Cut open compressor with undamaged windings.

Compressor Failure in HVAC Systems

DonanEngineering HVAC Forensics, Lightning/Power Surge

Heating, ventilating, and air conditioning (HVAC) systems consist of several components that work in unison to create the desired comfort level for our homes and offices.  A well- maintained system is designed to last about 15 years, but the life of the system can be dramatically reduced when proper installation procedures and routine maintenance are not followed.

System Components

The HVAC refrigeration system is composed of the outdoor coil, compressor, lineset, metering device, and indoor coil.

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The compressor is the main component of the system and commonly referred to as the “heart” of the system because it pumps the refrigerant through the coils and the lineset.  The inside of a compressor contains a rotor, pump or scroll, and stator (windings) with leads that are connected to the terminals through the hermetic seal.

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A failed compressor can be observed with a direct short to ground of the windings, a winding to winding internal short, a hermetic seal rupture or a mechanical issue.  These observations provide clues about the cause of failure, but electrical meters cannot be the sole means used to determine cause of failure.  There are two main reasons further investigation is needed to draw a conclusion about cause of failure: 1) the compressor is hermetically sealed, and 2) the compressor has electrical and mechanical parts that commonly fail.  A mechanical failure can cause an electrical failure inside the compressor, which is not the result of a lightning-induced power surge (although it is commonly mistaken for lightning failure).  The following sections will detail how a compressor can fail.

Compressor Failures

The amount of refrigerant or charge in a system can affect the performance and wear on a compressor.  Too much refrigerant can cause liquid refrigerant to enter the compressor leading to mechanical issues.  Too little refrigerant can cause the compressor to overheat.  A properly charged system allows the returning suction gas to keep the compressor windings cool.  The oil in the compressor provides lubrication to its moving parts.  If a system loses its refrigerant and oil, the compressor will cycle on its thermal overload.  This loss of refrigerant and oil can cause the mechanical parts to lock-up without causing any electrical damage to the compressor’s windings.  The thermal overload is an internal protection device which opens the electrical circuit in the event an overheating condition occurs.  This keeps the compressor from catastrophically failing, but this mechanism has limits and may fail if the unit cycles on its overload many times throughout its life.  A compressor operating on thermal overload will be very hot to the touch and the service technician should allow it to cool prior to determining that the compressor has electrically failed.  If the compressor’s winding resistances are good, then the failed compressor is either locked-up mechanically or it is operational and there might be an issue with the system’s controls or refrigerant level.

Only two things should be present inside of the sealed copper tubing of the refrigeration system – refrigerant and refrigerant oil.  A service technician achieves this during installation of the system through the “triple evacuation method.”  The system is purged with nitrogen and then evacuated using a vacuum pump.  Performing this action three times and down to 500 microns ensures a clean system.  Contaminants like moisture or debris can cause premature compressor failure if not removed prior to start up.  Acid burnout commonly occurs with improperly evacuated and/or poorly maintained systems.  Acid occurs in systems when the moisture-contaminated refrigerant is compressed and circulated over a period of time, or excessive long-term heat causes the oil to breakdown triggering the unwanted chemical reaction.  A simple acid test can reveal the presence of the slow breakdown in the system.  The acidic refrigerant and oil breaks down the windings varnish and insulation in the compressor.  This also causes the oil to lose its lubricating properties resulting in the compressor’s slow death.  The compressor’s windings will either short to ground, experience a hermetic seal failure, or will mechanically lock up.  The unit’s circuit breaker will trip during this event and an HVAC contractor is commonly called in to diagnosis the failure.

Investigation Options

Compressors can fail for many different reasons:  wear and tear, age, improper installation or repairs, poor maintenance practices, power surge and others.  Extending the life of a compressor requires routine maintenance, proper refrigeration practices, and operating the system within the manufacturer’s specifications.  A properly maintained system involves keeping a clean air filter, keeping the outdoor coil clean, properly charging the refrigeration system, and regularly checking the electrical connections.  Properly diagnosing a failed HVAC system is crucial in determining the appropriate repair.  A failed compressor can be very costly to the homeowner often resulting in an entire system change.  Consequently many compressors are condemned by a service technician without determining the actual cause of the failure.

The compressor is the most expensive and the most commonly failed component (49% of the time).  Of failed compressors investigated by HVAC Forensics, lightning/power surge was the cause only 44% of the time.  Other causes of failure were acid contamination, normal wear/mechanical damage, warranty and manmade damage.

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Chart 1:  Cause of failure percentage to HVAC compressors.

Donan’s Forensic Technicians document the cause of failure to compressors in over 1,500 investigations each year.  They are experts in HVAC failure diagnosis, utilizing their field experience, meter readings, ampere readings and destructive testing capabilities when necessary.  It is important for the technician diagnosing the failure to have specialized training to determine cause of failure and to have the ability to deconstruct the compressor.  We destructively test the compressor onsite to ensure an accurate cause of failure and quick turnaround on the report.

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Photograph 1:  Compressor with undamaged windings.

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Photograph 2:  Compressor with an isolated point of failure (lightning-induced power surge).

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Photograph 3: Compressor with overheated windings caused by a mechanical failure; locked rotor.