How to Diagnose & Repair Air Conditioner Compressor Hard Starting

HARD STARTING COMPRESSOR MOTORS - Air conditioning compressor hard-starting - How to diagnose air conditioner start-up problems, Air conditioner stutters or makes noises at start-up, When to install a hard-start air conditioner repair kit with starting capacitor, Hard starting can indicate end of air conditioner life, or it may be just a small repair, Air conditioner compressor defect diagnosis & repair guide
REPAIR GUIDE - A/C or Heat Pumps, separate article
BURNED-OUT COMPRESSOR - separate article
CAPACITORS for HARD STARTING MOTORS - separate article
CONTACTOR RELAY DIAGNOSIS & REPAIR - separate article
CONTROL CIRCUIT BOARD, A/C - separate article
CRANKCASE HEATERS - separate article
ELECTRIC MOTOR DIAGNOSTIC GUIDE - separate article
HARD STARTING COMPRESSOR MOTORS
INSTALLATION ERRORS, COMPRESSORS - separate article
LEVELING REQUIREMENTS, COMPRESSOR - separate article
LONG-ON CYCLING AC COMPRESSOR - separate article
NOISES, COMPRESSOR CONDENSER - separate article
PRESSURE READINGS, COMPRESSOR - separate article
REPLACING A COMPRESSOR - separate article
SHORT CYCLING AC COMPRESSOR
TIGHT or SEIZED AC COMPRESSORS - separate article
Questions & Answers about diagnosing and fixing hard-starting A/C or heat pump compressors
References

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This air conditioning repair article explains procedures for the diagnosis and repair of hard-starting air conditioning compressors. If your air conditioner or heat pump compressor motor won't start or has trouble starting, humming at start-up for example, it may be damaged and at or near end of life. But a common repair performed to try to get such motors running and to keep them in use longer is the installation of an air conditioner or heat pump "hard start" kit - a start capacitor or a start/run capacitor that helps get the motor spinning. First it's useful to diagnose the cause of the compressor's hard-starting trouble.

Also see CAPACITORS for HARD STARTING MOTORS. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

HARD STARTING - How to Diagnose & Repair Air Conditioner Compressor Hard Starting or Intermittent Running or Stuttering

Stuttering or on-off at startup: A "hard starting" compressor may stutter or begin to cycle-on then stop, then restart. If a compressor is frequently tripping the circuit breaker (or blowing the fuse) which protects its circuit, compressor and wiring diagnosis and repair are needed.

Also check: the start relay and all electrical wiring connections to the compressor motor. Loose wires or a bad start relay can also explain stuttering or "on-off" at compressor motor startup. (Thanks to Texas Red and his HVACR Technician for this tip.)
A/C Compressor starts, runs just briefly, then shuts off: this condition too may be solved by adding a starting capacitor, but watch out: it may also be a sign of a compressor near end of life. See CAPACITORS for HARD STARTING MOTORS. As reader S.W. wrote:

I had a curious condition whereby the compressor would run for a period of time...at least 5 minutes but usually longer, then suddenly shut down. The condenser fan which is essentially in parallel with it across the line continued to run. I tried to determine what sort of [overload] was in the Copeland U9X CR28K7 compressor. It appeared that the OL was opening but I didn't determine why. The compressor was not drawing excessive current and the house seemed to stay at the thermostat set point. I was been unable to obtain any details on the construction of the Copeland compressor. ... The problem was resolved - a simple matter of adding a hard start kit. Apparently the compressor increased slightly in start friction over the last year or so which made the problem appear intermittent.
.Low line voltage: Abnormally low line voltage may also be causing a compressor to "hard start". Air conditioner supply voltage is typically checked using a VOM (voltage ohms meter) right at the air conditioner service disconnect box near the compressor/condenser unit. Voltage should be within 10% of the required number (on the compressor/condenser data tag). A service technician may also check line voltage again at the compressor terminals when the compressor has reached normal operating state to be sure that there is no voltage drop.
Starting an air conditioner against compressor head-pressure: When an air conditioning compressor has been running long enough to reach its normal operating condition, it has pumped refrigerant to a high pressure condition in the compressor head.

