Figuring out why a compressor is failing — or has failed — in a residential/light commercial cooling system can be a challenge for many contractors. That’s because there are a variety of reasons why a problem may occur, and by the time the problem is discovered, it is often too late for the compressor.
However, failures are often a result of installation mistakes and/or poor maintenance, which means that they can usually be prevented. OEMs note that product-specific training can help contractors and technicians take the proper precautions to ensure their customers’ compressors do not die a premature death.
Overheating is one of the main reasons why compressors fail, and it is usually caused by not enough refrigerant in the system. As Wayne DeCosa, commercial products training manager at Fujitsu General America, explained, hermetic compressors are refrigerant cooled, so a system with a low refrigerant charge — either due to a leak or undercharge — will cause a progressive increase in suction gas temperature.
“Excessive, superheated vapor temperatures at the compressor inlet will result in the inability to adequately cool the compressor windings,” he said. “Continuous overheating of the compressor could easily contribute to the breakdown of winding insulation. If you combine the degradation of insulation with compression, heat, and time, compressor oil can become contaminated and acidic, accelerating the likelihood of a ‘winding short.’”
Victor Gomez, vice president of operations at Fujitsu General America, added that a low refrigerant charge also reduces the suction gas velocity, which is required for oil vapor to be returned to the compressor. With low velocity, compressor oil pumped out of the crankcase and not properly returned can result in premature mechanical failure of lubricated components, such as bearings, journals, and compressor mating surfaces.
The symptoms of overheating may include compressor short cycling due to increased trips on protection devices, reduced heating and/or cooling performance, and an increase in current draw, noted Scott Riegel, director of component engineering at Trane Residential HVAC and Supply.
“Compressor operation outside the design limits can result in compressor damage in a relatively short period of time, and if not corrected, damage can progressively lead to compressor failure,” he said. “To prevent overheating from occurring, ensure the manufacturer’s installation, charging, and commissioning instructions are met. Once the problem exists, it is important to move quickly to identify the cause — such as a low refrigerant charge — and fix it.”
Any time a compressor is running hotter than it should, without proper lubrication, or generally outside of the design parameters, a failure is inevitable, said Phil Oglesby, manager of education and content development at Rheem.
“That is why airflow and refrigerant charge both need to be measured with the right tools and compared to the factory-provided charts,” he said. “Smart technology can also help. For example, Rheem’s EcoNet® enabled line of equipment can detect and provide alerts for issues that will ultimately impact compressor efficiencies and life expectancy. The ability to catch and fix functional issues in advance of system failure is a huge advantage.”
There is an art to charging a system properly, and different types of equipment require different procedures. That is why it is always important to follow all manufacturer guidelines when adding refrigerant to a system (see sidebar).
REFRIGERANT CHARGE AND PIPING
As noted above, a system that is undercharged with refrigerant may result in the compressor overheating, but overcharging a system can result in liquid floodback, which occurs when liquid refrigerant enters the compressor during system operation. When liquid refrigerant — even in small quantities — is returned to the compressor crankcase, it will flash and literally “boil the oil,” resulting in oil foam formation, said Gomez.
“Foaming of the oil can and will result in premature wear of lubricated surfaces inside the compressor,” he added. “Additionally, larger quantities of floodback can result in permanent mechanical failure from compression of liquid refrigerant.”
An overcharged system may also result in slugging, which is when liquid refrigerant — by itself or combined with oil — enters the compressor during transient conditions, such as charging, startup, or defrost operations. This condition can result in breakage of internal components, leading to loss of heating and cooling performance and/or compressor failure, said Riegel.
“Small amounts of compressor damage can occur with limited slugging, but slugging and flooding can be extreme enough for immediate catastrophic failure,” he said. “To prevent this from occurring, ensure the inside and outside units are AHRI-certified matches in terms of capacities and flow control devices. Also make sure the system is charged appropriately per the manufacturer’s guidelines considering the flow control devices (e.g., orifice, TXV, EEV).”
Scroll compressors, in particular, can be sensitive to the consistent presence of liquid refrigerant, which can cause pieces of the scroll to break off, said Jeff Preston, technical support manager of residential ducted systems at Johnson Controls. He noted that when a system is under- or overcharged or piped incorrectly, mechanical failure of the compressor typically occurs due to multiple on/off cycles and heat pump defrosts.
