Residential central air-conditioning systems with high seasonal energy efficiency ratio (SEER) ratings have become available in recent years from multiple manufacturers. In the common split system configuration, the top SEERs available from the major unitary air-conditioning system manufacturers now range between 18 and 24.5 Btu/Wh *, ([dagger]) across the 2 ton to 5 ton (7 kW to 17.6 kW) range of residential cooling capacities. Improvements in compressor, fan and expansion valve technologies, along with generous coil sizing, have enabled significant gains in SEER rating.
The minimum efficiency of central air-conditioning units is governed by U.S. law and regulated by the U.S. Department of Energy (DOE).  The minimum SEER for residential central air-conditioning systems was set at 10.0 beginning in January 1992. At that time, few people thought that SEERs in the range of 20 could be attained cost-effectively.
Effective for appliances manufactured after Jan. 23, 2006, DOE raised the minimum SEER standard to 13.  Although the SEER minimum standard of 13 is still in effect, DOE has finalized proposed, updated rules that both raise current minimum standards for residential HVAC appliances as well as introduce regional standards. 
The amended standards for residential central air conditioners are summarized in Table 1 and will take effect on Jan. 1, 2015. 
For cooling equipment performance, the annual number of operating hours and relative humidity during those operating hours are key regional variations. Consequently, DOE defined two regions, "hot, humid" and "hot, dry" (Figure 1).  For single package ACs, DOE determined that regional standards would not produce significant energy savings compared to a single national standard. Single package systems with a minimum SEER of 14, as well as split systems with SEER of 14.5, meet the most recent ENERGY STAR program requirements. 
The technologies that enable increasing SEER ratings are those that reduce the total energy input required for a given cooling capacity, e.g., technology improvements to:
* Fan systems; and
* Expansion valves.
Table 2 presents a summary of technology improvements within the mentioned broad categories and their associated impact on energy consumption. 
In addition to these technology improvements, larger outdoor coils enable lower condensing temperature, increasing both...
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