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Furnace venting

vit99 Member Posts: 1
Am I able to vent my 90% furnace with B vent instead of running PVC? The one I have now is 80% currently trying to swap for a 90% 


  • Jamie Hall
    Jamie Hall Member Posts: 21,524
    What does the installation manual require? That's what you go by.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 10,785
    edited March 2022
    No, condensing anything cannot use B vent.

    I have used the B vent as a chase to run the PVC or approved pipe up thru the roof.

    You would have to use inside basement air for the combustion air.
    Not all 90% furnaces allow that.

    And if water heater is using the B vent then even the chase method is out.

    Back to the install manual.
  • pedmec
    pedmec Member Posts: 719
    you have to use an approved pvc or similar approved material in massachsetts. its called fgv approved. approved under ul listed system 1738 rated for 149 degrees or system 636 rated for 194 degrees.
  • Ironman
    Ironman Member Posts: 7,173
    No B vent with a condensing (category 4) appliance. B vent is for category 1 non condensing, negative pressure venting. Category 4 is condensing, positive pressure which is what a 90%+ furnace is.

    You also cannot “orphan” the water heater if it’s common vented with the existing gas furnace.

    As stated: read the manual! It gives detailed instructions of how the venting must be done.

    I’d highly recommend getting a good technician envolved, because even though run pvc pipe is not difficult, doing correctly and properly setting up the furnace is. Improper installation could cause CO poisoning! 
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.