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  1. #1
    Hello, I have a question regarding my furnace exhaust...

    My gas furnace has 4" exhaust pipe. I am wondering if I can use 3" exhaust pipe for about the first 4' or so off the furnace. Im trying to gain a little room for a bathroom im putting in my basement and this extra 1" will help get me there. After the first few feet of 3", it will go to 4" for the vertical run to the roof. Also, is there any rules on how close the exhaust pipe has to be from the main supply duct/column or can I but the exhaust pipe right up against it?

    Also, can I use flexpipe for this short span?

    Thanks for any help you can give.........

    If you need pictures, just let me know.


  2. #2
    Join Date
    Feb 2005
    Winnipeg, Canada

    Vent size

    In a word NO. Most equipment is designed to be vented full size. Read the installation instructions. I AM SURE all your questions will be answerd.

  3. #3
    Join Date
    Aug 2003
    Need more info but i would call a pro to look at it,venting is either right or wrong and really wrong can be devastating

  4. #4
    Join Date
    Mar 2004
    Grottoes VA
    What kind of pipe?

    How old is your furnace? It might be beter to replace with an 90% eff furnace and the vent will be PVC at 2" or 3". This PVC can be in direct contact with a combustible surface.

    If the vent is single wall metal than you need to keep 6" or more clearance from combustibles (depending on code in your area). If the vent is B-vent (double wall) than the clearnce shrinks to 1" or more clearnce (depeding on local code).
    Karst means cave. So, I search for caves.

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