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  1. #1
    Join Date
    Sep 2008
    Posts
    577

    Confused lead lag control

    So I'm doing a box that will have redundant systems including controls (24vac). I'm looking for a controller to keep the units running alternately without having to play with tstat adjustments.

    If at all feasible I'd love to figure a way to make the 2nd unit staged as well or I believe the redundancy will be lost in the controller if the lead unit fails.



    Feel free to tell me I'm over-thinking this and not do any lead lag controller.

  2. #2
    Join Date
    Jul 2000
    Location
    Guayaquil EC
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    10,266

  3. #3
    Join Date
    Sep 2008
    Posts
    577
    Ice,

    Thanks I looked at those but the 600 uses it's own tstat and doesn't go below 55° and the 602 only has a single input. In either case I'm relying on a single circuit board to keep both systems going effectively killing the redundancy.

    What I need is a communicating lead lag 2 stage controller...yeah right

    I think I might just need to refer to KISS.

  4. #4
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,361
    Quote Originally Posted by CoolWine View Post
    In either case I'm relying on a single circuit board to keep both systems going effectively killing the redundancy.
    a possible solution.....

    Use a lead-lag controller set to your target temp. Then install two mechanical t-stats set to a couple degrees warmer than your target temp. wire each stat to the cooling demand input of each system (in parallel with the controller's outputs)

    In the event of a controller failure, your case will revert to mechanical temp controls at a slightly warmer temperature.

    or ditch the controller completely, use 3 mechanical or electronic stats and an alternating relay.
    wire as above, but replace the controller with a stat and alternating relay.

  5. #5
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,219
    I have done this with two modulating Honeywell T775 temperature controllers and a simple time switch from Graingers and several multiple pole relays...

    Simply run the control wiring from the stats to the "non Common" of each relay pole and the commons to another relay for each unit using the "non Common" poles for each machine being controlled.

    When switched the setpoint from one controller swaps to the other boiler...

    I did this to two 3.5MBH Cleaver Brooks boilers with series 90 Modutrol actuators and start stop, it also ran the isolation valves.

    This was about 5 years ago and as far as I know, it's still working.

    edit; BTW, I don't think I spent more than $150 for the material, the T775 controllers were already there.

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  6. #6
    Join Date
    Sep 2008
    Posts
    577
    Quote Originally Posted by craig1 View Post
    a possible solution.....

    Use a lead-lag controller set to your target temp. Then install two mechanical t-stats set to a couple degrees warmer than your target temp. wire each stat to the cooling demand input of each system (in parallel with the controller's outputs)

    In the event of a controller failure, your case will revert to mechanical temp controls at a slightly warmer temperature.

    or ditch the controller completely, use 3 mechanical or electronic stats and an alternating relay.
    wire as above, but replace the controller with a stat and alternating relay.
    Great idea thank you. Simple cost effective and will allow the 2nd unit to act as stage 2 in case of failure.

  7. #7
    Join Date
    Oct 2002
    Location
    Lexington, NC
    Posts
    5,137
    Except when the breaker kicks
    The main thing is to keep the main thing the main thing!

    If "the grass is greener on the other side", it likely has been fertilized with Bull$hit!

  8. #8
    Join Date
    Jul 2007
    Location
    Mount Airy, MD
    Posts
    7,281
    Quote Originally Posted by nchvac View Post
    Except when the breaker kicks
    Gotta love single point failure stuff

    Keeping it cheap and keeping the redundancy............

    Why not (2) transformers in parallel from 2 different breakers with the same polarity running your control circuit?

  9. #9
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,361
    Quote Originally Posted by nchvac View Post
    Except when the breaker kicks
    nope, because the two backup stats are wired directly to each each systems cooling demand input (usually a liquid solenoid valve)

    Each system should have its own breaker and then a separate breaker for the normal operating/alternating control.

    it is the most failsafe design you can get because in the event of a control failure it reverts to two completely independent systems.

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