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1 pipe gravity water system upgrade.

I have a 1 pipe gravity water system. (See pics) I need to propose a new boiler for.

I am thinking of setting up as pri / sec pumping. The original loops are 3” and 3 1/2” so only about 1ft head in the main loops. If I install a system loop pump at 20 gpm at 1ft head. This should still allow the radiators to begin heating by gravity correct?

Note the supplys to each radiator are off the top of the run and the returns tie in to the side down low just after the supply.
System load is 200,00 btu so that is we’re the 20 gpm came from.

Is this the best way to go about this? Thanks for the help.


  • Jamie Hall
    Jamie Hall Member Posts: 22,918
    Single pipe gravity hot water depends on very low velocities in the mains. Otherwise you get turbulence and the water mixes too much for there to be the necessary temperature differential.

    It's a very ingenious system -- but unless I am mistaken you will have to pipe separate returns if you want to pump it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jumper
    jumper Member Posts: 2,208
    I don't think your sketch is correct.
  • Ironman
    Ironman Member Posts: 7,350
    It's a Phelps system.

    Here's an article that will help:

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    J Heritage
  • Steamhead
    Steamhead Member Posts: 16,726
    How many square feet of radiation does this system have?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • J Heritage
    J Heritage Member Posts: 27
    Steamhead said:

    How many square feet of radiation does this system have?

    1277 sqft standing radiation

  • leonz
    leonz Member Posts: 1,051
    As Jumper said your sketch is not correct it does not show radiators plumbed for gravity hot water.

    Why do they want a new boiler?
    Has anyone checked to see if the radiators have air in them or bled them if needed?
    Is this boiler leaking?
    does the old boiler have a low water cut off system installed in it.
    When was the last time the combustion system was cleaned/tested for efficiency?

    The gravity hot water system works using simple gravity and there are no moving parts other than the bleeders on the radiators.

    Your talking about adding more plumbing that is not needed as it would require a very large amount of work and piping to change it when the simple system that is there works.

    The hot water will rise from the new boiler tappings into the current piping and simply travel upward so it will still work with the new boilers smaller tappings by using bell reducers, short nipples and unions.

  • FranklinD
    FranklinD Member Posts: 399
    His diagram is probably correct except for how the run outs connect to the main...supplies come off the top and returns connect at the bottom/side...from dan’s books, they rely on the slow movement of water in the mains, causing the hot water to stratify at the top and the cooler water at the bottom - thus a “1-pipe gravity system”. And I agree with it being a Phelps-type system.

    I know a read a posting from a frequent contributor on here who was looking at adapting one of these systems to pumped flow (and I think he succeeded - very, very carefully)...but I can’t recall which user it was.

    Really cool system!
    Ford Master Technician, "Tinkerer of Terror"
    Police & Fire Equipment Lead Mechanic, NW WI
    Lover of Old Homes & Gravity Hot Water Systems
  • Henry
    Henry Member Posts: 996
    The pic is not correct. Depending on the system, it should have top feed and bottom return. Except most systems here used a main that was at the top floor that fed downward mains. The mains had two special TY fittings somewhat like a monoflow fitting that directed flow to a radiator and also sucked partially the flow out. As heat loss was greater in the top floors, it worked very well. When an engineering firm decided to pump the mains, it go all screwed up. It was at least a months worth of explaining and using old books to get it right!
  • leonz
    leonz Member Posts: 1,051
    Did they return it to the original method or keep they keep circulators?