“For heat pumps, damage to the compressor typically occurs within the first few days of the heating season,” he said. “Fortunately, refrigerant problems can usually be fixed and prevented by following OEM refrigerant piping guides and the use of accessories such as accumulators and crankcase heaters.”
Refrigerant piping size and length are also critical for proper compressor performance, which is why OEMs are very careful to specify line diameters, as well as minimum and maximum allowable lengths, particularly for single zone, multi-zone, and VRF systems.
“Unlike unitary [ducted] systems, inverter-driven [ductless]
systems can significantly vary the mass flow rate of the refrigerant in the system, thus requiring exact line diameters specified by the manufacturer’s design program or application guidelines,” said DeCosa. “Too-small refrigerant lines can cause excessive pressure drop, loss of unit capacity, and starve the compressor of suction gas cooling. If vapor line diameters are too large, they won’t provide adequate refrigerant velocity to entrain the oil back to the compressor, which will cause premature wear and eventual seizing of compressor components.”
Refrigerant lines that are too long will also reduce system capacity and may compromise suction gas cooling to the extent that the line length is exceeded, added DeCosa. Conversely, lines that are too short may not provide sufficient system volume for a given outdoor unit refrigerant pre-charge, which could potentially result in liquid floodback to the compressor.
Contaminants such as moisture or oxidation can wreak havoc in a cooling system by causing the oil to become acidic, which can eat away at compressor windings. Employing best practices when installing refrigerant piping can help keep a system free of contaminants, which can shorten the life of a compressor.
“In most air conditioning and heat pump systems, the practice of high temperature brazing is used to join copper refrigerant piping and fittings,” said Gomez. “As the inside of refrigerant lines must be kept to a near sterile condition during installation, a very low pressure nitrogen purge must be introduced while brazing. The nitrogen purging displaces oxygen, thus preventing oxide formation. When proper nitrogen purging is not performed, oxides — in the form of scale — are circulated through the system. This can clog strainers, expansion valves, and contribute to acid formation of the refrigerant oil, which ultimately leads to premature compressor failure.”
Keeping the piping ends sealed during storage and installation until brazing is performed is also crucial, in order to keep any debris, such as dirt, dust, sand, and other contaminants, from entering the lines, said DeCosa. The installation of a liquid line and or suction line drier — as prescribed by the OEM — should also be considered for most installations, in order to “mop up” any contaminants that enter a system.
Contaminants in a system can be difficult to diagnose until the compressor fails, noted Preston, but taking pressure measurements can give an indication if they are present in a system.
“If contamination is light, then the condition can last over a few seasons; however, compressor failure is inevitable if not treated,” he said. “Treatment involves removing the contaminated refrigerant and oil, and evacuating and replacing with clean products. Ultimately, following best practices for nitrogen use, proper evacuation, and good brazing skills will exponentially reduce the chance of a compressor failure.”
INCORRECTLY SIZED AND MATCHED
As mentioned earlier, another issue that can cause problems for the compressor is if the split system is mismatched, meaning that the indoor coil is the wrong size for the outdoor coil. When an undersized indoor coil is mismatched to the outdoor coil, for example, this can cause the compressor to experience liquid floodback, said Preston.
“This condition is diagnosed through diagnostic temperature and pressure readings and can be fixed (or prevented) by reviewing technical guides for proper OEM matched equipment and installing per guidelines of AHRI matched models,” he said. “This problem can cause most types of residential split system compressors to fail.”
Contractors should also always perform a complete load calculation for the building, which ensures that the system is correctly sized for indoor and outdoor design conditions. According to Gomez, systems that are too large or too small will place unnecessary stress on a compressor, resulting in poor conditioning of the space.
POOR INSTALLATION AND MAINTENANCE
Most of the compressor problems discussed so far can be attributed to poor installation and/or maintenance practices. In fact, Oglesby notes that improper installation and/or application is the reason for most compressor failures, followed closely by lack of maintenance.
That is why it is essential for installers of both ducted and ductless systems to follow all OEM guidelines for application, installation, startup, and service, said DeCosa.
