Modeled and measured effects of compressor downsizing in an existing air conditioner/heat pump in the cooling mode Page: 2 of 23
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MODELED AND MEASURED EFFECTS OF COMPRESSOR DOWNSIZING IN AN EXISTING
AIR CONDITIONER/HEAT PUMP IN THE COOLING MODE
#H-1048
William P. Levins
C. Keith Rice
Van D. Baxter
Oak Ridge National Laboratory
January, 1996
ABSTRACT
It is not uncommon to find oversized central air conditioners in residences. HVAC contractors sometimes
oversize central air conditioners for one reason or another - some to the point that they may be 100% larger
than needed to meet the load. Retrofit measures done to improve house envelope and distribution system
efficiency also contribute to HVAC oversizing, as they reduce house heating and cooling loads. Proper sizing
of an air conditioner or heat pump allows more efficient operation and provides a more comfortable
environment than a highly oversized unit. Another factor that lowers operating efficiency is an improper'
refrigerant charge. Field inspections have revealed that about half of the units checked were not properly
charged.
An option available to homeowners with oversized air conditioners is to replace the existing compressor with
a smaller, more efficient compressor, rather than purchasing a new, smaller unit. Such a retrofit may be
economically justified, especially during a compressor failure, provided the oversizing of the existing unit is
not too great.
A used, 15-year old, single-package heat pump with a capillary tube expansion device on the indoor coil was
purchased and tested in a set of environmental chambers to determine its cooling performance at various
conditions. The system was also modeled to estimate its existing performance, and that with two different
types of retrofitted state-of-the-art (SOA) efficient compressors with about 30% less capacity than the
original compressor. This reduced the overall system cooling capacity by about 20%.
Modeling estimated that the retrofit would increase system EER at 95*F by 30%, SEER by 34%, and reduce
power demand by 39% compared to the existing unit. Reduced cycling losses account for the higher increase
in SEER
The proper refrigerant charge of the as-received unit, determined using superheat, operating pressures, and
EER as guidelines, was 22% higher than the nameplate charge. After testing, the existing compressor was
replaced with one of the 30% smaller SOA compressors that had been modeled. Further testing confirmed
that a 33% increase in EER was attained, compared to the predicted 30%. Power demand was reduced 38%
compared to the predicted 39%.
We found that the surest way to obtain a proper refrigerant charge on our unit was to use a set of gauges
coupled with superheat measurements.
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Levins, W. P.; Rice, C. K. & Baxter, V. D. Modeled and measured effects of compressor downsizing in an existing air conditioner/heat pump in the cooling mode, article, May 1, 1996; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc667956/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.