Results 1 to 9 of 9
  1. #1
    Join Date
    Apr 2006
    Posts
    35

    Confused Distech VAV AHU code?

    I'm trying (somewhat unsuccessfully) to write code for a free-prog AHU application using terminal load from some VAV boxes. The sample code from Distech is rather interesting, it doesn't seem to throttle down the chilled water valve as supply air temp drops, it seems to react purely off terminal load in the boxes. Anybody have VAV AHU code they're willing to share? I'm about ready to abandon this and start from scratch.

  2. #2
    Join Date
    Aug 2006
    Location
    New England
    Posts
    520
    Aside from high and low limits (which I believe are included in the sample code), what's wrong with reacting off terminal load? The beauty of Distech controls on VAV systems is the weighted average terminal load function. It can be biased for cooling when you've got reheats or balanced between heating and cooling any way you want. It makes much more sense than controlling discharge air temperature reset by return temperature which is how most systems are programmed.

  3. #3
    Join Date
    Apr 2006
    Posts
    35
    Unfortunately the engineer's sequence I have to meet is to maintain a constant 55 degree supply air temp which can be reset upwards as the spaces satisfy. I guess what I'm thinking about is using a controller on the chilled water valve, this controllers input is SAT, and trying to use a variable as a setpoint input. Then using another controller whose input is terminal load, and comparing that to another setpoint. Then possibly putting a limit on the output to give it up to a 10 degree swing. This could be added to a 55 degree constant to come up with a variable which would go between 55 and 65 degrees to feed into the controller handling the chilled water valve. I haven't experimented with it to see if I can use a variable for a setpoint in a controller or not, but so far that's what I am thinking. ....unless there is a easier way to accomplish this. Again, I haven't messed with Distech's sample code that much yet, but at first appearance it seems as if it would drive the supply air down to the low limit if the terminal load is high enough, and then when SAT hit the low limit it would shut the chilled water valve off. If this is the case, it won't work for my application.
    Last edited by ccnster; 07-07-2007 at 03:03 PM. Reason: more clarity

  4. #4
    Join Date
    May 2007
    Location
    Somewhere, WI
    Posts
    351
    You should be able to do that with one controller. Are there other outputs you need?

    Each FP controller has 18 nvi's and 18 nvo's, it sounds like that should be enough.

    I found a doc hidden under a big blue blanket of hidden truths (aka JCI). It helped me alot with these controllers. Send me your address and I'll send it to you, its to big to attach.

  5. #5
    Join Date
    Apr 2006
    Posts
    35
    I added my email address to my profile

    I'm figuring on probably using the 400 series controllers. The sequence has the usual stuff, VFD supply fan, CO2 control, dampers, 2 or 3 stages heat, chilled water valve...

  6. #6
    Join Date
    Aug 2006
    Location
    New England
    Posts
    520
    I don't have the code written for this application, but just to get you started, you'll want a linear equation to do the reset (there's an editor built in - just right click on the code page). Something like:

    DischTempSpt = -1 * ReturnTemp + 1300
    DischTempSpt = Limit (DischTempSpt 0 100)

    CO1 is the output of the controller for discharge air temp. (direct acting assumed here)

    Buffer = CO1 * 2
    HeatSig = 100 - Buffer
    HeatSig = Limit (HeatSig 0 100)

    HeatStg1 = Switch (HeatSig 70 100)
    HeatStg2 = Switch (HeatSig 40 70)
    HeatStg3 = Switch (HeatSig 10 40)

    CoolSig = Buffer - 100
    CoolSig = Limit (CoolSig 0 100)
    ChwValve = CoolSig

  7. #7
    Still have that warm blanket with the document? Mind sharing?
    Intelli-Building = Less Stress, commissioned with diligence!

  8. #8
    Join Date
    May 2007
    Location
    Somewhere, WI
    Posts
    351
    Add your email to your profile or send me an email and I'll send it over.

  9. #9
    Quote Originally Posted by ddcfreek View Post
    Add your email to your profile or send me an email and I'll send it over.
    Done! Thanks, rp
    Intelli-Building = Less Stress, commissioned with diligence!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event