When an A/C system compressor is running and is unexpectedly shut off, perhaps by a human testing a thermostat or switch, if the system is immediately turned back on, some compressors, particularly older window and wall units, may be unable to re-start against this high head pressure.

Simply waiting a few minutes for pressures to equalized may be all that's needed. So if the compressor is not starting in this condition we simply turn it off and wait. If this problem is happening often, a service technician may install one of several versions of "hard start kit".
Do not just install a larger fuse or circuit breaker as doing so risks a fire or equipment burn up. Do not bypass fuses such as by installing copper tubing in place of fuses as some A/C service people recommend. This is a crazy electrical hazard risking fire, shock, and equipment burn-up. If new circuit breakers are to be installed to replace an older fuse-protection on an air conditioning circuit, the electrician should remove any obsolete, unused electrical equipment or devices to avoid future confusion or possible shock hazards.
A "hard start kit" is basically a capacitor which gives an extra electrical "jolt" to the compressor motor to get it moving. A starter relay may also need to be installed or replaced. Some air conditioning compressor brands do not usually need a hard start kit. GE, Trane, and Manurope are examples that do not usually take this kit. See CAPACITORS for HARD STARTING MOTORS for photos, wiring diagram, and installation instructions for air conditioner compressor, fan, blower, refrigerator motor, freezer motor, or other electrical motor starting booster capacitors.

Electric motor starting capacitor (C) Daniel Friedman

Bad air conditioner starting capacitor: your air conditioning compressor may already have a starting capacitor installed, but the starting capacitor itself may have failed. While electric motor starting capacitors can fail in a variety of ways that may not be obvious without performing some electrical tests, on occasion the failed starting capacitor may be visually obvious.

Most electric motor or compressor starting capacitors used on air conditioning equipment are in a round cylindrical shape. The two ends of the cylinder should be flat. If your starting capacitor is visibly bulged at its end(s) or anywhere else, or if it looks split or burned, it's almost certainly damaged and needs replacement.

Watch out: other problems can cause the starting capacitor to fail, and some starter capacitor failures may not be visibly apparent. If your starting capacitor is replaced and fails again, further diagnosis of the failure source is needed.

See CAPACITORS for HARD STARTING MOTORS where we explain how to select, install, or test an electric motor starting capacitor. There we also comment that in addition to hard starting or a non-starting air conditioner or heat pump compressor motor, a bad starter capacitor can also disable the fan in the outdoor compressor/condenser, or the blower fan in the indoor air handler unit. Thanks to reader George Fazio for this air conditioner start-up diagnosis suggestion and for the photo of an un-damaged starter capacitor (above left).
Tight or Seized Air Conditioner Motors can be hard to start and may make a "humming" noise while trying to start-up and the compressor motor may fail to start at all, eventually tripping the circuit breaker or blowing the circuit fuse.

It might be possible to get a slow or hard-starting tight or even seized AC compressor motor going again - see TIGHT or SEIZED AC COMPRESSORS for more details on how this is done. But in any case such a compressor is probably near the end of its life.
End of air conditioning compressor life may be near: A compressor which has difficulty starting might be fixed by installing a "hard start" kit, but depending on the reason for hard starting it's possible that the entire compressor will have to be replaced soon. On a more optimistic note, some hard start kit manufacturers assert that installing a "hard start kit" on a compressor will extend its life.

By assisting the compressor in starting at up to ten times faster than normal, Kickstart® hard start devices significantly reduce the amount of damaging heat that is generated in the motor windings with each and every start. Over time, this reduced stress on insulation, wiring, and other critical components of the compressor has the effect of increasing its reliability and extending its useful life." -- www.kickstartoem.com

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Questions & answers or comments about diagnosing and fixing hard-starting A/C or heat pump compressors

Question: Could Loose Electrical Wiring (or a bad A/C start relay) Explain a Hard-Starting Air Conditioner or Heat Pump Compressor Motor?

