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Thread: VAV Air balancing

  1. #1
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    VAV Air balancing

    Hi

    I am new to hvac world.
    I want to know about air balancing in VAV system.
    I mean what is the process and how to accomplish it.
    If any one having documents about it please share.

    Also what is diversity VAV system ?
    Sorry if this post seems vague, I didn't want to get to wordy.

  2. #2
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    Quote Originally Posted by DMHBMS View Post
    Hi

    I am new to hvac world.
    I want to know about air balancing in VAV system.
    I mean what is the process and how to accomplish it.
    If any one having documents about it please share.

    Also what is diversity VAV system ?
    Sorry if this post seems vague, I didn't want to get to wordy.



    Contact these organizations for educational information.......

    https://nebb.org/

    https://www.aabc.com/



    This is an extremely simplified explanation, .........DIVERSITY in a system is the intentional under sizing of the mechanical equipment....basically, with the intent to save energy and operating cost........



    WAYNE.........where are you.....?

  3. #3
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    Artrose You gave him as good advice as I can. There is too much to cover to advise him in this thread as to how balance should be done on a VAV system. He has to calibrate the VAV's and how to do that depends on the equipment he has and the control program.
    He also needs to know how the diversity is intended to work. An example is that in some portions of the building at a given time during the day the airflow needed to maintain temperature set point will be less than others.
    NEBB is a good place to get the information he needs.
    No man can be both ignorant and free.
    Thomas Jefferson

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  5. #4
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    I do a lot work on this stuff.

    In a nut shell, here's how it goes . . .

    On a small system like an Intellipak, there is no balancing. Volume controllers have been tweaked to be entirely useless.

    On a multi-story building, especially with one LARGE blower, balancing is critical. You need prints to get it right.
    I do a triple evac with nitro to remove non condensables.

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    Quote Originally Posted by BBeerme View Post
    I do a lot work on this stuff.

    In a nut shell, here's how it goes . . .

    On a small system like an Intellipak, there is no balancing. Volume controllers have been tweaked to be entirely useless.

    On a multi-story building, especially with one LARGE blower, balancing is critical. You need prints to get it right.


    Please explain what you're talking about with that Intellipak........?? No balancing...?? What are volume controllers...??

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    If all you are doing is resi work, then not worth the time to go into.

    There's no VAV's in resi.


    Quote Originally Posted by Artrose View Post
    Please explain what you're talking about with that Intellipak........?? No balancing...?? What are volume controllers...??
    I do a triple evac with nitro to remove non condensables.

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    Unless specified many systems call for 80% of the VAV's to be open when adjusting the air handler. The idea is that at any one time some will be closed. Once that's done the process is to balance the trunk lines. Was that what you asked. What powers your boxes?
    Tech stuff on the brand of the boxes can be found on the net.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

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    Quote Originally Posted by hvacker View Post
    Unless specified many systems call for 80% of the VAV's to be open when adjusting the air handler. The idea is that at any one time some will be closed. Once that's done the process is to balance the trunk lines. Was that what you asked. What powers your boxes?
    Tech stuff on the brand of the boxes can be found on the net.


    The 80% comment is bogus......In a pressure independent system, it is only necessary to have sufficient pressure in the main supply duct to satisfy the needs of the box, or group of boxes that you are balancing......

    To balance the airflow on series fan powered vav boxes, it's not really necessary to satisfactorily balance these boxes with any airflow from the air handler.....

    After the series fan style of box is balanced, you set the box pressure to neutral or slightly negative......

    Once you have completed any vav system box balancing, then is when you drive the entire system to max cool and do your traverse.

    Any need for sheave changes is then determined.

  10. #9
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    Quote Originally Posted by BBeerme View Post
    If all you are doing is resi work, then not worth the time to go into.

    There's no VAV's in resi.

    Sorry, there are also variable volume systems in residential too......

    Maybe not the traditional vav's we have in commercial, but vav none the less.....

  11. #10
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    The first thing I did was drive all the boxes to full cool because I wanted to traverse the main duct first. If low on airflow I drove the freg drive to where ever needed above 60 hz to determine if the AHU was capable of delivering air at design. It is also easy to determine sheave sizes required to obtain the needed AHU speed at 60 Hz. On systems with diversity I always set the AHU airflow about 10% above design (which is acceptable) in case the engineers design wouldn't do the job.
    In a VAV system with diversity all boxes are balanced to max design airflow. The cooling load determines the airflow required to cool each space. Traditionally in the morning the east boxes will be at full cool if the temperature requires for cooling and the west boxes will not be at full cool. In the evening this will reverse. In short you don't balance for diversity the controlls take care of it if working properly and balance is correct.

    All VAV'S and FPB'S have to have the calibration checked. You need to study how to balance and calibrate prior to starting.
    No man can be both ignorant and free.
    Thomas Jefferson

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    Quote Originally Posted by WAYNE3298 View Post
    The first thing I did was drive all the boxes to full cool because I wanted to traverse the main duct first. If low on airflow I drove the freg drive to where ever needed above 60 hz to determine if the AHU was capable of delivering air at design. It is also easy to determine sheave sizes required to obtain the needed AHU speed at 60 Hz. On systems with diversity I always set the AHU airflow about 10% above design (which is acceptable) in case the engineers design wouldn't do the job.
    In a VAV system with diversity all boxes are balanced to max design airflow. The cooling load determines the airflow required to cool each space. Traditionally in the morning the east boxes will be at full cool if the temperature requires for cooling and the west boxes will not be at full cool. In the evening this will reverse. In short you don't balance for diversity the controlls take care of it if working properly and balance is correct.

    All VAV'S and FPB'S have to have the calibration checked. You need to study how to balance and calibrate prior to starting.

