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  1. #1
    Join Date
    Dec 2007
    Posts
    4,685

    b-vent temperature

    What is the acceptable temperature of the exterior pipe on b-vent exhaust.
    I have a 100k btu Lennox furnace with about 6 feet of b-vent connecting to a c-vent "Y" with my water heater and continuing horizontally about 10 feet before going vertical about 15-20 feet through the roof.

    I ask because the a/c pvc drain is near, 1.5 inches away, the exhaust and shows signs of scorching. It has happened over a long period of time, but I just now noticed it.

    Should we just replace the scorched pvc and not worry. The temp of the b-vent with the furnace running is about 148*F, done with a k-type probe on my multi-meter.

    Thank you

  2. #2
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    Inches Matter...

    1" for double wall and 4" for single wall vent - from any combusable material including PVC. Hope it helps.

  3. #3
    Join Date
    Dec 2007
    Posts
    4,685
    thanks, but looking for temp.
    As I stated, pvc is at least 1.5 inches from b-vent = double wall
    c-vent= single wall.

  4. #4
    Join Date
    Nov 2007
    Location
    Austin Texas
    Posts
    54

    very well

    I think the temperature will vary with many situations including effiency of the burn - but that being said - I would say that if you (cant) put and keep your hand on the B vent... its too hot. Thank you for explaining the b/c vent situation. I am always looking to learn something.

  5. #5
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    80 plus temp runs about 300 to 400 degrees inside the b-vent
    the outer temp of the pipe will be determined by the inside temp and the amount of heat the aluminum liner can absorb

    atmicphereic draft can be higher

    i have seen cases where you could not put your hand on the b-vent this is why 1in clearance is required. It will give off heat

    te single wall with a lennox unit and most that i am aware of is 6in min clearance
    water heaters also

    i would replace the piece of pvc and try to move it farther from the pipe

  6. #6
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,164

    Cool

    By design, B vent is capable of withstanding 400F above room temp. in the stack. The temp. of the outer casing varies with a lot of factors and is not part of the UL 441 test. That test allows a temp. rise of 90F plus ambient on plywood, painted flat black, encasing the venting. Usually, this means there must be a 1" clearance to combustible to meet this max. temp .rise . Room air currents and pipe construction will cause variances in vent surface temps.

    HTH,
    Hearthman

  7. #7
    Join Date
    Jan 2007
    Location
    N.E. Ok.
    Posts
    1,364
    If you replace it you could use a 3/4" galvanized pipe. Female adapter to go back to pvc.

  8. #8
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,164
    Clearance to single walled vent connector is 6" for gas; 18" for oil and wood. Straight out of NFPA 211.

    I don't know why mfrs put PVC drains right where they know they are too close to vent pipes but 98% I see are wrong.

    Hearthman

  9. #9
    Join Date
    Aug 2006
    Posts
    74

    many recommend...

    many recommend running B-vent directly from furnace - maintains exhaust gas temps, and takes care of your clearance issues. We've had to do that in many communities for that very reason.

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