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new water heater venting question

I'm getting my water heater replaced. i was told the hood and vent used on my unit now was small for it(3 inches).
I have a chimney with liner installed. If new heater requires 4 inch vent and the opening to my chimney is 3, will installer adjust that? is that something that is common?


  • Jamie HallJamie Hall Member Posts: 12,864
    Ah... depends on the chimney liner itself (not the opening in the chimney). If the chimney liner is correct as to type and size, then the installer can adjust the connections.

    If it isn't... you have a big problem.

    You should verify the type and size of the liner in the chimney -- you may need to have a chimney sweep to do that -- before proceeding.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • sonicboominsonicboomin Member Posts: 4
    thanks for reply. not sure to be honest, chimney not used as fireplace as we dont have any, runs from center of house from basement to roof.
    i have steam boiler and water heater connected to it from basement.
  • STEVEusaPASTEVEusaPA Member Posts: 4,143
    You have both a steam boiler and a water heater connected into a 3" liner?
  • JUGHNEJUGHNE Member Posts: 6,932
    I would guess he has a masonry chimney with a liner that is large enough now for the boiler and WH. Old WH has 3" vent, new might have 4" vent, 3" is maybe WYEd into liner......or liner is old school masonry with only a 3" hole in it.
  • sonicboominsonicboomin Member Posts: 4
    Hi no, opening for steam boiler is much larger than 3 inches.
  • JUGHNEJUGHNE Member Posts: 6,932
    A picture showing the whole set up would clear this up.
  • sonicboominsonicboomin Member Posts: 4
    i will take pic once im home. FYI new boiler is AO smith 50 gallon 40000 btu,
    anyone know if 3 inches will work for that model?
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,058

    Flue pipes and liners are sized for each individual job. They are base on btu input of water heater, btu input of steam boiler and height of chimney and type and size of the existing liner

    We would need all the information to do that, but that is up to your installer. 3" flue may be fine for the water heater
  • Larry WeingartenLarry Weingarten Member Posts: 1,803
    Hi, I’d like to see the spec sheet for the heater. Normally 3” vent is fine for 40,000 btus.

    Yours, Larry
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,058
    I could be wrong but I think a natural draft water heater that normally can use a 3" vent has to be 4" under some conditions I think if vented with a draft assisted boiler
  • JUGHNEJUGHNE Member Posts: 6,932
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