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  1. #14
    Join Date
    Jul 2005
    Posts
    717
    Biiscuitnt; but there will still be more rpm carried into the 8" because of the 10" (2" diam. gain in volume.)
    Air don't abruptly slow down when confronted with a streamlined resistance, it slows gradually, and depending on the (tel)total euivalent length of the run, it may well achieve some added gain in air flow at its termination point.... As to exactly how much, only instruments (air measure)will tell.

  2. #15
    Join Date
    Jul 2004
    Posts
    322
    Originally posted by davidr
    I would say increase the size of the duct all the way to the boot,or add an additional branch.

    Depends on how much CFM you are short to the room. [/B]
    I'm sure this would work better. The problem is that this duct (to master bedroom) travels between first and second floors and cannot be accessed without tearing up the drywall ceiling. The plenum and air handler are in an attic above the kitchen area and garage which is only one story. I can get to them but the duct disappears between the floors after a few feet.

    I thought of a duct booster fan but the pros here don't seem crazy about them and it would have to be at the start of the run. I also thought of putting the duct on top of the plenum (it's a vertical upflow system) but that idea seems risky. Right now all the ducts (which all branch into 2 to four smaller ducts) come off the sides of the supply plenum. I've partially closed some of the other registers but that only seems to reduce airflow from them without helping the two in my bedroom that needs more air. Note: I was wrong in my opening post. These two registers are fed by a 9 inch, not an 8 inch, but I'm still wondering about increasing the takeoff on this duct.

  3. #16
    Join Date
    May 2005
    Location
    Mississippi
    Posts
    102
    I am not going to beat this one to death, take-offs are cheap, go ahead and give it a shot, it wont hurt anything. You could also try installing some manual volume dampers in the other ducts coming off the plenum to restrict the airflow to those trunks. just be careful if you do this and dont get carried away with closing off ot much adn restricting the entire system.

    regards

  4. #17
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    1.Move it to the top of the pleunum,add a damper if it's too much air flow.



    2. http://www.fantech.net has booster fans that work and with static readings.current and desired cfms ,will size the fan for you.

    [Edited by dash on 08-02-2005 at 03:29 PM]

  5. #18
    Join Date
    Jul 2005
    Location
    Orange County, NY
    Posts
    936
    Just how much flex duct is there on that run?

  6. #19
    Join Date
    Jul 2004
    Posts
    322
    Originally posted by core
    Just how much flex duct is there on that run?
    Guessing 25-30 feet of 9-inch flex, then one of those awful triangle boxes, then two 6-inch flex a few feet to the registers.

  7. #20
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Ducted return large enough,or a good retuen path??

  8. #21
    Join Date
    Jul 2004
    Posts
    322
    Originally posted by dash
    Ducted return large enough,or a good retuen path??
    I'm okay there. Rarely close the door.

  9. #22
    Join Date
    Jul 2005
    Location
    Orange County, NY
    Posts
    936
    FatBob

    That sounds like your problem.

    1. To much flex.
    2. If designed at (.1"WC friction) then 9" run = +(-) 320 CFM. 6" run = +(-) 110 CFM.

    110 + 110= 220 CFM
    320 - 220= 100 CFM (that may be availible in that duct)

    Increasing the 9" to 10" would be a huge waste of time and money.

    What does the 2 6" runs feed??

  10. #23
    Join Date
    Jul 2004
    Posts
    322
    Originally posted by core
    FatBob

    That sounds like your problem.

    1. To much flex.
    2. If designed at (.1"WC friction) then 9" run = +(-) 320 CFM. 6" run = +(-) 110 CFM.

    110 + 110= 220 CFM
    320 - 220= 100 CFM (that may be availible in that duct)

    Increasing the 9" to 10" would be a huge waste of time and money.

    What does the 2 6" runs feed??
    The 6" go to two 6x8 one-way registers. The registers were the 3-way stamped kind but I replaced them which helped a little. I don't think the registers are the problem. I also think the installer was pretty careful to seal the ducts and avoid unnecessary bends and kinks. I just need to get more air into the 9" trunk.

  11. #24
    Join Date
    Jul 2005
    Location
    Orange County, NY
    Posts
    936

    Originally posted by fat bob
    The 6" go to two 6x8 one-way registers. The registers were the 3-way stamped kind but I replaced them which helped a little. I don't think the registers are the problem. I also think the installer was pretty careful to seal the ducts and avoid unnecessary bends and kinks. I just need to get more air into the 9" trunk.

    Why would you think the installer was careful???? To me 20 to 30 feet of flex duct is careless! To much friction loss in flex duct.
    To maximize these 6" ducts you may have to remove the flex and run insulated hard pipe.

    How big are the rooms that are feed by the 6" ducts?

  12. #25
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by dash
    1.Move it to the top of the pleunum,add a damper if it's too much air flow.



    2. http://www.fantech.net has booster fans that work and with static readings.current and desired cfms ,will size the fan for you.

    [Edited by dash on 08-02-2005 at 03:29 PM]
    If you can repalace the tri-box with a wye,that would help,if is all unaccessable ,number 1 or 2 above will solve the problem.


    The TEL of a Tri-box is high enough that ,just chaging the takeoff,will not make enough difference,I had previously assumed this was one 8/9" run to a grille.

  13. #26
    Join Date
    Jul 2004
    Posts
    322
    Thanks Dash et al. for your suggestions.

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