“Many inverter-driven variable-speed systems today use flare connections, which can be a new challenge for installers to master if they aren’t familiar with flaring and the use of torque wrenches,” he said. “Careful attention must be given to refrigerant line sizing and initial pressurization and evacuation to ensure the absence of leaks. It’s critical to follow the correct minimum unit clearance and refrigerant charging procedures, because they’re essential to the longevity and life expectancy of any system.”
He added that because of the complex nature of inverter-driven HVAC systems, any unitary “rules of thumb” that may have worked in non-inverter systems must not be used at any time.
“It’s very important that all start-up and commissioning processes are followed — just as with unitary systems,” said Gomez. “This allows close analysis of many operating points through sensor readings to qualify a system’s operation. Contractors who are new to inverter systems should seek the support of a technical service advisor (offered by many distributors and/or manufacturers) when installing their first few systems to ensure there are no mistakes during installation or startup.”
In all ducted systems, DeCosa said that airflow checks must be performed, such as external static pressure (ESP), to verify the total pressure loss of the duct system does not exceed that of the blower capability.
“A quick ESP measurement at the indoor unit will quickly determine if the correct CFM range is obtained across the indoor coil,” he said. “Heat pumps are especially critical, as the indoor coil in the heat mode will typically be required to reject more heat than absorbed by the outdoor coil. Improper airflow can and will result in poor system performance, and in severe cases, potential refrigerant floodback to the compressor.”
Proper routine maintenance can also improve long-term reliability of the system by allowing technicians to identify issues before they can cause a catastrophic failure, said Riegel. He added that maintenance including basic tasks such as changing filters to ensure proper airflow to the indoor and outdoor coils and pruning outdoor landscape can help keep a system operating as designed.
Indoor coils that are not regularly maintained can adversely affect airflow and delivered capacity, as well as cause potential floodback to the compressor, said DeCosa. Outdoor coils also require regular maintenance to ensure proper airflow, so that they can provide total heat rejection of heat absorbed by the evaporator, as well as the heat of active compression within the compressor.
“A dirty outdoor unit coil will result in the inability to reject heat, thus increasing compressor operating temperatures,” he said. “An increase in temperature typically results in an increase in pressure. If this happens, the compressor then has to overcome a higher compression ratio — expend more energy while providing less capacity.
Unfortunately, many home and business owners neglect to have their systems properly maintained, which is why contractors need to educate consumers about the need for routine maintenance. They can also make sure the system was installed properly in the first place by obtaining the correct training for the types of products they are selling.
“I encourage contractors to attend training on an ongoing basis,” said Oglesby. “For example, Rheem offers in-person, hands-on training programs at its five state-of-the-art Innovation Learning Centers for contractors to learn proper installation of HVAC equipment and how to be most efficient in the field. The learning centers are located throughout the U.S. and Canada and help effectively guide HVAC contractors through the quickly evolving industry.”
Gomez agrees that manufacturer training and support play a key role in ensuring a system is installed correctly, noting that there is no substitute for product-specific training.
“While some may claim that many HVAC systems have similar operation, and features, the specifics of line sizing, refrigerant charging, system start-up, and function settings for both indoor and outdoor units are proprietary,” he said. “Regardless of the type of HVAC equipment, manufacturers readily agree that compressor (and other component) failures are substantially reduced when there’s a concerted effort to promote and host training about the proper equipment installation and startup procedures.”
Charging Methods Differ By System, Manufacturer
Methods of refrigerant charging can vary greatly, especially between ducted and ductless systems. For example, inverter-driven (ductless) HVAC equipment modulates to meet an instantaneous zone demand, said Fujitsu’s Wayne DeCosa, so the system modulates to meet the aggregate (or collective) zone demands.
“Refrigerant pressures and flow rates may not remain constant as the compressor and fan(s) increase and decrease speed,” he said. “Also, many inverter systems use subcooled heat exchangers that provide two to three times the subcooling values compared to non-inverter systems, with liquid temperatures well below the outdoor ambient temperature. For this reason, ‘conventional’ methods of system charging used in the past must not be used with modern, technologically advanced systems.”
That is why for all Fujitsu inverter-driven systems, regardless of type, the refrigerant charge must be weighed in and calculated as a function of the total installed liquid line length, said DeCosa. This is the only method that may be used, as charging by any other method may result in a significant under- or overcharge.
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