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  1. #1
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    Alerton VAV-SD Questions

    I have a building I was asked to review that has a bunch of Alerton VAV-SD controllers in it. From what I'm seeing basically every VAV in the different air handler systems have air flow sensor issues. All the air flow sensors read a substantial amount of flow even with air dampers closed or the air handlers serving them being off. When I say substantial the values the VAVs will read is more then the minimum flow setpoint of the VAVs. In some cases its half the nominal flow of the VAVs.

    I would guess the controllers are 7-10 years old. I see in an Alerton manual programming manual there is a variable AV 251 that is described as the zero cut off. The description is that adjusting this will make the VAV report zero when this value is increased and is used to adjust for temperature drift when the VAV has no flow.

    Is zeroing the VAVs with this variable an effective way to calibrate the zero of the flow transmitters?
    The description of this variable is worded almost like it just changes the displayed value when the fan is off and is not a true zeroing of the transmitter.


    Consequently I will mention that this in not the first time I've seen a building that has had Alerton VAV controllers that exhibited this behavior.
    It took actually learning something to realize how much I had to learn.

  2. #2
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    I don't think that would fix your problem.
    If they still have flow when the air handler is off that kind of worries me. I would check some hoses and make sure both are secured properly. There should be a setting called "Box Size" that you can look at, if that point got messed up somehow that would cause a lot of flow issues. I would check boxes that say the damper is 'closed' and actually go see if the damper is closed, sometimes if its a floating actuator the times get messed up.
    Those are a few ideas to start. Sounds pretty strange to me though.

  3. #3
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    Quote Originally Posted by willf650 View Post
    VAV report zero when this value is increased and is used to adjust for temperature drift when the VAV has no flow.
    What I would assume as well. Not going to fix anything but fool the end user into not complaining.

    Must be a way to zero those sensors. They all drift around over time.
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  4. #4
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    If the sensors have drifted, there is a calibration factor, AV-252 that can be used to compensate. The default is 1 on a new VAV controller.
    I would agree with punk0r1f1c, you should make sure the airflow really is zero.

  5. #5
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    Quote Originally Posted by deadheaddave View Post
    If the sensors have drifted, there is a calibration factor, AV-252 that can be used to compensate. The default is 1 on a new VAV controller.
    I would agree with punk0r1f1c, you should make sure the airflow really is zero.
    Can you explain what that variable specifically does. I saw that variable and it is set to some different values dependent on each particular unit.

    I read that variable to be a calibration factor for balancing purposes and not a variable to zero the sensor.


    Trust me when I say I have no flow. I'm not physically there and surveying the building remotely. When my air handlers shut down the trends show my static is going to zero and my VAVs still read flow. The flow is unchanged regardless of the air handler running or not. As stated the only time the flow changes from this false zero is if you do drive the damper open.

    Obviously there is the possibility for some of the VAVs to have damper and tubing issues but I would find it hard to believe that all of them have issues. Generally if you have tubing issues you will either read no flow or the the sensor will be pegged. The fact that the flow will rise when you drive the damper open leads to the fact that the zeros have drifted. This is also backed up by the fact the VAVs still read flow with the air handler off.

    Consequently the dampers could also have problems but I don't think loss of calibration is a concern. These are floating point actuators and I see they are set up to continuously drive either open or closed at the end of their stroke. So dampers out of sync is not an issue unless they are physically busted or slipped on the shaft. But once again either of these scenarios would be relegated when the air handler shuts off. It would also be hard to believe ever damper would suffer this affliction.


    As I said, this is not the first time I have seen this issue exhibited with Alerton VAVs. I set up a BACnet trending software up in another building a couple years ago and pulled data for a week and some of those VAVs exhibited the same problem just not to the extent I'm seeing in this building.
    It took actually learning something to realize how much I had to learn.

  6. #6
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    That variable is the K factor. The controller uses K factor, box size, and differential pressure to calculate flow on the box. If the box is reading zero dp across the flow tubes it should read zero flow regardless of box size or k factor.

    I saw some other threads talking about how the hot wire for a flow sensor could get dirty, but I'm pretty sure the VAV-SD doesn't use that type of flow measuring.

    I install a lot of these controllers so I'm kind of curious to find out what the issue is.

  7. #7
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    Sounds like every other controller. That is just adjusting the slope of the sensor reading and has no effect on the zero point.

    All these sensors drift and toss the zero point around, and this amount can be enough to fake min CFMs. When your measuring such small pressures (especially @ min) the zero point needs to be close. If not, you get exactly what you describe. Box reporting its at min cfm with the damper closed. It can just as easily toss the zero point negative as well. Just harder to spot on a casual look of the system.

