Two-Stage High Efficiency Air Conditioners: Laboratory Ratings vs. Residential Installation Performance

John Proctor and Gabriel Cohn, Proctor Engineering Group, Ltd.

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Abstract

The increased installation of high Seasonal Energy Efficiency Ratio (SEER) air conditioners along with utility program rebates for these units prompted a study of the measured performance of these systems. This project assessed the performance of these systems in the climate zones found in the mid-Atlantic region of the U.S. Similar studies in hot dry climates have indicated that laboratory SEER ratings may not properly predict the actual impact of these systems. This project monitored four high SEER air conditioners with dual-stage compressors, TXV metering devices, and high efficiency air handlers with ECM fan motors. One system with a single-stage compressor was also monitored. Data included capacity, power consumption, Energy Efficiency Ratio (EER), indoor/outdoor temperature and relative humidity. The data were analyzed to assess the relationship between laboratory testing and real world performance. This study found causes for concern including: actual seasonal energy efficiency ratios between 59% and 84% of the rated SEERs, constant fan operation substantially degrading seasonal efficiencies and reducing dehumidification, latent loads that exceed Manual J estimates, and sensible loads substantially lower than Manual J estimates. In addition there may be an energy and peak load penalty if dual-stage air conditioners are downsized to near the buildings’ actual loads. The study illustrates the intricacy of the whole building system. The air conditioners in the two leakiest building shells were unable to adequately control the indoor humidity.