I had a tech come out and install a hard-start kit to keep a 4-year-old 3-1/2 ton Carrier compressor from tripping the circuit breaker. The compressor failed this summer. A second tech was suspicious that a compressor less than five years old would be tripping the breaker. He discovered a loose connection in the cut-out switch. Would the hard-start kit have been necessary had the loose connection been tightened prior to it's installation? I'm wondering if the failure could have been avoided with proper diagnosis by the first tech

In 2009 tech # one had come to respond to my problem of a 4-year-old compressor kicking the circuit breaker. Without even looking at the connections in the shut-off box adjacent to the unit, he went straight into control panel, checked compressor current draw at start-up and installed a hard-start kit. I thought it odd that a compressor less than five years old would be drawing too much current. Had no problems in 2010 but last month the circuit breaker started kicking off again late in the day.

Tech #one returned, checked coolant pressures and return temp, added some freon and started to pack up. I asked if he wasn't going to check the current draws. He reluctantly removed the control cover, checked draw and voltages and pronounced the unit was running perfectly before leaving with my $168 check. When the circuit breaker kicked off again the next day, I started to suspect the breaker might be worn out. Called an electrician a few days later.

He replaced the breaker switch, again without checking the connections at the shut-off box, and within eight hours my compressor had died - coils shorted to ground - confirmed by tech # one who quoted me a price to replace the unit. Now I called a contractor I knew asking if he knew any good AC techs. He referred me to tech # 2 who, in my first call, was immediately suspicious that my original problem may have been in the wiring, not the unit. His reasoning was that a compressor less than 5 years old shouldn't have been giving me problems to begin with. He came over to confirm my compressor was shorted out. He then did what no other tech had done since I owned the house - popped the cover off the shut-off box and tightened the leads at the terminals - black wire loose, white wire "very loose."

My question is whether that may have been my problem all along and would a hard-start kit have temporarily relieved the symptom -tripped circuit breaker- without curing the problem.

Reply:

Tx Red: you and your second HVACR Tech have come across a useful diagnostic step: check for loose, or even burnt or arced wiring connections. I can't be sure of the answer to your question, as time has passed and we can't know for sure what conditions the first tech saw. A bad relay can cause compressor start problems, and I pose that its possible that arced, burned, or loose connections at the relay could make it misbehave.

Your thesis that a loose wire could have been misdiagnosed as a hard-starting A/C or heat pump compressor motor is reasonable but not, IMHO, absolutely proven by events. A compressor start relay (or most other switches) that are loosely wired will typically experience arcing at the loose connection (an event that can, by the way, when the circuit is active or the switch closed, draw higher amps than normal due to the resistance at the point of arcing and corrosion.)

Adding a start-capacitor is adding a device that gives a big voltage surge just during motor start-up. A voltage surge that normally is overcoming inertia in a still motor might also overcome a start relay contactor resistance, caused by damaged or burned connecting points, or by a loose connection that was in effect acting in that same fashion. The start capacitor is delivering voltage and pushing the motor into rotation.

So without being an E.E. but with some experience around this topic, my reply is "could be" but not that we can say for sure.

Also FYI even a brand new A/C compressor could become hard-starting due to other system problems that cause internal damage to the device or even by (perhaps less likely) a manufacturing defect, events at the property, even general power delivery conditions such as abnormally low voltage. To research the topic you'd look for events that can damage the start winding on an electric motor.

Finally, it's not a surprise that the first tech didn't check nor re-check for loose wiring but I agree it would have been a useful step. Often service techs, contractors, and some investigators become habituated to a short cut of trying the "fix" that usually works successfully and fast.

IF the original start relay or contacts that were found to have been loose is still installed, one could investigate further by inspecting that device for evidence of arc burning or damage.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

George Fazio, reader, contributed comments on failed starter capacitor diagnosis by noting the bulged capacitor ends. 09/25/2009
Thanks to reader Skip Wallenburg for discussing repairs to hard-starting and intermittent-shut-down air conditioning compressors, August 2010.
ALUMINUM WIRING HAZARDS - aluminum electrical wiring often burns-up at high-amperage electrical connections such as air conditioner wiring.
Federal Pacific Electric (FPE) FPE Stab-Lok Circuit Breakers can increase the risk of a fire if these breakers are used on air conditioning or heat pump equipment
Zinsco Electrical Circuit Breakers: overheating, failure to trip, burn-ups involving Zinsco and certain Sylvania electrical panel components. These components can also increase the risk of a fire if these breakers are used on air conditioning or heat pump equipment

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

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Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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