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  1. #1
    Join Date
    Aug 2002
    Posts
    391
    I am trying to understand ductwork sizing as it relates to residential AC. I have read Manual D cover to cover but sections 3 and 4 make no sense to me, I just dont get it, call me stupid but those sections are very hard to understand the way its is written.
    Are there any suggestions for other written material that explains those two areas? Thanks



    [Edited by jrc2905 on 02-20-2005 at 09:55 AM]
    The obvious is obvious

  2. #2
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Ask some questions,maybe we can help.


  3. #3
    Join Date
    Sep 2002
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    3,708
    I have one..Question.Lets say you go to a house that has three story.one unit and only one distribution system.

    You head to the third floor to see if you are getting any air.You have lots of noise and what appear to be good air coming out of the vents.

    So knowing that the duct are to small,because you have the jet noise in the house,you decide to change the duct to a
    bigger size to reduce the noise.
    But you can only change the ducts in the first floor,which
    you do so.

    Now you have sucessfully reduce noise,but you have created another problem on the third floor because not only is there
    no noise,theres no air as well.

    So what happen to the air?






  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,171
    Did you replace the trunk line and supply runs to the first floor, or just the supplies.
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    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Aug 2002
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    Air follows the path of least resistance,which after the change ,is to the first floor.

  6. #6
    Join Date
    Sep 2002
    Posts
    3,708
    What if I only increase the size of the duct main trunk,supply and return just before it ties into the riser
    on first floor.Total of 30 feet for both supply and return
    coming off the package unit.
    No change on the sizes of the outlets and only thing added was a 12inch return.Which added another 25 feet.








  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Increasing the size of any of the ducts ,to the first floor,reduces the restistance(static) of that section of ducts,air that went to floors above,now has a less resistive path to follow.


    The blower ,only "knows", air in(return) equals,air out supply,other than leakage,it will follow the least resistive path,from and back to the blower.

    [Edited by dash on 02-20-2005 at 02:37 PM]

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,171
    You should put balancing dampers in the first floor supplies.
    Thats about the only way you'll get the air flow back to the third floor again.
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    How many times must one fix something before it is fixed?

  9. #9
    Join Date
    Aug 2002
    Posts
    391
    What is confusing how to figure the fan operating point?
    The obvious is obvious

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Originally posted by jrc2905
    What is confusing how to figure the fan operating point?

    Manual D,shows fan curves,most all residential "fan Data" from mfrs. is a chart,not a curve as shown in Man.D.

    Look at your mfrs fan data ,if it's a chart ,convert the chart to a curve for one speed,then the other speeds.This should help you understand.Just use a sample chart and plot the points on the fan chart,then connect the dots into a "curve".

    When you get to the actual Friction Loss worsheets,to determine the FR,you see that all you need is the cfm at what ESP,from the fan chart.

    My guess is that mfrs Fan data ,was in the form of fan curves ,years ago.Today the only fan curves a see is for Variable speed fans.

  11. #11
    Join Date
    Jan 2005
    Posts
    44
    Men,
    as a HO, I'm very impressed with the knowledge you have about your craft. You definitely deserve the rates you charge! Dash can you install my system for me? Just kidding, I'm sure the person I have is just as qualified.

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