be adjusted for 2-pipe vapor systems?

rhodebump Member Posts: 152
My following question pertains to the manual shutoff valves, not thermostatic valves.

So for quite a while now, I have read "on the internet" that steam shutoff valves were "on or off" only. More recently, I read that "on or off" was only for the case for 1-pipe steam. With 2-pipe steam systems, manual valves can be adjusted. By adjusted, I mean to allow for high/medium/low volumes of steam and corresponding heat for the radiators.

Can someone please clarify if my understanding is correct?

Thank you,


  • Zipper13
    Zipper13 Member Posts: 229
    edited January 2020
    I partially close valves to balance my 2-pipe heat.

    though I find that these original valves appear to offer better modulation (https://heatinghelp.com/assets/documents/89.pdf)

    than these ones that have been replaced

    With a 1-pipe system, the incoming steam and the outgoing condensate use the same valve/pipe. So a partially closed valve reduces the diameter through these two media can flowagainst each other and can lead to the steam having to bubble up through the pool of condensate waiting to get out rather than flowing around it. So air vents are used to regulate how quickly steam can enter the radiator by regulating how quickly is is allow to displace and evacualte air in the radiator.

    2-pipe systems have a separate return line for the condensate so slowing the inflow of steam to the radiator by partially closing the valve is OK since steam and condensate are not competing for space. Though some valve are more effective at this than others
    New owner of a 1920s home with steam heat north of Boston.
    Just trying to learn what I can do myself and what I just shouldn't touch
  • Jamie Hall
    Jamie Hall Member Posts: 22,905
    Yes they can, and often - if not usually -- are. The older Hoffman valves which @Zipper13 noted are wonderful that way, but any inlet valve can be adjusted, although not as accurately.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,849
    Those bulbous globe valves are really designed for 1-pipe radiators, where the demand is to allow maximum space for the steam and water to pass in opposite directions. In a two-pipe system, you won't see very much throttling until the valve is almost closed, so you end up using only something like 10–20% of the full range of adjustment, and within that range it's likely to be a little touchy.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • dopey27177
    dopey27177 Member Posts: 887
    See attachment

    From my book Steam the perfect fluid for heating and some of the problems.

    In the pictures you will see why a regular radiator valve will not adjust the steam flow properly. In addition regular radiator valves have hard seat washers where the adjustable valves are metal and almost never wear out.