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Gravity system

The Marathon pump in my 4 story urban home's boiler feeder failed after 8 yrs. of use (leaks, doesn't move water, and just buzzes). A replacement pump will cost $800. As an alternative, I'm wondering whether or not the boiler could instead run via gravity. The system is a 2-pipe vapor system that is driven by a Weil McLain LGB-6. The mains and risers are really big, so we use a 4 oz. cut-in. I could install a steam valve to have the condensate by-pass the boiler feeder and go directly into the boiler to test it, but I would appreciate any thoughts on why or why not gravity might work? To where on the boiler would I pipe the condensate? Were I to change to gravity versus pumping, would the steam traps at the end of the mains stay in place?


  • Jamie Hall
    Jamie Hall Member Posts: 21,512
    A vapour system at your pressures should not need a condensate return or boiler feed pump. There are many variations on how these things should be piped; if you don't have a copy of The Lost Art of Steam (available from the store on this site) you should get one and study it.

    Will you need steam traps at the ends of the mains? Depends on how the system is piped. You may need crossover traps to the corresponding dry returns; if there aren't corresponding dry returns, you need big vents at those locations. There is usually a drip at the ends of the mains ti take any condensate from the mains back to the boiler (there is a possibility that the mains are counterflow, but that's not as good an arrangement. Condensate from the radiators, along with the air that was n them, goes usually into dry returns, which need to be vented near the boiler where they too drop into the wet returns. But, as I say, there are many ways to do this.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Post a diagram of the present piping layout for the system, and we can advise on the changes.
    If you go to the Weil McClain website, and find the manual for your boiler, it will have diagrams of various piping arrangements, which you can compare with what you have now.
    Gravity is much better for steam systems.--NBC
  • KC_Jones
    KC_Jones Member Posts: 5,429
    Pictures of the boiler and associated piping would help also.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    Each main is dripped to F&T's which then go to return piping. The mains each also have Gorton vents. I'll check the Weil McLain manual for piping diagrams, and I'll post some images later today. Thanks to all.
  • Dave in QCA
    Dave in QCA Member Posts: 1,777
    Gravity return on a 2-pipe system is limited by the "B" dimension. That is the difference between the Normal Water Line in the boiler and the lowest elevation of the horizontal condensate return piping, before it drops to the boiler return line. The water column conversion to psi is 1 psi = 27.68" of water column, or round it off to 30". That means if your boiler NEVER exceeds 1 PSI, and on most 2-pipe vapor systems, 8oz is satisfactory. But, figuring 1 psi as an absolute upper limit, a "B" dimension of great than 30" would allow for gravity return of the condensate.

    It appears to me that the installation of condensate tanks, while largely driven by ignorance, was also affected by stock controls supplied with boilers, those being a pressuretrol, and those cannot accurately control pressure much lower than 2-3 psi. With 2 psi being the assumed pressure on low pressure boilers, a condensate return tank was necessary in many installations in order to push the return into the boiler and also to prevent the boiler water from backing out of the boiler.

    Use of a vaporstat, which costs a LOT less than a condensate tank and pump setup, solves the whole issue.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    Dave et al, the bottom of our lowest (basement) radiator is 34" above our boiler's normal water line. Is that what you meant by the expression " the lowest elevation of the horizontal condensate return piping, before it drops to the boiler return line?" Here is an image of the piping side of the boiler. I propose to eliminate the corroded pipe at the bottom of the Hartford Loop and to connect the return there, as well as the fresh water supply. By the way, the installing plumber used one piece of galvanized vs. black pipe in the Hartford Loop? Should this be replaced, or it okay? We use a vaporstat, and as a point of correction, it looks like it's set at 12 oz. Main and 6 oz. Differential. Maybe lower settings would work. Do I need to remove the steam mains' F&T traps for a properly functioning gravity system?imageimageimage
  • Jamie Hall
    Jamie Hall Member Posts: 21,512
    No, you don't have to remove them, but they can be a nuisance and may get in the way at very ow pressures.

    Shame about that piece of galvanized. I uppose it's what he had handy. It will rust...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England