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  1. #27
    Join Date
    May 2006
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    334
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    Sys, you are good!

    In a serials of Tom's articles, he recommended REPLACING PID CONTROL WITH INTELLIGENT ITERATIVE CONTROL.

    Quote Originally Posted by sysint View Post
    I think Tom's theories are more suited to an Iterative control structure and not the industry-standard PID structure.

  2. #28
    Join Date
    Feb 2005
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    Perhaps - but the overlying concept is the same, iterative simply means a task will take small corrective steps instead riding a corrective ramp of response.

    I'm still not sure how this global melting pot of data in and data out (the "System") translates into an actual "if,then,else" sequence of operations.

    I'm all ears though...

    Nikko

  3. #29
    Join Date
    May 2006
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    334
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    Nikko,

    That bothers me too. I will revisit what I've read and try to put all puzzles together again. Good to have someone to discuss with along the road!

    Quote Originally Posted by nikko View Post
    Perhaps - but the overlying concept is the same, iterative simply means a task will take small corrective steps instead riding a corrective ramp of response.

    I'm still not sure how this global melting pot of data in and data out (the "System") translates into an actual "if,then,else" sequence of operations.

    I'm all ears though...

    Nikko

  4. #30
    Join Date
    Feb 2012
    Posts
    398
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    Quote Originally Posted by twisted pair View Post
    Assuming this is for a rooftop unit or air handling unit. It means the supply air temperature or some people refer to as discharge air temperature (leaving air temperature) from the unit will reset as needed to maintain temperature. If this is on a DDC system you would have a process variable like space temperature or return air temperature.

    So the heating & cooling will work as needed (depending on lockouts, limits, deadbands, etc.. in the program) to reset the SAT / DAT based on a variable like space temperature or return air temperature.

    A simple reset would look like this;

    Space Temp
    75
    70

    Supply Air Temp
    55
    65

    So if the space is 75 deg. we want a 55 deg. supply in a cooling mode, and on the other end if the space was 70 deg. we would want a 65 deg. supply. Many variables. Does system have reheat, is it constant volume or VAV etc.. The reset would usually be proportional in between those setpoint values.

    Gotta get back to work. Hope that helps.
    Necro posting here. I'm trying to understand the idea behind temperature resets so please bare with me (if the answer isn't obvious enough already)

    What's the difference between a unit that has a reset program and one that doesn't? Wouldn't both units stage up/down/load/unload their cooling capacity based on demand either way?

  5. #31
    Join Date
    Jan 2003
    Location
    USA
    Posts
    3,343
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    Quote Originally Posted by Jigjoe View Post
    What's the difference between a unit that has a reset program and one that doesn't?
    One changes its setpoint based on something, other is fixed.

    vav unit holding 55F DAT 24/7 even when it -10F outside, vs one that sees the spaces are freezing and backs off to 68F DAT.
    Propagating the formula. http://www.noagendashow.com/

  6. #32
    Join Date
    Oct 2015
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    124
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    Quote Originally Posted by Jigjoe View Post
    Necro posting here. I'm trying to understand the idea behind temperature resets so please bare with me (if the answer isn't obvious enough already)

    What's the difference between a unit that has a reset program and one that doesn't? Wouldn't both units stage up/down/load/unload their cooling capacity based on demand either way?
    1. Without SAT Reset, the unit will try to maintain a constant 55 degree SAT all the time. Without it, the unit will always try to make 55, and to your point, work a bit harder when load increases. This is also why you see space heaters and blankets in some offices in some cases. Swing seasons can be tricky if you're constantly supplying 55 degF and have no means of terminal unit reheat downstream. Resets are generally viewed as a means to save energy as well, as they can back off their setpoints when the load drops a bit due either to weather or occupancy.

    2. New record for digging up old posts? August of aught-seven, that was the beginning of a very slow time for new construction jobs.


    Sent from my iPhone using Tapatalk

  7. #33
    Join Date
    Apr 2009
    Location
    chicago
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    93
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    Quote Originally Posted by Jigjoe View Post
    What's the difference between a unit that has a reset program and one that doesn't? Wouldn't both units stage up/down/load/unload their cooling capacity based on demand either way?
    Yes, they both operate the same. Attempting to match the capacity to the load.

    reset is for stability, why it was created, if there is lag, transmission or otherwise, then reset is essential to achieve stable operation.
    this is its primary function. some say its for energy reduction but that may or may not be true.

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