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  1. #1
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    Oct 2011
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    12" trunkline on 2.5 ton AC...lol.

    I dont usually do new construction but I am going out to help on a rough-in on Saturday to help catch us up. I was looking at the equipment and the associated trunkline sold. 12" main supply trunk and 2.5 ton equipment...lol.

  2. #2
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    That's not out of the question, provided the trunk isn't wire flex, and the unit is spec'd for higher statics.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

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  3. #3
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    Thread Starter
    Quote Originally Posted by BBeerme View Post
    That's not out of the question, provided the trunk isn't wire flex, and the unit is spec'd for higher statics.
    Dont have the prints in front of me but I think it is about 20' of 12 then 90s and reduces down to 10 and goes another 15-20'.

    I dont know man....doesnt look right to me. And this isnt high static equipment. Standard .5

  4. #4
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    Nov 2006
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    Quote Originally Posted by Adlerberts-Protege View Post
    Dont have the prints in front of me but I think it is about 20' of 12 then 90s and reduces down to 10 and goes another 15-20'.

    I dont know man....doesnt look right to me. And this isnt high static equipment. Standard .5
    So it is the duct system we find in 90% of the houses.
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  5. #5
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    Thread Starter
    It is, but we actually have a quality guarantee that the system will be designed within system specs, including proper static. This one worries me.

  6. #6
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    Jul 2009
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    South Carolina
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    There is a common misconception about how much airflow can be achieved under static pressure because TEL of 100 ft. of duct is assumed. 25ft. of a 12" supply can easily handle 1000 CFM's at .1 static but the velocity will be high. It all depends on the rest of the duct system. Manual D compensates for the whole duct system by using the TEL of the longest run & uses the standard of static per 100 ft. of duct. It's an excellent system but it is not perfect.
    I worked some of those numbers out on this ductulator: https://oceanhvac.com/ductulator/
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  7. Likes CEAS-AC-TECH liked this post
  8. #7
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    Thread Starter
    Quote Originally Posted by garyed View Post
    There is a common misconception about how much airflow can be achieved under static pressure because TEL of 100 ft. of duct is assumed. 25ft. of a 12" supply can easily handle 1000 CFM's at .1 static but the velocity will be high. It all depends on the rest of the duct system. Manual D compensates for the whole duct system by using the TEL of the longest run & uses the standard of static per 100 ft. of duct. It's an excellent system but it is not perfect.
    I worked some of those numbers out on this ductulator: https://oceanhvac.com/ductulator/
    Ya, I used to teach Manual D to a lot of companies when I was in Seattle. I know that our salesman are using Wrightsoft, and that they are supposed to be using it. Highly doubtful on this one.

  9. #8
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    Dec 2012
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    Thats pretty typical on a mobile come. Static pressure will be high no doubt.


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  10. #9
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    Jan 2004
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    Quote Originally Posted by garyed View Post
    There is a common misconception about how much airflow can be achieved under static pressure because TEL of 100 ft. of duct is assumed. 25ft. of a 12" supply can easily handle 1000 CFM's at .1 static but the velocity will be high. It all depends on the rest of the duct system. Manual D compensates for the whole duct system by using the TEL of the longest run & uses the standard of static per 100 ft. of duct. It's an excellent system but it is not perfect.
    I worked some of those numbers out on this ductulator: https://oceanhvac.com/ductulator/
    20' of 12" with 2-90s. When you include the transition to round at the plenum. Will most likely be close to 100' of TEL.
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  11. #10
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    Quote Originally Posted by beenthere View Post
    20' of 12" with 2-90s. When you include the transition to round at the plenum. Will most likely be close to 100' of TEL.
    No way is it going to push 1000 cfm.

  12. #11
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    Quote Originally Posted by Adlerberts-Protege View Post
    No way is it going to push 1000 cfm.
    Why not?


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  13. #12
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    Mar 2015
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    Quote Originally Posted by Core_d View Post
    Why not?


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    Put enough blower in it will
    I don't have a ducalator but IIRC 12" @ .06 or .08 is 700 cfm . It's too darned small unless it's a MH with hi static blower. Run a 10 to a couple of close boots will help

  14. #13
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    Quote Originally Posted by oldguy1949 View Post
    Put enough blower in it will
    I don't have a ducalator but IIRC 12" @ .06 or .08 is 700 cfm . It's too darned small unless it's a MH with hi static blower. Run a 10 to a couple of close boots will help
    Any constant torque motor will put out its design cfm at .9 static or higher with one of the upper speed taps used. Even a psc motor with most systems should do good enough. It happens everyday.


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