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  1. #1

    New A/C unit vs return duct size

    Our fire station had two new a/c units installed today.I have a question regarding one. I'm posting here because the station is more residential in style than commercial.

    In our barracks side, which measure 30' x 18' qith another 10' x 10' room they installed a York 2 ton unit on the roof replacing the old 2 ton unit. We noticed tonight it doesn't want to cool below 72 degrees and it's in the mid 80's outside. We think the old return duct is too small at 13.5" x 15.5".

    Could this be a problem and what should the proper size be? In the large room there's two 10" x 10" registers and in the small room a 10" x10". Just some general info would help us out a lot.

    Thanks,
    Dave

  2. #2
    Join Date
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    Is that the size of the inside of the duct, or the size of the return grille?
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  3. #3
    Join Date
    Aug 2007
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    Northern VA 38 degrees N by 76 degrees W
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    Quote Originally Posted by engineco16 View Post
    Our fire station had two new a/c units installed today.I have a question regarding one. I'm posting here because the station is more residential in style than commercial.

    In our barracks side, which measure 30' x 18' qith another 10' x 10' room they installed a York 2 ton unit on the roof replacing the old 2 ton unit. We noticed tonight it doesn't want to cool below 72 degrees and it's in the mid 80's outside. We think the old return duct is too small at 13.5" x 15.5".

    Could this be a problem and what should the proper size be? In the large room there's two 10" x 10" registers and in the small room a 10" x10". Just some general info would help us out a lot.

    Thanks,
    Dave
    If that is the size of the return duct dimension then it is not the problem. That will handle 800 cfm at 0.06 and a velocity of 600 fpm. Not bad at all.

    Was both of the indoor units replaced?

  4. #4
    Join Date
    Aug 2012
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    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; 08-24-2012 at 10:24 PM. Reason: non AOP member

  5. #5
    Quote Originally Posted by toocoolforschool View Post
    Is this a package unit???
    If you mean a all-in-one, then yes. Everything is on the roof in one unit. The return duct tube size is 13". Still dumbfounded wny it's so slow to down. Took over four hours to cool from 73 to 69 from 7pm to almost midnight. Seems terrible to me. It b lows like hell, just doesn't cool.

  6. #6
    Join Date
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    Quote Originally Posted by engineco16 View Post
    If you mean a all-in-one, then yes. Everything is on the roof in one unit. The return duct tube size is 13". Still dumbfounded wny it's so slow to down. Took over four hours to cool from 73 to 69 from 7pm to almost midnight. Seems terrible to me. It b lows like hell, just doesn't cool.
    have them check both the airflow and charge. It should have dropped 4 degrees in one hour.

  7. #7
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    How many people were in these rooms last night when you were trying to get the temp down to 69?

    At 7PM, wasn't it still pretty warm outside?
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  8. #8
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    May 2012
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    Your duct are not undersized. How many people are in the rooms? Residential units defer because they assume not many people and there isn't any outside air requirements. Your 2 ton might be a little small if you have a lot of people in those rooms and the fact that you need outside air.
    You can call me Sam

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  9. #9
    Join Date
    Sep 2008
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    Western PA
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    toocoolforschool

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