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  1. #1
    Join Date
    Jun 2005
    Posts
    2
    We just had A/C and forced air with new duct work installed in an older house. The air flow out of the each duct varies considerably. On duct will not blow off a piece a paper and another feels like a wind tunnel. So my question is there a device to measure the air flow at the registers. What airflow should we be looking for at each register and how much difference between registers is acceptable. All the registers are the same size 3.5" x10"

    Thanks for your help.

    Brian

  2. #2
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    go to WWGRANGER.com & ck $ for air flow hood = $$$

    forget the amount of air velocity --
    are the rooms comfortable?
    are the rooms within 3F of each other just after blower shutoff?

    if you think any one supply has too much velocity, then just have another added, or get bigger pipe & outlet.

  3. #3
    Join Date
    Jun 2005
    Posts
    2
    Originally posted by cem-bsee
    go to WWGRANGER.com & ck $ for air flow hood = $$$

    forget the amount of air velocity --
    are the rooms comfortable?
    are the rooms within 3F of each other just after blower shutoff?

    No the room (35'x18' west facing)where there is a lack of air velocity isroughly in the afternoon about 15 to 20 degrees hotter than the rest.


    if you think any one supply has too much velocity, then just have another added, or get bigger pipe & outlet.
    There is just one main supply trunk line in the basement, the duct with the most velcity is ay the end of it. Is there a rough formula I could plug in to see how big the ducts should be. (i.e. one 3x10 register needs a 6" round duct, 2 6" round duct need a 4x10 duct an so on.

    Thanks
    Brian

  4. #4
    Join Date
    Nov 2001
    Location
    East Texas
    Posts
    883
    Install dampers at each take off of the main duct then you can tweak it.

  5. #5
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Manual D

    >>Is there a rough formula I could plug in to see how big the ducts should be.

    There is ACCA MANUAL D, which calculates precisely how big each duct should be. It is rather more complicated than a formula but the pros who use it tell us it is a lot more correct. You can buy the book around $40, but I think it would serve you better to hunt down an AC technician who knows how to do Manual D calculations. That guy will be more knowledgeable than 90% of the techs out there.

    Best of luck -- P.Student

  6. #6
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579
    If you have a heat pump or aux electric elements you can use this method.

    http://www.bacharach-training.com/norm/cfm.htm


  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Cheap method

    Just remembered something... I have heard someone on this board tell about holding a big garbage bag up to the vent and measure with a stopwatch how many seconds it takes to fill.

    You might have to do some math to get actual CFM but if you compare one vent with another you certainly would identify which is flowing more, and by how much.

    I would really appreciate hearing the criticism of a pro for this method. Has this been documented before?

    Best of luck -- P.Student

  8. #8
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247

    Re: Cheap method

    Originally posted by perpetual_student
    Just remembered something... I have heard someone on this board tell about holding a big garbage bag up to the vent and measure with a stopwatch how many seconds it takes to fill.

    You might have to do some math to get actual CFM but if you compare one vent with another you certainly would identify which is flowing more, and by how much.

    I would really appreciate hearing the criticism of a pro for this method. Has this been documented before?

    Best of luck -- P.Student
    This is a little trick the scientist at Lawrence Berkley National Laboratory cooked up.
    Personally I view this method to be at the least very inaccurate.
    When placing a trash bag over any diffuser you immediately change the airflow ditribution of the duct system due to an unidentifiable back pressure.
    Even with the use of a flow hood there will be some back pressure but usually not enough to effect the readings on residential type diffusers.
    IMHO the only reliable way to test airflow through ducts is by use of a flow hood or a traverse of the duct feeding the diffuser while the system is in actual operation.
    Plus when you add the fact that there is no trash bag that has NIST calibration certification you can see my reasoning for questioning it as being valid.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  9. #9
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Originally posted by b-astill
    We just had A/C and forced air with new duct work installed in an older house. The air flow out of the each duct varies considerably. On duct will not blow off a piece a paper and another feels like a wind tunnel. So my question is there a device to measure the air flow at the registers. What airflow should we be looking for at each register and how much difference between registers is acceptable. All the registers are the same size 3.5" x10"

    Thanks for your help.

    Brian
    Brian check out http://www.nationalcomfortinstitute.com for contractors who have been trained to deal with the problems you are expieriencing.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  10. #10
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,509
    size of the regester is not much of a factor. what size pipe is ran to the regester. size of the trunk line and how many runs off it between each reducing transition in the main trunk. how many cfm does the room need and btu
    just feeling the register is not going to tell you any thing. are the rooms reaching temp. if not how much differance is there in temp. balanceing is not hard to do.
    since you just had the system installed call the contractor and ask them to balance it for you.
    if the system was sized and installed properly this should not be a problem and they should know how to balance it out. may take two or three trips but it is thier job

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,755
    Are you sure the reg with no air coming out, isn't a return.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  12. #12
    Join Date
    Mar 2004
    Location
    Gone
    Posts
    5,340
    Originally posted by tinknocker service tech
    if the system was sized and installed properly this should not be a problem and they should know how to balance it out. may take two or three trips but it is thier job
    If the system was installed properly, it would not need to be balanced out like it does at the present time. If they could not install it correctly, I guarantee you they do not have the equipment to balance it out correctly either.

    It only takes one trip to balance a system if you have the proper tools and knowledge.

  13. #13
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Originally posted by tinknocker service tech
    size of the regester is not much of a factor.
    The selection of proper registers is very important & often overlooked.
    This selection will determine the throw & spread of the conditioned air at a given CFM.
    Take a look at ACCA Manual T for more info on this.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

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