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Thread: Sizing issue

  1. #1
    Join Date
    Jul 2005
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    Sizing issue

    Greetings, I have some questions concerning sizing of two separate residential split systems in the same house. I have a job that is 2/3s complete and just needed some professional advice about sizing. Both systems R-22 3000 sq ft house two separate split systems, natural gas 90afue furnaces with straight cool, all brand new. It seems most of the hard work is done but owner had to pull contractor off for financial reasons. Now trying to sell house. Anyway first system has 3 ton blower , 4 ton indoor coil . Second system has 4 ton blower and 5 ton coil. Not sure what size condensing units to install or what the contractor had in mind when he oversized the indoor coils. Also wondering about piston size, do I size to indoor coil or outdoor unit? Thanks in advance.

  2. #2
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    Very common to have lager coils then condensers.
    Ask him for the other contractors load calcs. Or do your own.
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  3. #3
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    7 tons for 3000 sq ft?

  4. #4
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    Maybe its all glass walls.
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  5. #5
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    The best way in my opinion is to size the condensers to the furnaces, 4 and 3 ton respectively. Size the pistons to the condenser size and enjoy the SEER boost of the larger evaporator coils. Personally, I would use TXV's instead of pistons to be assured maximum efficiency. Seven tons should do a real good job and save jingle on power bill - shorter cycles...Watch out for humidity if not variable speed furnaces with that high a BTU rating.
    Last edited by leonmiller; 07-08-2008 at 11:46 AM. Reason: afterthought!

  6. #6
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    Quote Originally Posted by leonmiller View Post
    The best way in my opinion is to size the condensers to the furnaces, 4 and 3 ton respectively. Size the pistons to the condenser size and enjoy the SEER boost of the larger evaporator coils. Personally, I would use TXV's instead of pistons to be assured maximum efficiency. Seven tons should do a real good job and save jingle on power bill - shorter cycles...Watch out for humidity if not variable speed furnaces with that high a BTU rating.
    Over sizing leads to many issues.Not going into all of them,just savings.

    Saving will not be there,think of it this way,if you needed two tons and installed four tons,it would run half as much ,at twice the cost.No savings.

  7. #7
    Join Date
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    Quote Originally Posted by leonmiller View Post
    Seven tons should do a real good job and save jingle on power bill - shorter cycles...
    Not correct.

  8. #8
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    Quote Originally Posted by leonmiller View Post
    T Seven tons should do a real good job and save jingle on power bill - shorter cycles...Watch out for humidity if not variable speed furnaces with that high a BTU rating.
    I thought he was kidding *(save jingle on power bills - shorter cycles) & I am still laughing.
    A variable speed blower is not going help much with that setup.

  9. #9
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    Do a load calc, and quit guessing at size.

    If the first contractor was planning wrong. Why do you want to follow in his footsteps.
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  10. #10
    Join Date
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    Quote Originally Posted by udarrell View Post
    I thought he was kidding *(save jingle on power bills - shorter cycles) & I am still laughing.
    A variable speed blower is not going help much with that setup.
    No, he wasn't kidding. You'll save jingle; a few coins, that is.

    AM

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