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  1. #1
    Join Date
    Sep 2008
    Posts
    5

    Simple question about final register in HVAC duct system

    Hi all,
    So, I have a simple question about the final register in our ductwork.
    The ductwork in our 1600 sq ft condo consists of a 12' run of ~14"x10" ducts with 1 T split at the beginning. The ducts don't taper off anywhere except at the last register on one end, where it's shrunk down over a length of 4" to an 8"x6" register. Unfortunately, this last register uniquely serves almost half of our condo (poor design, I know, but it's a retrofit to an even worse design). I was wondering if we increase the size of the final register to a ~14"x10" register (so we're no longer tapering the duct), will it increase airflow through the final register? Is this short term tapering needed for airflow through the rest of the ducts in the house?
    Thanks a bunch,
    David

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,327
    Not really.

    Enlarging it will reduce air flow to the other areas.

    How much? Can't tell from here.
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  3. #3
    Join Date
    Sep 2008
    Posts
    5
    we're fine reducing air flow to other areas.
    As it is, there is easily a 5+ degree temperature differential between the rooms, simply because 1/3 goes to one bedroom, 1/3 to another, and 1/3 to the kitchen+dining room+living room through the one duct.

    I looked at the final duct again this morning, and it doesn't taper off to the smaller register, but rather a 6"x4" hole is cut into the end of the ~14"x10" duct, and a 4" duct is attached to this hole, which then increases in size over the 4" until it hits the 8"x6" register.

    So, I'm wondering if this barrier is unnecessarily reducing potential air flow to the final register.

    Thanks,
    David

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,327
    A 4" duct, moves very little air.
    So yes, its slowing a lot of air.
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  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,358
    Is the 8 x 6 register of the stamped steel variety? This register does not have adjustable blades, only a damper with movable vanes behind the fixed blades. These registers are restrictive to airflow.

    Depending on where this supply register is located, either a curved blade or an adjustable blade register might net you more airflow without any modifications to the ductwork. Since you're already on record as stating the other rooms receive too much airflow, changing the register in the room lacking enough air to a better design might get you a little better performance.

  6. #6
    Join Date
    Sep 2008
    Posts
    5
    Quote Originally Posted by beenthere View Post
    A 4" duct, moves very little air.
    So yes, its slowing a lot of air.
    Sorry, didn't mean the connector duct is only 4" diameter, but 4" in length.
    This 4" connector connects to the main duct as a 6"x4" duct, then expands in size over the 4" to connect the 8"x6" register.

    I understand that I'm probably not being very clear here, sorry.
    as an fyi, all of the other registers in the system are 8"x3"

    maybe this simple diagram will help?

    HTML Code:
                                    Bedroom 1
                                         ^
                                         |
                                         |
                                  (14"x10" duct)
                                         |
                HVAC --(14"x10"duct)---->|
                                         |--- (14"x10" duct)-->Bedroom2
                                         |
                                  (14"x10" duct)
                                         |
                          (abrupt change to 6"x4" duct)
                                         |
                               (final 8"x6"register)

  7. #7
    Join Date
    Sep 2008
    Posts
    5
    Quote Originally Posted by shophound View Post
    Is the 8 x 6 register of the stamped steel variety? This register does not have adjustable blades, only a damper with movable vanes behind the fixed blades. These registers are restrictive to airflow.

    Depending on where this supply register is located, either a curved blade or an adjustable blade register might net you more airflow without any modifications to the ductwork. Since you're already on record as stating the other rooms receive too much airflow, changing the register in the room lacking enough air to a better design might get you a little better performance.
    Hi,
    no the 8x6 register is an adjustable blade like this one:


    [/IMG]

  8. #8
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    You've got a 10" X 14" main trunk line running the lenght of your layout, what is being used for the branch runs to the various rooms 6" or 8" round going to the floor vents using a 3-1/4" X 10" or 3-1/4" X 12" for your floor registers ? Your end run that you say your having a problem with, it would be best to extend the 10" X 14" trunk run past or as far as you can go beyound the end run floor register and then branch off with a 6" or 8" round depending on the size room it needs to serve. It wil give you much better airflow and will help balance air velocity thru out the rest of your house. Of course thats just my opinion. Also might want to take a good look at the type of floor registers being used as they may need to be changed out to increase the air pattern and throw of air to the room. Also may want to take a look at your return air register at the sametime as you may need to add one or more return air registers depending on what you already have as we do not know the layout of your house but if the furnace is not taking in enough return air based on the amount of air it has to provide to all the rooms of the house could also be a problem.

  9. #9
    Join Date
    Sep 2008
    Posts
    5
    Hi,
    the 10"x14" main line runs the length of the ductwork, and the 3.25"x10" wall registers come directly off of this. In both bedrooms, there are two of these registers. Unfortunately, due to the layout of the condo, I can't extend the trunk past where it is now, without tearing down the entire ceiling.
    The return vent is large enough, and like with extending the main trunk, nothing can be done to that without tearing down walls..
    Thanks.
    David

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