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  1. #1
    Join Date
    Jul 2011
    Sahuarita, Arizona

    Troubleshooting Puzzle: A Heat Pump That's Not Working

    In this troubleshooting situation, you have a customer who has called to say that the heating/cooling system for their office is “not keeping people comfortable”.

    When you arrive at the strip mall-type office complex and locate the customer’s suite, your first step is to confirm the type of equipment; an eight-year-old split system heat pump in which the outdoor unit is on a pad at the rear of the building.

    You also confirm that the indoor air handler (located above the ceiling in the office) and the outdoor fan motor, are operating normally.

    As you are removing the outdoor unit access cover, you also notice that the outdoor fan motor slows down momentarily, then speeds up again, indicating that the compressor has made a failed attempt to start. With this condition confirmed, you determine that this is a PSC operated compressor, and what you see is the following wiring configuration:

    A single-pole contactor.

    A run capacitor wired from T1 on the contactor, in series with the start winding of the compressor.

    Upon a close inspection, you also determine that the run capacitor you see wired to the compressor was the component installed in a previous repair, which the customer confirms was performed over a year ago. The reason you notice this is because the mounting bracket was "modified" to hold the replacement part in place. When you disconnect the power supply and isolate this component, your meter shows the correct microfarad rating as it is printed on the capacitor case.

    Since your troubleshooting steps thus far lead you to consider the compressor as the source of the problem, you consult the wiring diagram on the access panel for information, and, at that moment, you note that the wiring configuration for the compressor, as shown on the manufacturer's diagram, is:

    The single-pole contactor.

    A capacitor identified as "Ca" wired from L1 on the contactor to the compressor start winding.

    A capacitor identifed as "Cb" wired from T1 on the contactor to the compressor start winding.

    With this circuit understood and your compressor test procedure confirming that it is mechanically stuck and unable to start under any circumstances…. here is your troubleshooting question:

    What is the underlying cause of this compressor failure?

    Jim Johnson

    Learn from yesterday….Live for today….Look forward to tomorrow

  2. #2
    Join Date
    Feb 2007
    Lack of run capacitor?

  3. #3
    Join Date
    Jul 2011
    Sahuarita, Arizona
    You're on the right track. Here's my take on the complete explanation on this one.

    A failure of the original capacitor led to the wiring modification that eliminated the capacitor wired from L1 to the start winding.

    Without that capacitor, which provided a trickle charge through the start winding in an off cycle, gravity allowed liquid refrigerant to migrate to and lay in the crankcase of the compressor, which caused oil foaming on start-up.

    The lack of lubrication led to a mechanical failure of the compressor.

    Jim Johnson

    Learn from yesterday......Live for today.....Look forward to tomorrow

  4. #4
    Join Date
    Dec 2005
    Cincinnati, Oh
    Whats probably not helping the situation, is the HP is on a pad, instead of pump ups.

    Winter Damage.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

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