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Thread: c.f.m. per ton

  1. #1
    Join Date
    Sep 2003
    Ocean County, N.J.
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    c.f.m. per ton

    Back when I was learning about air flow the thinking was 400 c.f.m. per ton. Has that changed? I overheard a conversation the other day and the subject was that they were talking about a job they were going to do and size the duct work ( supply and return ) at 450 c.f.m. per ton. What's that about? Positives and negitives.

  2. #2
    Join Date
    Dec 2009
    Roswell, Georgia
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    350 to 450 depending on how much humidity you want to remove. Areas in southern USAand humid areas lean towards the lower side, drier climates the higher side. Always have return sized at .08 static pressure or lower if possible.

  3. #3
    Join Date
    Dec 2002
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    Its really based on your SHR ( sensible heat ratio). In Phoenix ( Arid climate) with a high SHR your 450 cfm per ton would be good, however in Miami ( Humid Climate) with a low SHR you would be looking at more like 350 cfm per ton. Where do you get your sensible heat ratio? That would be manual J
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  4. #4
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
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    In the end all green grass climates need air flow that gets you to <50%rH in the home. If you run the stat at 75^F, 50%RH is a 55^F dew point. To provide a 55^F dew point you need a 45-48^F coil temperature. Measuring air flow is difficult has to many variables. With the home at your desire temp and outside at typical cooling temps, slow the air flow until you get a 45^F coil temp. Then check for sweating spots on the supply and insulate with close cell rubetex.
    This will get you a comfortable home. For low/no cooling loads and high outdoor dew points, add a small whole house dehumidifier. Also consider the filtered fresh air ventilating option like the Ultra-Aire has for when the home is occupied. This will purge indoor pollutants and renew oxygen.
    Regards TB
    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"

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