    Easy enough to test, pull the tubes off and see if the reading changes. If not, the zero is hosed by more than enough to matter.

    I see this more often on large boxes with low min flow rates (aka oversized or stupid mins). Had to really dig in on a site with this issue and found that if the sensor was not zeroed out to three decimals, large boxes could meet mins with errors something like > 0.007". Dealing with pressures that small, temperature drift on these sensors easily can cause that much error. Your not working with metrology grade sensors on your lowly VAV controller... It was a hassle back and forth with the OEM who thought two decimals was enough. After handing them two dozen examples they reluctantly came up with a kludge to deal with the problem. In the end, I created a script to play with the K and read the reported airflow & pressure then worked backwards to calculate the offset with the resolution I needed. Ever since that fiasco, if the controller allows software to zero I do it after the unit has been off for more than 5 minutes....plus one for fully programmable controllers and software definable sensor offsets.

    Hard to believe they wouldn't have an offset or way to force zero. If it cannot be done, they must be doing something in the firmware to deal with this. If that is the case, could be your conditions are hosing up their "automatic" zeroing. 24/7 occupied maybe?
    Last edited by orion242; 02-04-2017 at 06:31 PM.
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  8. #8
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    Try painting around the air sensor block with nail polish, I had several VAV-SD controllers about that age with the same problem and it seemed the plastic housing wasn't making a good seal to the board.

  9. #9
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    There is a VAV configuration utility in the VLC DDC editor, it's a bit klunky but there is a zero cutoff factor. From the BACtalk Programmer Guide:

    Velocity zero cutoff: This value is entered in percent of full airflow (0-100).
    Airflows lower than the cutoff are reported as zero. This feature eliminates nonzero
    airflow readings due to ambient fluctuations when the main fan is off.

    I've uploaded a picture of it in action as well as the Programmer guide to file convoy (expires in 21 days)
    http://www.fileconvoy.com/dfl.php?id...0f8b2eb358970b

    Hope this helps!

  10. #10
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    Quote Originally Posted by willf650 View Post
    I have a building I was asked to review that has a bunch of Alerton VAV-SD controllers in it. From what I'm seeing basically every VAV in the different air handler systems have air flow sensor issues. All the air flow sensors read a substantial amount of flow even with air dampers closed or the air handlers serving them being off. When I say substantial the values the VAVs will read is more then the minimum flow setpoint of the VAVs. In some cases its half the nominal flow of the VAVs.

    I would guess the controllers are 7-10 years old. I see in an Alerton manual programming manual there is a variable AV 251 that is described as the zero cut off. The description is that adjusting this will make the VAV report zero when this value is increased and is used to adjust for temperature drift when the VAV has no flow.

    Is zeroing the VAVs with this variable an effective way to calibrate the zero of the flow transmitters?
    The description of this variable is worded almost like it just changes the displayed value when the fan is off and is not a true zeroing of the transmitter.


    Consequently I will mention that this in not the first time I've seen a building that has had Alerton VAV controllers that exhibited this behavior.
    zero cutoff will just raise the value that the card will register as zero. for example 50cfm will be read as 0 but 51 will be read as 51. you probably want to change the calibration factor and ensure the box diameter are correct.

  11. #11
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    The zero cut off is not the correct property to zero the transmitter. It's just a property to trick the display value as mentioned. It will actually make the box control even worse as in my case the values are reading 300+ cfm. The controller will attempt to control and jump from reading zero CFM to 300+ as soon as the damper opens if the zero cut off is adjusted to "zero" the transmitter.

    Unless the calibration factor is different in these controllers then every other controller I've dealt with it is also not the correct property to zero a transmitter. According to what I have previously read it is what I say, a calibration factor, not a zero. Basically if the flow is 110 and you are reading 100 you adjust the calibration factor to 1.10.

    I would hope these devices could self zero like many other manufactures I've dealt with but that doesn't appear to be the case. The building was occupied 24/7 due to some misprograming from another vendor that had circular logic. Once the building was occupied it stayed occupied and could never unoccupy. I corrected that logic upon my first review of the building a few weeks ago and yet the transmitters still haven't zeroed.
    Last edited by willf650; 02-20-2017 at 09:33 PM.
    It took actually learning something to realize how much I had to learn.

  12. #12
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    Quote Originally Posted by masonlc View Post
    This feature eliminates nonzero airflow readings due to ambient fluctuations when the main fan is off.
    and what they don't say is it can totally hose your control @ box minimums. With temperature these can drift enough to require zeroing.
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  13. #13
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    I am having trouble with the same problem with one of our Alerton buildings. I am the property manager, and the controls guys are telling me we have to replace all of the controllers with this issue. Is there no other way around that?

    Thank you.

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