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Jim Pompetti
Jim Pompetti Member Posts: 552
Go a call today , lady had a chimney relined, the new liner 6" boiler venting 7'' .The smoke pipe comes out of the boiler 7" , 90's then goes to  7" smoke pipe,barameter damper  then reduces to 6" a the chimney. All looks fine. Home inspector told her , it must reduce at the boiler to 6" (would not pass the boiler) . Your response .


  • Eric_32
    Eric_32 Member Posts: 267
    There are...

    some instances when the chimney can be a size smaller than the appliance draft hood or outlet.

    Very tall chimneys can reduce the diameter down,

    Fields Power Venters can also.

    We use NFPA 54 here in MA and has vent tables in the back to reference, but if the manufacture recommends a particular venting installation in the manual, that usually supersedes our code.

    What kind of boiler is it and what was the call for?
  • Tom_133
    Tom_133 Member Posts: 773
    In Gas Class

    I heard of a scenario similar to this, they called the company who built the boiler their engineer gave them the thumbs up and provided necessary data, perhaps you can call manufacturer and see if that would work in this scenario.
    Montpelier Vt
  • Bob Harper
    Bob Harper Member Posts: 882

    Jim, I'm right in your area. The 2009 IRC requires the chimney be at least the same size as the appliance collar. Now, most AHJs will allow you to downsize one diameter if the smaller liner still meets the sizing charts in the code.Don't forget corrugated liners must be derated 20% plus another 20% if offset. However, there are no charts for oil in the code, only in the supplemental info. in '31. In general, you want the larger diameter off the appliance with as little restriction as possible until it reaches the vent. In fact, the sizing charts for the vent connector will often require a larger connector than common vent such as a 4" connector required for a 40mbh water heater if
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