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  1. #1
    Join Date
    Feb 2016
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    Sizing question -- too small or too big?

    What is the worst possible consequence to undersizing a central A/C,
    and what is the worst possible consequence to oversizing it?

    I'm talking about a difference of a half ton.

    Thank you!

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
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    UNDERrsizing: room temperature rises when approaching environment design conditions

    OVERsizing: short cycling, inadequate humidity and comfort control

    Short cycling presents a distinct possibility of decreased equipment life.

    SIGNIFICANT Lack of humidity control may lead to mold issues.


    HOW DOES ONE EVEN KNOW & can Define
    " Half ton over/ under sized"
    before construction completion and commissioning ?
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by Homeowner60 View Post
    What is the worst possible consequence to undersizing a central A/C,
    and what is the worst possible consequence to oversizing it?

    I'm talking about a difference of a half ton.

    Thank you!
    Do you mind not being able to maintain 75^F during a design or real hot day? Undersized means the temperature will be probably be 2-4^F above the desired temp used in the Manual J calc.
    Also less cycling during part load conditions with lower %RH.
    When there is less then a 30 minute cooling load in a 90 minutes, like overnight/early morning under-sized a/c will not remove significant moisture. Also on rainy days neither unit will provide real humidity control.
    Oversized a/c do slightly less moisture removing with more cycling during part load cooling. If you routinely raise the a/c temp when away from your home, the sightly oversized a/c will regain control of your desired temperature when you return. With occasional extra visitors during hot weather, the extra .5 ton will maintain the desire temperature.
    Neither under or oversized a/c will maintain 50%RH during low/no sensible cooling loads and high outdoor dew points plus the latent load from the occupants.
    If comfort is important, the a/c sized to your max sensible/latent. This will assure you of the desired temperature/%RH during most of hot days. Than add a small whole house dehumidifier will maintain the desired %RH during low/no sensible cooling loads. When you away from your home, you have superior %RH control to avoid any mold/dust mites/sticky a/c odors while the a/c may not be needed.
    So it is real all up to your tollerence level or desire for comfort and good indoor air quality.
    Keep us posted.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
    Join Date
    Feb 2016
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    Thread Starter
    Thank you both for your responses! I had an independent contractor do a manual J, and it came to 3.5 ton. Yet every single contractor I've gotten bids from has insisted on selling me a 4 ton!

  5. #5
    Join Date
    Aug 2002
    Location
    Southold, NY
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    Quote Originally Posted by Homeowner60 View Post
    Thank you both for your responses! I had an independent contractor do a manual J, and it came to 3.5 ton. Yet every single contractor I've gotten bids from has insisted on selling me a 4 ton!
    Now have a Manual "D" performed to see what if any duct modifications are needed.

  6. #6
    Join Date
    Feb 2016
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    Thread Starter
    Thanks, pecmsg. I think one problem is that I have never had central air before, so I don't know
    if I'll only use it on our hottest days. The contractor says that when it gets up to 110 or 115 outside,
    the 3.5 ton will struggle. Maybe I should size it based on the assumption that I'll only use it during
    our worst heat waves?

  7. #7
    Join Date
    Nov 2004
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    SW FL
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    Quote Originally Posted by Homeowner60 View Post
    Thanks, pecmsg. I think one problem is that I have never had central air before, so I don't know
    if I'll only use it on our hottest days. The contractor says that when it gets up to 110 or 115 outside,
    the 3.5 ton will struggle. Maybe I should size it based on the assumption that I'll only use it during
    our worst heat waves?
    ALMOST all forms of life and equipment would struggle at 115'F.

    I wonder WHY this one HOT place is known as Death Valley?

    ZIP _ _ _ _ _
    or
    Saudi Arabia
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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