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I have a two story house with mechanical closet in basement, housing a water heater and furnace. 140k BTU total which requires a 46.6 sq. in. combustion air duct to attic space.

This is currently being served by 6" and 5" vertical round pipes that run straight up through the middle floor (inside a coat closet) to reach the attic. This is done in accordance to IRC G2407.6.1 (304.6.1) Two-permanent-openings method.

G2407.6.2 (304.6.2)One-permanent-opening method requires a single opening. I'm wondering if I could use the wall cavity to create a rectangular duct (3.5x15.5" = 52.5 sq in) to the attic. This would be done similar to a return air duct and would allow me to remove the two round ducts that cut through the coat closet.

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    Walls usually have fire blocks half way up. This might go against local fire/building codes if you have an open space between the basement and attic.
    – crip659
    Dec 28, 2022 at 23:30
  • I also have a suspicion this may be against fire code, and/or you'd have to run continuous metal ducting through the wall all the way to the outside. You could call a local HVAC company or call the city.
    – P2000
    Dec 28, 2022 at 23:40
  • Doesn't the "open space" already exist from the 5 & 6" pipes in place now?
    – Nick
    Dec 28, 2022 at 23:45
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    The metal pipes should keep flames from catching the walls on fire for a certain amount of time. Just a straight opening to the attic without flame stops will start every floor burning in no time. This is mainly to give your family a few extra minutes to get out, not to save the house.
    – crip659
    Dec 28, 2022 at 23:55
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    A simple call to your building or fire departments should let you know if your idea is allowed. It can be different just between two local governments(town/city/county). On one hand your idea should work if there is an open space all the way up, but most walls do not have an opening from the basement to all the way to the attic. There usually are fire/nailing blocks and floors/ceilings in the way.
    – crip659
    Dec 29, 2022 at 0:04

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