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  1. #1
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    Non-Balancer question for a Controls Guy

    Dumb question about Balancing a Change Over-Bypass (COB) system. The system has a constant speed fan for cooling and a somewhat reduced speed for heating, i think.

    Can I get the Readers Digest explanation (Thumbnail) of how a COB gets balanced ?

    My understanding is that the motorized Zone Dampers have to be fully open and then the registers/Diffusers need to be measured and set (cut at the takeoff's) to Max design Flow. Then the total Flow of the RTU has to be adjusted, maybe, to match the total flow requirement, right ?????

    I believe then, and only then, can the Flow bypass get setup to open based on Total Static pressure.

    Please correct my ignorance on this.

    Thanks in advance.
    If sense were so common everyone would have it !
    Stupid should Hurt !
    If people took care of their cars like they do their HVAC Systems you'd see a lot more people walking
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  3. #2
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    Are you going to bring in a balance contractor? If not do you have the test equipment to balance the system? These systems are a pain to balance. The bypass should be controlled by supply duct static not total static. It has to be balanced in the max cooling mode in order to know the required static pressure set point and that depends on where the sensor is located. I've seen a lot of sensors located at the fan discharge.

  4. #3
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    Thread Starter
    WAYNE3298,
    Odd situation. The Balance contractor has done his stuff, correctly I believe. Now we have an equipment guy telling us that the balance is not correct and changing stuff (D'oh !).

    I have talked to 2 of my local Balancing experts and I am leaning towards the equipment guy not quite understanding how a COB system actually has to work.

    I am just going to step back and wait to see how this all turns out.

    Thanks for your reply, though.
    If sense were so common everyone would have it !
    Stupid should Hurt !
    If people took care of their cars like they do their HVAC Systems you'd see a lot more people walking
    !

  5. #4
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    If you work for the owner demand a meeting with all parties and make the equipment guy explain what is wrong and what needs done. If the balancer did his job the equipment guy will look stupid. What is wrong that he is screwing with it?
    The purpose of the bypass is to maintain the manufacturer's required minimum airflow across the coil and that is done by adjusting the maximum static pressure required to deliver design airflow to all the units served.

  6. #5
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    Thread Starter
    "And now for the rest of the story".
    The balancer, right or wrong (wrong in my mind) set the branch ducts by making the DDC damper max position what was needed per the Design drawings. My argument is that the DDC damper actuators do not have any real repeatability and the branch flows should have been set with the Balance Dampers.

    A comment was made that the system should be "Self Balancing" (???), I still don't see how that is possible. There were also references made about the "ESP" getting too high. I think they meant the Duct Static pressure.

    Correct me if I am wrong, but on a single zone constant volume duct system the branch ducts/diffusers have to be "Balanced" to supply the designed flows. If that system happens to have a Change Over Bypass control system added to it the Bypass operates to maintain the duct static at (actually when it increases above) the pressure that gets measured when all of the "Zones" are at full design flow. On this system I recall that pressure was .39" WC.

    Am I missing something here ? It all seems pretty straight forward to me.

    I need to add that all the "Zones" are pressure dependent on this system.
    If sense were so common everyone would have it !
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    If people took care of their cars like they do their HVAC Systems you'd see a lot more people walking
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  7. #6
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    Your system is relatively unique. It appears the intent is energy savings accomplished by operating the fan at a very low supply static. This design works best if CO2 control is used rather than minimum motorized damper set points. That is because the zone dampers have to be full size to deliver design max airflow and the velocity pressure at minimum is too low for control. I say that realizing you are not likely using VP at all. This system works best if damper position is controlled by the stat using space load rather than a CFM set point. This system normally has a diversity which means with all the zone dampers open the connected load exceeds the AHU design.
    This may sound like BS to you because I have balanced hundreds of systems and have only seen two or three like I'm describing and that assumes my description is correct. Somebody should explain the objective to you. The first one I saw threw me for a loop.
    I balanced this system with all dampers driven to 100% open then proportioned the airflows using the zone dampers. I then calculated the maximum supply static pressure needed that would supply design airflow to any area and had it set to that. That assured that as areas away from the sun cooled the static pressure remained sufficient to supply design airflow to the areas with the most load.
    Admittedly this is a foreign concept but the facility owners swear it works and saves energy.

