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  1. #14
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    Well I've been reading up on PID control and I think this problem is due the derivative part of the PID algorithm. From what I've learned, PID control can be the best but it appears that with thermostats, under certain conditions it can become unstable.

  2. #15
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    If it's become unstable in it's control response, the issue is likely not the controller, it's probably something like thermostat location. Garbage in = garbage out. Maybe a water heater tuns on at that time creatign a negative draft that draws air into the wall cavity behind the thermostat, creating a draft across the temprature transmitter that creates a small shift in the local air currents. You'd be suprised. Always make sure any wall penetrations behind a thermostat are sealed. Poor thermostat location can cause a lot of issues. I've relocated on of mine for just that reason and need to relocate another that lcoated too far from the return duct and in a corner. I think the PO wanted it in a less conspicous location.

  3. #16
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    Quote Originally Posted by motoguy128 View Post
    If it's become unstable in it's control response, the issue is likely not the controller, it's probably something like thermostat location. Garbage in = garbage out. Maybe a water heater tuns on at that time creatign a negative draft that draws air into the wall cavity behind the thermostat, creating a draft across the temprature transmitter that creates a small shift in the local air currents. You'd be suprised. Always make sure any wall penetrations behind a thermostat are sealed. Poor thermostat location can cause a lot of issues. I've relocated on of mine for just that reason and need to relocate another that lcoated too far from the return duct and in a corner. I think the PO wanted it in a less conspicous location.
    That could be the case here. I was using a wired remote temperature sensor in an interior wall, which I have stopped using. It's possible that this wall could get cold inside. I'll check it out with my IR meter. The thermostat is also on an interior wall, but is closer to the returns than the remote sensor, so maybe air mixing is better there.

  4. #17
    Join Date
    Jun 2009
    Location
    Albuquerque NM
    Posts
    2,476
    Oh, and I see that the VisionPro uses P&I, not PID. Oops. But I"ll take a guess that it's continually adjusting the integral period. Maybe there's some lag there when the outdoor temp drops as fast as it does here at night.

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