Performance and Reliability Degradation of a Residential Unitary Air Conditioning System Resulting from a Mismatch of a 1992 Air Conditioning Unit with a 15-20 Year Old Evaporator

Fred Keller Split Systems Director The National Appliance Energy Conservation Act has established a minimum efficiency of 10 SEER for residential split system air conditioners and heat pumps. Approximately 50% of residential split system air conditioners are sold as replacements for units having much lower SEER levels, typically 5-9 Btu/W·h. This investigation assessed the detrimental affects that could occur when a 1992 vintage 10 SEER air conditioner is used to replace a 15-20 year old outdoor unit without replacing the indoor evaporator section. An evaporator constructed with similar geometry as a 15 year old, 30,000 Btu/h (8,790 W) evaporator was tested with a typical 1992, 10 SEER residential air conditioner. The performance of the 1992 outdoor unit with the old evaporator experienced 5% degradation in capacity and 11% degradation in SEER. The system also operated at compression ratios typically 20% higher than the system with the properly sized evaporator; thus increasing the stresses on the compressor and hence reducing its life. INTRODUCTION The National Appliance Energy Conservation Act has mandated that all residential split system (central) air conditioners and heat pumps built on or after January 1, 1992 have seasonal efficiencies of at least 10 SEER. This is a substantial increase from older systems which typically operated between 5 and 9 SEER. By redesigning the three primary components of the system, the compressor, condenser, and evaporator, manufacturers are able to attain minimum efficiency standards. Although manufacturers focus on capacity and SEER while designing new air conditioners and heat pumps, they are also very aware of design parameters which influence the reliability of their systems. Product and more specifically, compressor reliability is highly dependent upon the proper design and installation of the air conditioning system. Historically, nearly one-half of first year residential split system component failures are compressors. Primary causes of these failures include: (1) liquid entrainment in the suction gas during start-up (slugging), (2) liquid entrainment in the suction gas during steady-state operation (flooding), (3) excessive cycling, ( 4) oil starvation and dilution, (5) operation at excessive compression ratios, and (6) overheating.