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  1. #1

    Possible lack of return causing AC not to keep up.

    I have a ranch with a walk out basement around 4500 square feet. I built in 2012 and on 90 degree days the upstairs can't keep up. When they ran the computations we said a 4 ton would do the job, but when it hits 85 or above the unit can't keep temp. Here are some basic facts. Bryant evolution, 4 ton, 4 zones. Two upstairs two downs stairs. My contractor swapped out the other unit and replaced with a five ton unit. I think there is not enough return and the unit just can't keep up. He is telling to cover all my windows and that will help but it is a contemporary home and it was not designed to have blinds and drapes everywhere. We spray foamed the entire house. My question is how much return capacity should I have and what questions should I be asking. He just cranked up the blower and the system is a whistling., which is unnerving.

  2. #2
    On those nights even when the sun goes down it often takes it till 3 in the morning before the not can catch up. I live in Indiana.

  3. #3
    Join Date
    Jan 2004
    Under sized duct system is probably the main problem.

    What size return do you have.
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  4. #4
    Join Date
    May 2012
    San Diego, CA
    9 tons should be more than enough for your house. How is your airflow from your supply registers? What temperature is coming from those supply registers. Lastly, does your ac keep running constantly or does it seem to switch off even though the room isn't cooled enough?.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  5. #5
    Join Date
    Jul 2006
    time to call someone else . i would ask for a copy of the manual j load calculation and the manual d duct design . then have a test and balance company check the airflow

  6. #6
    Join Date
    Mar 2009
    Mount Holly, NC
    my bet is on leaky or undersized or both supply ductwork. the evolution handles restrictions well, but leaky ducts cause a multitude of problems. have an energy audit done that checks duct leakage.

    the house sounds about the same size as my parents home, but 1/2 of your footage is underground... mine's all above. and it's only got 4.5 tons on it. in NC the temps climb and the humidity blankets everything... it handles it fine.
    The TRUE highest cost system is the system not installed properly...
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    The A/C repairman

  7. #7
    Join Date
    Dec 2010
    The Quad-Cities area (midwest).
    Quote Originally Posted by catmanacman View Post
    time to call someone else . i would ask for a copy of the manual j load calculation and the manual d duct design . then have a test and balance company check the airflow
    Ya, right. Good luck. If the contractor had that, why did he install a 4 ton and then switch it out for a 5 ton?

    The ductwork probably isn't enough for the original 4 ton, and then add the zoning factor!

    He should have over sized the supply duct 25% but probably didn't thinking the modulating dampers will take care of the poor duct design. The modulating damper will keep the A/C from freezing, however, the zone calling isn't getting enough A/C.

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