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  1. #1
    Join Date
    Dec 2003
    Posts
    28

    Furnace input duct size

    My old Lennox 75,000 BTU furnace has an input duct about 30% smaller than the mfr marking on the side panel. The marking suggests a duct of "24 x 15", but my duct is only 24" x 10". The furnace was replaced about 20 years ago and the original ducts were used without modification.

    It seems to me that the mfr markings would be important to follow and I'm curious what the experts on this board think.

    I'm thinking about a new furnace and I am guessing that some installers may think the input duct needs replacing and that will probably add significant cost.

    Thanks

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,158
    Dealer needs to determine the correct size furnace for the house. Not guess. Not base on what's there. Consider size of A/C. Then look at the amount maximum airflow needed by the system. In northern climates, it likely may be heat needs to move more air. But anyway, after determining how much airflow is needed, he can tell you if the duct size is sufficient. It is more than the trunk line, it is the branch ducts as well. And the return.

  3. #3
    Join Date
    Jan 2008
    Posts
    2,023
    I'm thinking the old furnace needed 24 X 15 filter area. Thats different from duct size.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  4. #4
    Join Date
    Aug 2010
    Location
    West Monroe, LA
    Posts
    1,535
    Quote Originally Posted by 2old2rock View Post
    I'm thinking the old furnace needed 24 X 15 filter area. Thats different from duct size.
    Agree! Is the current furnace Loud? Just makes you wonder how the furnace has lasted this long if the wrong return was used?

    I would get a pro out to overlook the system and get a load cal. Done to size the furnace as needed not go by what is their now. Too many times we all see oversized furnaces with return air grill undersized which makes for loud unit, decreased performance and shorter life span of the unit.

    Is it a heat only system or do you have a a/c unit also for cooling? Cooling often requires more cfm/return to work propely compaired to heating system only.

  5. #5
    Join Date
    Dec 2003
    Posts
    28
    Quote Originally Posted by duckman06 View Post
    Agree! Is the current furnace Loud? Just makes you wonder how the furnace has lasted this long if the wrong return was used?

    I would get a pro out to overlook the system and get a load cal. Done to size the furnace as needed not go by what is their now. Too many times we all see oversized furnaces with return air grill undersized which makes for loud unit, decreased performance and shorter life span of the unit.

    Is it a heat only system or do you have a a/c unit also for cooling? Cooling often requires more cfm/return to work propely compaired to heating system only.
    The furnace doesn't seem loud to me. When I had the A/C installed maybe 15 years ago the tech was a little concerned but he said his pressure checks were good.

    When I get estimates for a new furnace I would like to have some "independent opinions" (you guys) about the need for a bigger input duct.

    Obviously, you don't have much information to give an opinion so maybe you can say if it is typical to use an input duct sized according to the furnace markings on the side panel?

  6. #6
    Join Date
    Jan 2012
    Location
    California
    Posts
    86
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 03-07-2013 at 07:08 PM.

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