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  1. #1
    Join Date
    Aug 2008
    Posts
    4

    Furnace output opening Vs A-Coil opening…

    Hey All,

    Up here in Minnesota there is only a small need for AC and I would appreciate your opinion on this.

    My home is a two stories and finished basement with around 900 Sq Feet per floor.
    About a year ago my AC went out and I had a large installer put in a new one. (1.5 Ton) It never seemed to work as well as the old one about a month ago I asked them to come out and look at it. They said the A-coil was a little dirty, cleaned it and told me it was fine.
    I remember the old A-Coil was much larger then the now A-Coil.

    Here is my query:
    My furnace/heater has an exit air opening of 20x20”. My plenum that the A-coil sits in is 19x21”. However when I opened it up to see if it was dirty today, I noticed something odd. The New a-coil sits very loosely on top of some makeshift brackets. The Shelve/Bracket is obviously new and put in to hold the a-coil in there. The part that confuses me is with the smaller A-Coil and shelving the opening for the air to flow through is 13x6”.

    Is that OK? 20x20 restricted down to 13x6???

    Any thought or recommendations would be greatly appreciated!

    Thank You for taking the time to read this.

  2. #2
    Join Date
    Jul 2007
    Location
    Chicagoland
    Posts
    474
    This is most likely NOT OK. In addition to the poor cooling you are experiencing, that great a restriction can create high static pressure resulting in excessive temp rise in heating mode. Your furnace model tag will list a "max. external static pressure" and a temp rise range for safe operation, have the installing co. measure these values and prove to you they are within spec.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,808
    What size is the furnace.
    Model and brand.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Aug 2008
    Posts
    4
    Quote Originally Posted by beenthere View Post
    What size is the furnace.
    Model and brand.
    Dayton GF9S
    http://www.grainger.com/Grainger/items/4NA53

    After looking through the install book of the furnace, it looks like the recommendation is an air supply opening minimum of 14x20” or 280 in/Sq.

    The restricted 6x13 is only 78 in/Sq… (I think I am starting to understand this…)
    Thank You.


    Quote Originally Posted by Delta-T View Post
    This is most likely NOT OK. In addition to the poor cooling you are experiencing, that great a restriction can create high static pressure resulting in excessive temp rise in heating mode. Your furnace model tag will list a "max. external static pressure" and a temp rise range for safe operation, have the installing co. measure these values and prove to you they are within spec.
    I will ask them to come out.
    Thank You.



    Could this condition cause the heat tubes in the furnace too crack?

  5. #5
    Join Date
    Nov 2007
    Location
    Westlock, Alberta, Canada
    Posts
    57
    It could cause your furnace to cycle off and on on high limit. Your exchanger should be ok with the higher temps as the high limit should stop it before it gets too high.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,808
    Quote Originally Posted by uaplumber View Post
    It could cause your furnace to cycle off and on on high limit. Your exchanger should be ok with the higher temps as the high limit should stop it before it gets too high.
    A gas furnace that rides the high limit uses a lot of extra gas.
    And the heat exchangers don't last long.

    Thats a 100,000 BTU input furnace. Not good to restrict it as much as that coil does.
    Contractor locator map

    How-to-apply-for-Professional

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