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  1. #1
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    OK Control dudes--

    Got a 100% MAU for kitchen hoods, starts when any hood is turned on.

    Has an outside air damper then, HW coil, CW coil, discharge air sensor about 4' down the duct.

    Johnson FEC controller, in unocc HW valve opens all the way

    controller looks at DAT to maintain temp delivered to space

    upon startup, oa opens, HW coil starts to modulate closed (hw @180 deg.).

    problem?

    well , control loop has HW valve all the way closed and DAT is still reading about 100 deg., With recent OA temps around 10 deg., this trips the freezestat mounted after the HW coil and before CW coil.

    Obviously the CW coil has absorbed alot of the heat and is still giving off its heat, messing up the DA sensor.

    Avoiding a freezeup is everybodies biggest concern so slowing down the control loop isn't very desireable as we need the HW valve to be able to respond quick enough to avoid this from happening.

    Johnson guy is working on it but I was wondering what you guys would do?


    I'll let you know how things work out.
    5 out of 4 people don't understand fractions

  2. #2
    Join Date
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    Setup the hot water valve to maintain 70F DAT during unocc, instead of blasting the valve 100% open.

    Double check the fstat and make sure that every bit of it is directly in front of the HW coil. Sometimes cold air leaks around the edges of the coil and can cause issues.

    If the fstat is tied to the controller, you could add a time delay before total shutdown. This would be my last resort, if at all.

  3. #3
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    Quote Originally Posted by orion242 View Post
    Setup the hot water valve to maintain 70F DAT during unocc, instead of blasting the valve 100% open.
    Same here-slow the loop down. Best bet is to add a DAT after the HW coil and control to that when in heat.
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  4. #4
    Join Date
    Mar 2009
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    In the race between the control loop and the freeze stat the freeze stat always wins !

    If the guy that is programming it is STILL messin' with it tell him to get some help. From a controls stand point this is not hard. 30 minutes and he should have it dang close. If he has spent more than 30 minutes I would guess he is either sorta green or doesn't really have any experience with 100% OSA controls. Either way he needs someone with more experience to get it straight.

    At a minimum he needs to slow down the OA Damper.

    (and not have the HW valve wild in the off mode)

    Just my 2 cents.
    If sense were so common everyone would have it !

    All opinions expressed are my own. Any advice provided is based on personal experience, generally accepted fact or publicly available information. As such, it is worth exactly what you paid for it, not a penny more not a penny less !!

  5. #5
    Join Date
    Oct 2006
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    116
    What I have found with the FEC's is the AI's are filtered quite a lot. When you need a control loop to respond quickly you need to eliminate the filtering. In the UI setup change the Filter setup to monitor only, then the time constant to 1 or 0 seconds. It should have a faster reaction time.

  6. #6
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    Oa damper is on-off, end switch makes before fan starts.

    As far as controlling HW valve in unocc, what are you controlling it with?

    fan is not running, so DAT is useless.


    freezestat tied directly to fan, also sends signal to FEC, opening HW and closing OA.

    xarralu-- I see great minds think alike:>)
    5 out of 4 people don't understand fractions

  7. #7
    did the geniuses at JCI write the control sequence? The damper should be proportional at minimum. open that puppy slow. This could be done easily with a different actualtor on that controller.

    Is this make up for fresh air requirements or building static control or both? Pump? 3-way valve?


    If the DA is in the hull of the unit ..it will naturally convect enough to give your valve a chance of remaining in fairly good control in unocc. With the damper PID opening slowly on occ command... the pid actions of the valve will be much easier
    The feezestat in front of the coil makes no sence... it would trip all the time.

    Ultimately.... you should probably had a HX and a glycol loop. also it is good practice to drain/valve off the CHW coil in the winter. Sounds like a fair weather engineer moved north to me.

    good luck

  8. #8
    Join Date
    Nov 2005
    Posts
    454
    Install heating coil discharge sensor before cooling coil. Control for pre-heat.

