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  1. #1
    Join Date
    May 2004
    Posts
    44

    Line for outside combustion air?

    Setup, current natural gas furnace for the first floor is in a closet on an outside wall of a finished basement (basement has it's own separate system). Due to the slope of the land outside, any lines going to the outside must be run parallel to the wall for about 8 feet before turning and going outside.

    One of the salesmen that I've had come out mentioned the need for two lines running to the outside of the house for the furnace, one line for intake of combustion air, the other for the exhaust. In talking to another salesman, he said his plan is to only run an exhaust line to the outside. He's planning on running a 2" intake line from furnace into the main part of the finished basement and pull the combustion air from there.

    The current 12 year old furnace is sitting behind a louvered door and therefore is actually pulling combustion air from the basement, but it seems like the second salesman is trying to cut corners by running the intake line a couple of feet into the basement rather than outside. What would you guys recommend in this situation?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,090
    Currently. Your basement furnace is using more fuel to keep the basement warm. Because the basement must now pull in outdoor air to replace the combustion air that the first floor furnace is using.

    By running a combustion air intake from the outside. You save of gas for both the first and basement.

    The second contractor may be trying to get the job by keeping install cost low.
    But it will cost you more in operating cost.

    If you want high efficiency, and fuel savings, 2 pipe system.
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  3. #3
    Join Date
    Feb 2007
    Location
    Bellingham, WA
    Posts
    115
    I assume that a 2" line would be applicable for a 90%+ furnace. What would be the size of the fresh air intake line be for a 80% furnace?

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    80% furnaces don't normally have sealed combustion.

    A 80% and 90% furnace of the same BTU input, would have similar air intake requirements. The burners should not be significantly different, only the additonal of a secondary heat exchanger and perhaps a inducer designed for high static pressure and more condensation would be the difference.

    Most 80% furnaces are not designed for a sealed combustion intake.

  5. #5
    Join Date
    Feb 2007
    Location
    Bellingham, WA
    Posts
    115
    If you have an 80% furnace in an enclosed furnace room (with a tube running to the outside for air), is this not somewhat equivalent to a seal combustion intake design?

    In the situation described above, should the intake also be 4" like the exhaust?

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,090
    If the furnace room door is solid.

    Intake size depends on furnace BTU.
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