    Good point Wayne......Doing the preliminary traverse is a very good idea because it gives you a good idea where you stand from the get go......If you have the time....

    And yes, all boxes need to be checked for proper operation, mechanical condition, flo tubes correct, rotation correct, stroke times correct, software programming correct, etc., etc........

    And most certainly with DDC, they must have undergone a start up procedure, in which the box goes through it's calibration period.....

    Thing is, when it's very hot, those unbalanced boxes will drive to wherever they think they need to be, and depending on time and money, it's only necessary to manually close enough boxes to achieve enough main duct supply pressure to satisfy what you're working on.......(in general)

    Without that preliminary traverse, as you proceed forward with your balancing, and you start to bring more and more boxes under control, you begin to know where you stand with respect to available duct pressure.......




    I'd still like to know....... Please explain what you're talking about with that Intellipak........?? No balancing...?? What are volume controllers...??

  13. #12
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    Now my head hurts.
    I do a triple evac with nitro to remove non condensables.

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    Quote Originally Posted by BBeerme View Post
    Now my head hurts.


    Come on now BB...this is a place of learning isn't it..??

    We need to pass on accurate information, and unfortunately a lot of this stuff is not easily learned from books, or by simply talking about it......

    Much of this requires hands on with a mentor guiding you......

    I apologize for jumping so hard sometimes, but when incorrect, or inaccurate information is passed, I have a hard time letting it go........

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    [QUOTE] We need to pass on accurate information, and unfortunately a lot of this stuff is not easily learned from books, or by simply talking about it......

    Much of this requires hands on with a mentor guiding you......

    You are dead on Artrose. I have seen experienced air balance guys stumped when the VAV'S wouldn't calibrate.

    Of course BBeerme likes to have a little fun.
    No man can be both ignorant and free.
    Thomas Jefferson

  17. #15
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    I was actually very serious in what I said.
    I do a triple evac with nitro to remove non condensables.

  18. #16
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    Quote Originally Posted by Artrose View Post
    The 80% comment is bogus......In a pressure independent system, it is only necessary to have sufficient pressure in the main supply duct to satisfy the needs of the box, or group of boxes that you are balancing......

    To balance the airflow on series fan powered vav boxes, it's not really necessary to satisfactorily balance these boxes with any airflow from the air handler.....

    After the series fan style of box is balanced, you set the box pressure to neutral or slightly negative......

    Once you have completed any vav system box balancing, then is when you drive the entire system to max cool and do your traverse.

    Any need for sheave changes is then determined.
    Opening the boxes is the first step in most systems. The 80% I wrote was like said, Unless specified.

    T&B is all about procedure. If procedure isn't followed it will result in more time and needing to wrestle a system.\
    into compliance.
    The beginning of a T&B is 1st do the paper work. In a constant volume system, all the outlets are opened.
    2nd adjust the air handler to spec. SP, amps, rpm, and all that applies to the fans. Do a traverse.
    In one case the spec asked for 1/2 plugged filters. This was done with cardboard.
    Different systems can require adjusting methods but usually not al that different. If specs call for 80% open before doing a traverse or wide open would depend on the engineer. Like said the idea is to save some energy by not needing a fan to be adjusted full bore when all the boxes won't be full open.
    A system has to be adjusted to spec before boxes can be adjusted.
    This is what I learned from SMACNA when I got certified.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  19. #17
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    Quote Originally Posted by hvacker View Post
    Opening the boxes is the first step in most systems. The 80% I wrote was like said, Unless specified.

    T&B is all about procedure. If procedure isn't followed it will result in more time and needing to wrestle a system.\
    into compliance.
    The beginning of a T&B is 1st do the paper work. In a constant volume system, all the outlets are opened.
    2nd adjust the air handler to spec. SP, amps, rpm, and all that applies to the fans. Do a traverse.
    In one case the spec asked for 1/2 plugged filters. This was done with cardboard.
    Different systems can require adjusting methods but usually not al that different. If specs call for 80% open before doing a traverse or wide open would depend on the engineer. Like said the idea is to save some energy by not needing a fan to be adjusted full bore when all the boxes won't be full open.
    A system has to be adjusted to spec before boxes can be adjusted.
    This is what I learned from SMACNA when I got certified.


    Correct....after you have been selected to do the balance job......before the install is complete, walk the job, visit the subs, prep your paperwork, spot check the finished installation, do your preliminaries, then begin your balance work......

    In a perfect world, with a constant volume, or a variable volume air system you must be certain that all dampers in the system are wide open, and if there are any splitters, etc., these devices should also be adjusted to allow for free flow of air in the system.

    A spot check of the system is also in order to determine if there are any obvious deficiencies. We find all kinds of issues, wiring, flow tubes crossed, off, pookie, missing, loose insulation, flex blown off, shipping cardboard left inside air handler discharges, incorrect programming, on and on...

    One of the best finds were the parallel fpb's that were hung inverted. Barometric dampers won't close when gravity is on their side.......Water systems are not immune either....coils piped incorrectly, plugged strainers, on and on...

    The ahu filters must be clean, and it's preferable to work with a system that has been running and stabilized ...if possible.......

    If you are carrying around a cert, then you know that there are several different designs of vav terminal boxes.....You also know the different methods necessary to balance the different designs.

    It is preferable to adjust your ahu to as close to design as possible before you start balancing boxes, but it is definitely not required before you can balance vav's......

    Yes, occasionally an engineer, or a building owner might ask for something special, or out of the ordinary, but that is rare......

    I've seen all kinds of T&B folks out there......some really good, some lost, and I watched one group escorted off the jobsite........

    As a T&B tech, I've balanced thousands of vav's, and as a Control man I've set up, programmed, and serviced thousands more..... Today, amongst the other things that I do in this career field, I teach this crap......

    You do what you got to do.......

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