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B-vent chimney and fire code

| Posted in Energy, Heating & Insulation on February 3, 2003 09:55am

I put a second floor on existing one story house.  Existing masonary chimney serves gas furnace and hot water heater.  There is no plan for a fire place.  Right now, the chimney ends in the middle of the second floor.  My plan was that the appliance gases will vent through a continuous chimney liner from the basement to the chimney cap.  The part that passes the second floor and out to the roof will be enclosed in the B-vent pipe.  At least, this was my idea, but no one seems to think this is possible.  Even the guy who is going to install the second furnace does not think this is possible.  His question is how will the liner and the B-vent connect to one chimney cap.

My idea is this.  The liner handles the gases so the cap must connect to the liner.  The B-vent, in this case, is for fire protection.  The connection point between existing flue and the B-vent will be encased in a concrete (what thickness for 2-hour fire rating?) .  The connection point between the B-vent and the cap will be made with just screws (if possible) or fixed with metal angles.  I think that the connection between the B-vent and the cap does not have to be factory tight since this point is outside the house.  I will tape vent joints (inside the house) and apply duct sealant.  The cap may be big enough to protect the B-vent from rain as well.  Do you think the inspector may accept this?

My code question is:  The International Residential Code accepts 1 foot clearance between listed chimney cap and the roof under certain pitch when chimney is only used for gas appliances.  My town inspector wants 3 feet and 10 feet away from roof peak, the tried and true.  This was about 6 months ago.  Now other code organizations no longer exist, how can I convince the inspector?  I have not talk to the inspector yet.  What information should I take with me?  Thank you all for the advice.

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  1. User avater
    rjw | Feb 03, 2003 12:46pm | #1

    Is the 2nd furnace going on a different level of the house than the existing furnace?  The codes are a bit ambiguous on this one, I have found, but it seems to me the best interpretation is that combustion appliances on different floors need different flues.

    Rather than try to connect the B-vent at the top of the existing furnace, why not run a metal flue liner down the existing flue and have an all metal flue.  (Very commonly done in my area when a natural draft furnace is replaced by an 80+, especially where the chimney is on an exterior wall.  Typical cost around me is $300-400.)   That way the connection to the B-vent is standard and the whole thing is pretty much something the inspector has seen - (inspectors love "standard")

    There are B-vent caps that allow the top of the vent to be only one foot above anything within 10 feet, but go for the extra foot or two. 

    (FWIW, the "general" rule is that the top of flues must be at least 2 feet above anything within 10'.  It wasn't clear from your post if that's how you were reading it.  That 3' might be a local variation.)

    _______________________

    "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

    1. UncleDunc | Feb 03, 2003 03:53pm | #2

      I thought it was 3' above where it emerges from the roof AND 2' above anything within 10'.

    2. KwanChoi | Feb 03, 2003 05:19pm | #3

      Thank you for your reply.  The second furnace is going into the attic.  The installer said that this furnace will have its own vent.  I think the attic furnace is above 85% efficiency.  The metal liner will be used all the way from the cap to the basement connector to the appliances.  However, this liner will pass through half of the second floor and the attic before it exits to the outside and I must make this part pass the fire code, therefore using B-vent.

      The code here in New Jersey is 3 feet above roof and 2 feet above and 10 away from the roof peak.  This is generalized code, I believe, covering all kinds of combustion material such as wood burning fire place as well as gas furnace.  The new International Residential Code says 1 foot is accepted for gas furnace and water heater.  But now, I think I read somewhere that chimney cap must be higher than roof peak to prevent back draft of vented gas caused by wind sweeping over the roof peak.  Is this part of the reason for the cap clearance?

      Any advice and all experience will be appreciated.  Thank you all very much.

        

      1. User avater
        rjw | Feb 03, 2003 07:11pm | #4

        You might a call to the local inspector, but I don't see why that plan wouldn't pass muster.

        What part of Joisey?  Bergan county boy, here.

        _______________________

        "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

        Edited 2/3/2003 11:11:50 AM ET by Bob Walker

        1. KwanChoi | Feb 05, 2003 11:15am | #5

          Thank you.  Same county here.  I got another advice using b-vent to liner connector.  I can connect b-vent directly to liner, put the b-vent about a foot inside existing flue, and firestop at the flue.  The b-vent from here on out to the roof and the b-vent takes care of gas and the fire code.  Again, thank you for the advices. 

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