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  1. #1
    Join Date
    Apr 2007
    Posts
    6

    cfm's of pipe sizes??

    I have been calling 5-50cfm...6-100cfm..7-150cfm...8-200cfm on flex & then a 10 hard pipe as 400 cfm..12 hard pipe 600cfm I think..meaning really how important is it to duct out exactly 1600 tons of air to a 4 ton unit?? What if I go over say 200 cfm meaning duct 1800 cfm on a 4 ton or use a 3.5 unit(1400cfm) with a 1600 duct system..where do I draw the line as to over ducting to a unit...Ive heard some people say a 6 is 125cfm & 7-200cfm & 8-225-250cfm..Are there any good charts or anything out there to know for sure how to duct per ton of air??
    Thanks,Brian

  2. #2
    Join Date
    May 2007
    Location
    Pittsburgh
    Posts
    43
    Not sure if this is what you are after, but there is a chart here and a diagram here.

  3. #3
    Join Date
    Sep 2006
    Posts
    1,253
    What you are not addressing is the velocity of the airflow. As the CFM goes up on a given size duct, the air velocity goes up too. While you could push 200 CFM through a 6" metal duct, it would be quite noisy with it moving at about 1,000 FPM.

    Manual D and a ductulator should provide you with the info you need.

  4. #4
    Join Date
    Sep 2003
    Posts
    1,311
    CFM is only part of it. There is also FPM and actual SP to consider along with grill size. A good airflow will be felt on a 6" pipe at 160 CFM through a 4x10 register. However on the very same system a 6" might also only put out 90 CFM through a 4x10 register and will feel gentle. Go with sizing by Manual D and you'll get what you want where you want it.

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Each job is different,more duct,90's ,takoff,distribution boxes,etc., the larger the duct will need to be .

    Plus with a PSC motor ,you need to set the fan speed buy checking the static and using a fan chart.

  6. #6
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,769
    If the load calc shows a high latent load, and you have 1800cfm going through a 4 ton system, you won't be able to keep the humidity under control.

    If you have a sensible load, and only 1400cfm on a 4 ton unit, you won't pull the temp down.

    So how important is it for the systems you install to do what they are suppose to do.
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