  8. #7
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    Quote Originally Posted by Cagey57 View Post
    the branch flows should have been set with the Balance Dampers.

    should be "Self Balancing" (???), I still don't see how that is possible.

    Correct me if I am wrong, but on a single zone constant volume duct system the branch ducts/diffusers have to be "Balanced" to supply the designed flows. If that system happens to have a Change Over Bypass control system added to it the Bypass operates to maintain the duct static at (actually when it increases above) the pressure that gets measured when all of the "Zones" are at full design flow. On this system I recall that pressure was .39" WC.
    1) Self balancing is a dirty phrase.
    2) Without seeing the system it would be hard to say with 100% confidence. But, if its what I think it is then yes. Use the hard balancing dampers and mark them once complete. What would happen if someone were to go through the BAS and take out all of those setpoints?
    3)Forgive my ignorance, but what is a COB system? Just by reading this, I'm assuming its like a dump damper system. So once duct static rises above a set point, the bypass opens and supply air is bypassed back to the return of the AHU? Sorry, we don't have anything down here known as a "Change Over Bypass".

  9. #8
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    Not being picky but there is a difference between a dump damper and a bypass damper. The dump damper dumps the excess air into the ceiling plenum and the bypass dumps the air directly into the return duct.
    This system has been operating a while now. How is it working?

  10. #9
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    Cagey57,
    You are correct. Needs to be balanced as a constant volume system. There should be manual balancing dampers in front of every VVT damper. 1st set all VVT dampers 100% open. 2nd. Proportion diffusers downstream of VVT dampers then, Proportion all branches to the VVT dampers. 3rd set supply duct static at bypass controller to a setpoint that achieves the design CFM for the system. Good luck
    AIRFLOW IS CRITICAL

  11. #10
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    airdata,
    VVT'S are not constant volume that is why the supply has static pressure control. VVT systems are balanced the same as pressure dependent VAV systems. They can be balanced with maximum and minimum airflow set points using the VVT'S. The setpoints are determined by damper position not velocity pressure. Some control programs do not have damper position set points. In that event the engineer needs to know minimum set points cannot be set. NEBB and other certifying agencies require that at least one balance damper on every zone or system be wide open. That prevents flow control by balance dampers and requires balance of the zones using the VVT'S. Individual zones are balanced after design airflow has been set. Static pressure final set point is established near the end of VVT set up. Proportional balance of the zones will also create the need for static pressure. The last thing you check after balance is complete is static pressure and the set point.

  12. #11
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    Wayne, I didn’t say the system was constant volume I said he needs to be balanced as a constant Volume system . But typically these are RTU units DX cooling and they need constant volume through the RTU. Also I said proportional balancing, that means one Zone damper 100% open and one branch damper 100% open , Key Outlet. I also said for the last step to set up the static pressure setpoint . I also have been balancing for many years and I’ve balanced many of the systems . Carrier VVT or Trane tracker. Or hybrids of these changeover bypass systems. The carrier 3V control system can be used as pressure dependent or pressure independent as they have transducers to except velocity pressure . Setting up the monitor thermostat along With the zone controllers with the System Mode Demand is just as important depending on the application.

  13. #12
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    airdata,
    Your write-up is confusing. The 3rd sentence states "set all VVT'S at 100% open". The next sentence says "proportion the diffusers downstream of VVT dampers. Having done that the system has been balanced constant volume not just the RTU. How would you balance with a diversity? Overall proportional balance on a system with a diversity using manual dampers instead of the controllable VVT damper doesn't work.
    I agree the controls on these systems can read airflow but the velocity pressure at minimum flow usually drops to or even below 0.005 inches and the minimum airflows cannot be controlled. Every VAV manufacturer gives that warning. Hot wire measurement also has a problem at very low airflows.

  14. #13
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    Wayne,
    I have never come across a changeover/bypass system that was designed with diversity,
    True VAV systems, yes diversity is used quite often. I agree with you that most controllers do not control well @ .005 VP or less., TAC controllers do pretty well. These systems are a challenge especially if you have conference rooms. It helps to have localized heat on the perimeter of the building as I am in a northern climate. Being that itΒ’s pressure dependent, We proportion balance to full load cooling then determine a duct static set point. At part load conditions it is unlikely to stay in proportion. Wayne, with diversity in the design how do you set up the system in a static mode to proportion the system? I am here to learn

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