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80% gas furnace

Good day, I have a few questions about my 80% Carrier LP furnace

#1...Can I come off the furnace with an elbow for venting

#2...Can I take the 4 inch vent piping into a 6 inch type A chimney (will be exclusively for the furnace



  • Spence
    Spence Member Posts: 316
    Furnace Venting

    Try to get as much straight pipe as you can from the furnace to your 1st elbow , remembering that your vent connector must have a lateral rise of at least a 1/4" per foot.

    Can you terminate your connector into a 6" common vent? Yes. Should you? Perhaps not. I'm not familiar with a "Type A" chimney,, so I would check the GAMA tables for single appliance verification of sizing based upon the parameters in the table for your common vent, BTUH input, lateral length, etc.
  • Bob Harper
    Bob Harper Member Posts: 882
    class A chimney

    Class 'A' chimney is considered "all fuel". Actually, it must be listed to UL 103HT in the US to meet that class as solid fuel requires a 2,100f rating whereas regular factory chimney carries a 1,700F rating. You are allowed to increase a gas vent up to 7x. With a 4" vent at a cross-sectional area of 12.56 " square, you could legally vent that appliance into a 10.5" vent if everything else is ok.

    the sizing charts in the gas codes, which were provided by GAMA, start with a 12" rise off the appliance collar. While it may be allowed, it certainly is not a good idea. Spence was correct in his info. A single walled connector for gas carries a 6" clearance to combustibles. This work is best left up to a pro.
  • Spence
    Spence Member Posts: 316

    And to key off Bob, even though the code says you can go up in sizing, that doesn't mean you should or that it is a good idea. A result of getting to 80% AFUE means a flue temperature that easily is 150 degrees cooler than an old wheezer. Lower flue gas temperature, bigger pipe could mean condensation. Ever wonder why you see so many 80s with chalk and rust on the connectors?
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