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zoned vapour steam system?

Jamie Hall
Jamie Hall Member Posts: 22,942
Don't ask. I am working on a largish house which has a very nice, three pipe, vapour steam system with almost all of the original fittings intact and in place. Thank goodness. And working. Here is the problem, and my bright (?) idea, and I would appreciate other suggestions, thoughts, etc. etc....
About 25 years ago, the old boiler was replaced and the system was split with valves on the steam main, dry return, and wet return. The north zone, about 500 square feet radiation, was not used and shut off. The south zone, about 700 square feet, was and is in use (and works fine, thank you) with a new boiler sized to fit. The new owner wants to reactivate the north zone, but operating at a lower temperature. My idea is to add a new boiler, sized properly for the north zone, feeding into the same main header (10 inch) as the existing boiler, thus using the same Hoffman loop and main air vent etc. (and eliminating the need to repitch everything). It would run only when the north zone wanted heat and the valve on the steam main (which can be motorised) to the north zone was open. If the north zone wants no heat, the valve closes and the new boiler is turned off.
Will it work? What do I need to look out for? If not, why not?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England


  • Good thinking

    There are LOTS of opinions on how to tie two boilers into a steam system. Am I right, you guys?

    I'm going to go out on a limb and express my opinion.

    The thing that you need to have in mind every step of the way is the water level in the two boilers.

    I mean that.

    Next is the pressure. I don't have the book open, but from memeory....Any system that has both high and low returns is a vapor system...Never forget that. (really, the book said,"never forget that").

    Since you said hoffman loop, I guess that you know where the water line should be. That device determines what pressure you need to shut off at.

    I think I would run the second boiler on steam pressure, and let it satisfy first. After the piping heats, one boiler may heat both zones. Both will run at first, and fast response times will result. If one won't keep up, the second would cycle on again, by steam pressure.

    Plan for your waterlines and you can do it.

  • Oh, by the way

    I think I would consider TRV valves, instead of zone valves. It can be done, I did just that. Put the operators in a concealed place, like the basement. Check these out...Noel
  • Steamhead
    Steamhead Member Posts: 16,732
    I'll second that, Noel

    TRVs are a much better way to go than zone valves on this system.

    Jamie, if I were doing that job, I'd use one boiler for the whole house and a TRV on each radiator, except in the room where the thermostat is. This will be easier to do and will give individual room control rather than just 2 zones.

    The thermostat should be in a main living area such as a living or dining room. These rooms are typically kept the warmest. The TRVs should be placed in the kitchen (due to heat input from stove etc) and bedrooms (which most people like to keep cooler than the main house), and in any other room which they want kept cooler or isn't used all the time.

    I wouldn't use TRVs on the bathroom radiators, due to the higher humidity and danger of freezing in these rooms.

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