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Thread: Intermittent erratic behavior

  1. #1
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    Intermittent erratic behavior

    The unit in question is a 3 ton system manufactured by Allied Air, model# 4SCU13LB136P3. Sometimes the unit operates normally, but sometimes it doesn't, which makes it difficult to diagnose.
    The normal operation is the thermostat demands cooling, the compressor and blower turn on and run until the thermostat is satisfied and the compressor turns off and the blower turns off 1-2 minutes later.

    Attached is a diagram that illustrates the abnormal behavior, and here's a description of it:

    The thermostat demands cooling, the compressor and blower turn on and run for a long time, then for no apparent reason (thermostat still demands cooling) the blower and the compressor turn off, the blower turns back on after about a second and runs for a minute or two, then it turns off for about a second and at approximately the same time the compressor turns on for a second. It then repeats, blower runs for 1-2 minutes then turns off, compressor tries to turn on for a second, and then off. I'm not sure how long that cycle will continue, but if it is noticed it can be stopped by increasing the thermostat setting until the thermostat no longer demands cooling.

    Any insight into the cause of this behavior would be helpful. Thank you for your interest.
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  2. #2
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    Excessive head pressure
    Low suction pressure
    Excessive amperage
    Low voltage

    Time to call for service!

  3. #3
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    That is not normal operation. But it is not something which can be diagnosed online.
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  4. #4
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    power stealing tstat?
    Col 3:23


    questions asked, answers received, ignorance abated

  5. #5
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    Thread Starter
    Yes, I planned to call my service company, but they tend to younger, less experienced than your group and may not have the time to consult with others, so I thought I'd try to help them out by doing that for them. And this seems to be a more unusual problem that they may not know how to address.

    There is a high pressure and a low-pressure switch, and a thermal protection switch in the control circuit which energizes the compressor contactor.

    Questions:
    If it was the low pressure switch that opened, could it run for an hour or more before tripping?
    Since it does run for quite a while before tripping it would seem more likely to be high pressure or high temperature issue.
    If it was the high pressure switch that opened could the contactor energize for just a second when the blower switches off for a second?
    If it was the thermal protector switch that opened would it reset in 1-2 minutes and trip again?

    I'm looking for a scenario that explains the behavior and then have the tech test for it.

  6. #6
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    Quote Originally Posted by Flash55 View Post
    Yes, I planned to call my service company, but they tend to younger, less experienced than your group and may not have the time to consult with others, so I thought I'd try to help them out by doing that for them. And this seems to be a more unusual problem that they may not know how to address.
    Then time to find a better service provider!

  7. #7
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    The questions you are asking are precisely the kind we cannot answer.
    You shouldn’t have to help the tech. THEY are responsible for finding answers.

  8. #8
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    What do you mean by blower?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  9. #9
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    .
    What thermostat are you using?
    ..
    Do not attempt vast projects with
    half vast experience and ideas.
    ...

  10. #10
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    Thread Starter
    Quote Originally Posted by pacnw View Post
    What do you mean by blower?
    The air handler in the furnace.

  11. #11
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    Thread Starter
    Quote Originally Posted by Sleuth View Post
    .
    What thermostat are you using?
    ..
    I have 2 different thermostats, tried them both and they give me the same results

  12. #12
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    Thread Starter
    Here's some new info: I tested the 24v supply coming into the condenser prior to the high and low pressure switches, and found the compressor was operating in concert with that signal, in other words when the 24v was present, the contactor was energized. That was the turning point and lead me to the problem's solution. The unit is fully functional now.

  13. #13
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    Quote Originally Posted by Flash55 View Post
    Here's some new info: I tested the 24v supply coming into the condenser prior to the high and low pressure switches, and found the compressor was operating in concert with that signal, in other words when the 24v was present, the contactor was energized. That was the turning point and lead me to the problem's solution. The unit is fully functional now.
    So
    What was shutting it down and why?

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  15. #14
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    So
    What was shutting it down and why?
    Elementary my dear Watson, but first don't you want to take a stab at it and solve the mystery with the clues given?

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