  9. #9
    Quote Originally Posted by mackinaw View Post
    Install heating coil discharge sensor before cooling coil. Control for pre-heat.
    That would be best, but whats the chance there is room to put a good avg element in there..short of major surgery and money? I would not trust a stick in that app...

  10. #10
    Join Date
    Nov 2007
    Posts
    24
    Oa damper is on-off, end switch makes before fan starts.
    Does the unit have a face/bypass damper on the coil.

    I recently configured a similar unit. Sequence was when the kitchen exhaust fans were turned on they opened the OA damper on the makeup unit. When that was 100% the fan started. When the fan started it sent a proof to the controller to start operation of the HW coil valve. Always tripped on freeze. I installed two relays. One is a proof point to start HW valve control when the damper starts to open. The other is an output to control when the fan turns on. This unit has a coil temp sensor so I used that to determine when to turn the fan on. It also maintains a minimum coil temp based on OAT when off. The discharge temp sensor maintains a fixed 70 degree discharge. Kitchen gets a blast of cold air when first turned on but the freeze stat hasn't tripped even in single digit temps.

  11. #11
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    Quote Originally Posted by freddy-b View Post
    did the geniuses at JCI write the control sequence?

    Sorry ,had to laugh at that

    Yea, but from what I understand they are in India, probably dont get many cold weather apps there.



    Originally Posted by mackinaw
    Install heating coil discharge sensor before cooling coil. Control for pre-heat.

    "That would be best, but whats the chance there is room to put a good avg element in there "


    Actually got very lucky There is a nice door between coils were freezestat
    is mounted. lotsa room.


    After turning this on the first time and saw what was happening I told him this will never work like they had it drawn up.

    He had an averaging sensor on his truck so I installed it with the freezestat and ran another wire to a spare UI on the controller.

    He was rewriting the control sequence to look at this sensor for heat and the DA sensor for cooling when I had to leave,

    Did not get there today but should get there tomorrow to see how he made out.

    was just curious how others have done this, I figured I wasn't the first to run across this.


    Wont even talk about our ongoing adventure with trying to control the condenser water temp with 2 BAC towers, VFDs and bypass valves
    5 out of 4 people don't understand fractions

  12. #12
    Join Date
    Nov 2005
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    454
    Quote Originally Posted by glennwith2ns View Post
    Sorry ,had to laugh at that

    Yea, but from what I understand they are in India, probably dont get many cold weather apps there.



    Wont even talk about our ongoing adventure with trying to control the condenser water temp with 2 BAC towers, VFDs and bypass valves
    You got something against Indians?

    If you're having trouble with you cooling tower loop, the "auto-tuning" process ID cooling tower control is way too slow in many cases. Put some manual values in there, or even try MA-T for a process ID. Many times MA-T works great. Good luck.
    Last edited by mackinaw; 02-02-2010 at 10:02 PM.

  13. #13
    Join Date
    Mar 2005
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    Interesting - especially the .5 hour comment, maybe I am still way too green and almost all 100% OA MAU's that I've dealt with have an on/off damper.

    Since it is a kitchen hood you can be somewhat sloppy on startup with the temperatures as the air is getting exhausted back out anyway. Had one system with gas heat exchangers (not direct fired) in which I had to do quite a bit of customizing to account for the delays in the gas firing up, etc. I had to use a software freezestat to make this work, but it's been working fine for about 4 yrs now. Granted they had a gas valve go bad and the software freezestat worked. I'd rather have a hardware freezestat but given the design, had to do software.

    I don't know the overall design of this system, but what about adding a time delay reset? OA<35 then valve at 100% when unoccupied. OAD end switch makes and you have two values one from the time delay reset and one from the valve PID in which the greater is always passed to the valve. From startup to 0-1 minutes time delay reset has valve at 100%, 1-4 minutes time delay at 50%, 4-6 minutes time delay at 25%, 6-7 minutes delay at 10% and at >8 minutes delay at 0%. Somewhere through these settings the valve PID will be greater than the time delay reset value and the valve PID will take over. Granted one could do some kind of cascade control based on OAT temp to make this much tighter, but it's just a kitchen.
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