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Thread: Sizing Furnace B-Vent

  1. #1
    DIY Junior Member
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    Jan 2010
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    Linden, MI
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    Default Sizing Furnace B-Vent

    Have been reading threads here for some time, and everyone has been most helpful in many projects. Currently insulating my attic and was getting ready to insulate around my b-vent when I noticed it seemed larger than need be. Went down in the basement and looked and here is my setup:

    80% 66,000 btu fan assisted Bryant furnace, 3' single wall connector, 2 elbows, no horizontal runs
    40 gallon, 40,000 btu with draft hood, 2' single wall connector, 2 elbows, no horizontal runs

    Both the furnace and water heater vent through 4" single wall pipe to a 6x5x4 sheet metal wye. The 5" outlet has a 4" reducer that connects to the furnace. The 6" end of the wye then connects to a 6" b vent that travels straight up, no offsets, through a wall chase and through the roof where it terminates, approximately 16'-17'.

    From all the sizing guidelines I have read, it looks like I should be running a 4" b-vent connector from the furnace and can use a 4" double wall for the common vent up through the roof. I currently have 6" b-vent for my common vent. Will this cause problems with condensation and furnace efficiency? If I'm going to fix it I want to do it right!

    Any help is much appreciated!

    Thank you to all in advance.

  2. #2
    Retired prof. engr. gator37's Avatar
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    See if you can go to a library and find the NFPA (National Fire Protection Association) 54. It tells you what you can and cannot do and it also gives you examples of how to size flue vent piping. Also the International Gas Code (IGC) will tell you a similar method. My preference is the NFPA.

  3. #3
    DIY Junior Member red22769's Avatar
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    Quote Originally Posted by agstaff View Post
    Have been reading threads here for some time, and everyone has been most helpful in many projects. Currently insulating my attic and was getting ready to insulate around my b-vent when I noticed it seemed larger than need be. Went down in the basement and looked and here is my setup:

    80% 66,000 btu fan assisted Bryant furnace, 3' single wall connector, 2 elbows, no horizontal runs
    40 gallon, 40,000 btu with draft hood, 2' single wall connector, 2 elbows, no horizontal runs

    Both the furnace and water heater vent through 4" single wall pipe to a 6x5x4 sheet metal wye. The 5" outlet has a 4" reducer that connects to the furnace. The 6" end of the wye then connects to a 6" b vent that travels straight up, no offsets, through a wall chase and through the roof where it terminates, approximately 16'-17'.

    From all the sizing guidelines I have read, it looks like I should be running a 4" b-vent connector from the furnace and can use a 4" double wall for the common vent up through the roof. I currently have 6" b-vent for my common vent. Will this cause problems with condensation and furnace efficiency? If I'm going to fix it I want to do it right!

    Any help is much appreciated!

    Thank you to all in advance.
    agstaff ,

    When multiple applicances are venting in a common stack, the common HAS to be larger than the vent connections of the appliances for proper draft.
    I sell commercial HVAC equipment for a living, and do sell quite a bit of B-Vent, and Pressure stack. I rep the Metal-Fab product line, and have a sizing program I can run for you to verify it was originally sized properly, but it is on my PC at my office...I should be able to run it based on the info you listed.
    If you can hold off until Monday I will check back with the results.
    Word of catuion: if you are changing the size of the flue pipe, double check the furnace manufactures installation instructions for what size they recommend so as to not void the warranty. Wrong size flue pipe can cause all sorts of problems, and they could be costly if you void the warranty!

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