Strange compressor behavior
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  1. #1
    Join Date
    Sep 2012
    Posts
    6

    Strange compressor behavior

    Guys,

    I have a 12 year old Carrier central a/c unit. The compressor (R-22, 230 Volts "scroll" type) has been displaying some strange behavior for the past year or so. When my a/c guy came over last year, he thought it just needed a charge and that the system might possibly have a very small leak (couldn't find one with the electronic "sniffer", but it was quite windy that day and he couldn't take proper readings around the compressor unit's coils).

    I hadn't at that time realized that it was acting strangely and that its cooling behavior seemed to depend on outside temperature. When it is very hot outside (say, >95 F) sometimes the compressor starts but doesn't cool properly. I placed an electronic thermometer in the first air duct output. Normal operation seems to happen when the air temperature in the duct is between 55-62 F (depending on outside temp). However, sometimes the compressor runs but the duct air temp stays around 68-72 F. Cycling the compressor (switching off, waiting a number of seconds, switching back on) normally seems to fix the problem. I also took "clamp" amp readings (I am in the electrical/electronic field) and each leg draws around 7-8 amps when it doesn't cool properly and around 12 to 14 amps when it does.

    We just got a cold front into Texas and the temperature yesterday night fell quite a bit. It is warming up during the day but the compressor refused to cool properly even after cycling this morning a few times (did not draw more than 8.5 per leg). Leaving it on for 20 minutes and then cycling again got it back to normal cooling (and normal amp draw) which seems to show it only wants to work normally when it is hot.

    Is this something you guys have seen before? Could a lack of charge (or maybe air/moisture in the system) cause this kind of symptom? Maybe the leak (if there is one) might have gotten worse? Or could this be a sign that the compressor is "shot"?

    Any input greatly appreciated. I am trying to find out more about this problem before I call my a/c guy back again.

    Thanks,

    Clovis

  2. #2
    Join Date
    Jan 2010
    Location
    Central Texas
    Posts
    155
    By cycling the unit, do you mean that you are shutting off the compressor at the thermostat, or by killing power to the breaker/disconnect? I've had some scroll compressors that would try to run backwards due to a weak capacitor or even in some rare cases, due to a bad time delay relay. The relay would open up momentarly to shut off the compressor and as the compressor spun backwards to relieve pressure from the high side to low side, it would restart and run backwards. Usually this is indicated by a very noisy compressor. The fact that you are drawing half the amperage indicates to me that it might be running backwards.

    You will need to have a service tech come out and check the unit.

  3. #3
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,410
    Sounds like you've got a compressor going bad to me!

  4. #4
    Join Date
    Sep 2012
    Posts
    6
    sgraefe,

    Think you are on to something... I had forgotten that the capacitor (and the relay) were replaced and the unit cooled just fine after that happened. However, a few months after the replacement this problem came back again. By "cycling" I mean at the breaker (that is where I am measuring the amps) but I always wait at least 20 seconds to restart it.

    Which means that the compressor might be doing something that is killing the capacitor and/or it is not the proper capacitor or relay for that unit. However, there wouldn't be any reason for it to start running backwards after being off for several minutes, would it?

    I will call my guy but am glad this kind of behavior has been seen before. Thanks for the input!

    Clovis

    Quote Originally Posted by sgraefe View Post
    By cycling the unit, do you mean that you are shutting off the compressor at the thermostat, or by killing power to the breaker/disconnect? I've had some scroll compressors that would try to run backwards due to a weak capacitor or even in some rare cases, due to a bad time delay relay. The relay would open up momentarly to shut off the compressor and as the compressor spun backwards to relieve pressure from the high side to low side, it would restart and run backwards. Usually this is indicated by a very noisy compressor. The fact that you are drawing half the amperage indicates to me that it might be running backwards.

    You will need to have a service tech come out and check the unit.

  5. #5
    Join Date
    Sep 2012
    Posts
    6
    Interesting!

    Following the wiring diagram, this unit (38CK036330) apparently is supposed to have a start relay and a thermistor (and/or a start capacitor). The dual-run capacitor is there, but the relay and thermistor are awol and there is no sc.

    This was a rental property and it looks like the wiring has been messed with (the renters did have an a/c tech come over a couple of times and he did change the main capacitor). No way to find out what the original wiring was like and/or if the design of the unit somehow changed and does not correspond with the wiring diagram on its plate. I did however find out that the unit is from 1994 and therefore almost 18 years old, so it might be about time to replace compressor and indoor unit anyway and not go any farther on this. It will likely be a huge waste of time.

    Thanks again for your help!

    Clovis

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