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Thread: Johnson Controls VMA1430 VAV Discharge Temp Reset Control Strategy

  1. #1
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    Johnson Controls VMA1430 VAV Discharge Temp Reset Control Strategy

    Can HVAC Pro configure a VMA1430 for VAV w/reheat to utilize discharge temperature reset control strategy? We have done it with programmable and newer terminal controllers from various manufactures.

    We are trying to utilize this strategy for limiting discharge temperature:

    https://www.ashrae.org/file%20librar...y_20200817.pdf

  2. #2
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    I have not done that specifically. However, if you add a sideloop, you can do something like that reset I believe. Then physically wire the output of the sideloop back to an input, and use that input as a DAT SP.
    That level of hokey pokey may not be necessary, you may find an easier way, however you may not have known that level of hokey pokey was possible. I seen it a few times when the HVACPro logic needed was more than was allowed in a single pass.
    Hmmmm....smells like numbatwo to me.

  3. #3
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    Thanks numbawunfela. I still don't see how that sideloop would change how the reheat valve and damper would sequence? I don't think it utilizes a DAT SP point in the sequence.

  4. #4
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    The idea is that your canned VAV program will have a DATsp somewhere. No reset. So to add that reset, you make a sideloop that does the reset. Have the sideloop output a 0-10v output. The input is whateva you want (OAT?). That 0-10v sideloop output is hard wired to an AI. your canned program is told to look at that AI for the DATSP.
    The sideloop is an auxiliary function that is layered onto the canned program.
    Maybe I am nossing something....?
    Hmmmm....smells like numbatwo to me.

  5. #5
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    I don't see that there is an ability to modify the "canned" program. The controller is only "configurable" through check boxes using HVAC Pro software. I don't have the software or controller in front of me so my question was to whether there was an option (check box) in HVAC Pro to configure that particular controller to sequence to discharge air for single duct VAV w/reheat. All of the documentation I found doesn't really answer that question for me. Thanks for the hints numbawunfela!

  6. #6
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    You got it.
    I don't have it in front of me either. Maybe there is a check box for that. Someone may post.
    The goal here is that even if there is not a check box for it, any canned program can get a side loop added to it. And with that one should able to add the functionality you seek.
    Basically walk away with 'I am 98.4% sure I can do this'. Maybe super easy (checkbox in the question and answer series) or slightly less so (adding a sideloop).
    Hmmmm....smells like numbatwo to me.

  7. #7
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    You can accomplish a reset using LCT.
    Law Of The Thermostat: He who has the thermostat wins!!!!!

  8. #8
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    Quote Originally Posted by numbawunfela View Post
    However, if you add a sideloop, you can do something like that reset I believe. Then physically wire the output of the sideloop back to an input, and use that input as a DAT SP.
    You cannot add a user defined sideloop in a VMA application....

  9. #9
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    Quote Originally Posted by bozo123 View Post
    You cannot add a user defined sideloop in a VMA application....
    Thanks bozo. Been a while since I played with a VMA in that way.
    Hmmmm....smells like numbatwo to me.

  10. #10
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    Quote Originally Posted by simux View Post
    You can accomplish a reset using LCT.
    Are you saying I can put custom logic into a VMA1430 using the LCT tool? Are you sure about that?

  11. #11
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    Quote Originally Posted by zmanvortex View Post
    Can HVAC Pro configure a VMA1430 for VAV w/reheat to utilize discharge temperature reset control strategy? We have done it with programmable and newer terminal controllers from various manufactures.

    We are trying to utilize this strategy for limiting discharge temperature:

    https://www.ashrae.org/file%20librar...y_20200817.pdf
    @zmanvortex,
    I just fired up the old XP-Pro laptop with Hvac-Pro 8.08b and after about 6-7 tries I will say that I cannot find any method to do Discharge Air Reset inside a VMA1420/1430.
    I quickly scanned the ASHRAE document and I would say "Don't bother to even try due to the fact that the VMA1400's are discontinued/obsolete/ and not capable of performing that function". JMO at this point.

    The only way I can see to "...utilize this strategy for limiting discharge temperature..." is trial and error by limiting the Reheat valve response. The reheat valve is controlled based on Space Temp deviation from setpoint. It has absolutely NOTHING to do with DA-T.

    My call on this would be to tell whomever "It cannot be done. Lets move on to hardware that was developed in this century ". I believe the VMA1400 was release in late 1998/early 1999 so you have that going for you (or against you).

    To answer your question in post #10; Nope, you cannot do that either. You can create external logic that may be able to influence the VMA but refer to my previous sentence !

    No disrespect but unless you have gobs of time and lots of dollars I say "Just walk away" !!!
    If sense were so common everyone would have it !
    You cannot protect the Stupid from themselves !
    "Experience is the ability to recognize a mistake Before you make it again!" (Stolen Quote)

  12. #12
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    LCT

    Quote Originally Posted by zmanvortex View Post
    Are you saying I can put custom logic into a VMA1430 using the LCT tool? Are you sure about that?
    The LCT program exists at the supervisory front end level.
    Law Of The Thermostat: He who has the thermostat wins!!!!!

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