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  1. #1
    I have two conflicting quotes from contractors. I want to change my duct system from a radial system to an extended plenum system. The extended plenum would be 8X18X14 feet long and have 5-6"round ducts and 1-9"round duct installed off of it. The order the duct take-offs are 6", 6",9",6",6",6",at 16" spacing at the last 6 ft of the duct. Each of them running up between the floor joists to their destination. One contractor told me that there should be a reduction in size after the 9"duct, the other contractor told me there is no need because it is such a short run, and they were just trying to run the price up. Who is right??

    I also asked to have the duct jog 1 foot to the side immediately after leaving the furnace, what is the best way to accomplish this? Curved ducts or some type of modified takeoff from the furnace plenum. I have a diagram that says it is from ASHRAE 1952 guide, Warm-air and return-air bonnet or plenum, figure D that I was thinking of. It angles from the furnace, and I thought I could have this made to eliminate adding 2 turns in the duct. I dont see it on the air duct calculator that I have unfortunately.
    I have been doing some research to figure this out but don't know the best answer.
    This work is being done in an OLD house so the system was retrofitted in the past to accomidate forced air, and is not set up the best. I would like to finish the basement and the extended plenum will clean up all the ducts running every where. Thanks for any help you can give!!

  2. #2
    Join Date
    Mar 2005
    Location
    Suppy NC
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    4,513
    yes there should be a transition after the 9 in run. also the trunk line should be 2ft longer after the last run for static pressur to be mantained
    if you leave out the transition or reducer the proper cfms will not go where you want them to go. you should also have damper installed in each of the runs so you can balance the system afterwards

  3. #3
    The takeoffs for the ducts are in consecutive joist spaces so the 2-feet you mentioned will be hard to achieve. I will check to see if there is some spacing to recover the cfm.

  4. #4
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    I agree with tinknocker about needing to reduce the trunk size. If you don't, the velocity of the air in the trunk line will likely drop to low.

    Need to follow Manual D in the design of it to be sure though.
    If more government is the answer, then it's a really stupid question.

  5. #5
    Join Date
    Apr 2004
    Posts
    7,680
    Yes, I too agree, reducing to a smaller duct actually costs LESS in metal.

    9"? What kind of register do you plan on installing here? Or does it split? Not that I need to know. But with that yes you better reduce to keep control of your static and things should be closely calculated. The other problem is the 16" spacing... by this you mean every joist space? That may give you a tight space to make your reduction.

    Manual D for sure.

  6. #6
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    There are only two reasons to reduce the size of the duct: (1) save money, as small duct is cheaper than large duct, and (2) reduce thermal losses by reducing the surface area of the duct.

    If you reduce the size of the duct, the velocity of the air will increase. This increases the velocity pressure at the expense of static pressure. Lower static pressure means less air into the branch ducts.

    Higher velocity also increases friction losses.




  7. #7
    Join Date
    Oct 2001
    Location
    Waterford Michigan
    Posts
    2,668
    With such a short run an extended plenum system should be fine I vote for contractor #2. Nine inch is a non-standard size try 8" or 10".

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    In your case either one,properly designed will wok fine.

    Reducing is required/needed in long trunks.

    When it's reduced the velocity is maintained,not increased,as the cfm has been reduced(in the trunk) by the air going to the previous branchs.

  9. #9
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Originally posted by dash
    Reducing is required/needed in long trunks.

    Why, Dash?

  10. #10
    The 9 inch duct serves 2 registers, in the dining room and living room on the same wall one on each side. Basically there are 2 vents on opposite sides of the wall in two adjacent rooms. All of the doorways are wide and the two rooms are basically one, because this is an old house and they like big room openings. So is a reduction really needed??!! It seems easier to install on uniform sized duct than to reduce it after the 9inch duct. How should the takeoffs be spaced, ie staggered back and forth along the extended plenum duct or from the center of the extended plenum duct.


  11. #11
    Join Date
    Jan 2001
    Posts
    7,748
    Doesnt manual D recommend a reduction in trunkline after you drop 50 percent of the air....like if you are putting out 1000 cfm from the air handler.... you size the duct coming off the air handler to handle 1000 cfm but after you drop 500 cfm thru branch lines you then reduce the duct to handle 500 cfm...then after you drop 250 more cfm thru branch lines you reduce the duct to handle 250....and so on.


  12. #12
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by Panama
    Originally posted by dash
    Reducing is required/needed in long trunks.

    Why, Dash?

    Longer single sized trunks,create large difference s in the lengths of the closer and farther away runs.This causes air not to "turn",into the runs closer to the fan and over feed those farther down the "single sized " trunk.

    Reducing the trunk makes the "fitting losses" more equal between the close and far branchs ,allowing the air to turn easier(similar) in all branchs.

    All this and other systems are covered in Section 1,Manual D,from ACCA.

  13. #13
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    first installing a reducer/transition is not the cheap way and truth be known adds to the cost of the duct. some one has to make it and that is time and metal.
    second air is the same as water it seeks the least ammount of resistance. so with out a trans. the air with los static presure and just flo any where it can. the reason for static presure is to push the air out of the trunk and into the runs and still maintain the same presure and fpm at the register. this way the air can be controled easer and balanced easer. with a 9 ince run that has two takeoffs on it you will need that control
    leaving out the trans will just make control of the air harder. also the blower is designed for a certan static pressure and this is why is it important to maintain or the blower can free wheel and not push the air as well

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