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Gravity conversion

My gravity boiler finally stopped working. Looking at tying in a new Lennox boiler that will have a pump on it. My fear is this is adding pressure to a system that never really had a lot of pressure on it. My house is 100 years old. The pipes look to be cast iron but I dont know for sure. Pictures attached. Anyone have any experience with a system leaking once you added a new boiler?


  • ScottSecor
    ScottSecor Member Posts: 854
    The 'new' boilers with circulators operate at the same pressure as your original system. The pump does not add any noticeable pressure to the pipes, it must moves the water a little faster.

    The pressure that fills the system with modern heating systems comes from the pressure reducing valve, that is typically factory set at 12psi. 12 psi will lift water approximately twenty seven feet above the boiler. This covers most two story homes and some three story (as heat emitters in most homes are near the floor).
  • ScottSecor
    ScottSecor Member Posts: 854
    By the way, your pipes look like pretty much every other house we work on from that era. Just plain old steel threaded schedule 40 pipe and threaded cast iron fittings.
  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    The only real problem with gravity conversions is rebalancing the system when one is done, but it the piping around the boiler is done with some thought, and provision made for balancing, they work out very well.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ScottSecor
    ScottSecor Member Posts: 854
    @Jamie Hall is correct about balancing the system.

    I just noticed in your first photo that there is a plug cock (old fashioned brass valve) on that horizontal pipe that likely goes to one of the radiators upstairs. I'm guessing that the original installers used that valve to balance the system. As with all heating systems really, the goal is to make all of the room about the same temperature (bathroom should be a few degrees warmer).
  • leonz
    leonz Member Posts: 1,122
    edited October 2022
    If gravity system is intact, has the open to air expansion tank in the attic or top floor and does not leak you do not need a boiler with a pump on it.

    Just buy a good boiler that has top tapping's of the same size that will connect to your hot water riser pipes and the same size tapping to the cool water return pipe that goes to the boiler sump.

    You may have to add a few elbows, pipe nipples and unions and a low water cut off for code compliance-I have a Bell & Gossett RB-122-E immersion type low water cut off and a new mechanical double aquastat like the Honeywell L8124L and that is all.

    If you have a good plumber you trust he will duplicate the piping you have and install the Low Water Cut Off in the side of the steam chest or the hot water riser pipe in a Tee with a reducer bushing.

    Can you provide us with a picture of your boiler and the controls? It would help a lot as your old boiler may just have 2 single aquastats which is even simpler to maintain the high limit temperature.