Central air conditioners and heat pumps are appliances designed to provide cool air, warm air, or both to an enclosed space. Residential systems are considered those with cooling capacities less than 65,000 Btu/h (19,000 watts). On a national basis, the saturation of households with central air conditioning (including heat pumps) has increased significantly.

Most models are manufactured as split systems, i.e., the outdoor unit is "split" from the indoor coil and connected to it via refrigerant tubing. The outdoor unit consists of a compressor, heat exchanger coil, fan, and fan motor. The indoor coil typically resides either on top of a furnace or inside a blower-coil unit and conditioned air is conveyed to the space to be conditioned (heated or cooled) via ducts. A flow control device (e.g., capillary tube, short tube orifice, thermostatic expansion valve) is located at the indoor coil.

In "mini-split" systems, the indoor coil and flow control device are located in a self-contained unit that is typically hung on a wall inside the conditioned space. Conditioned air is conveyed without ducts. "Mini-splits" are commonly found in Asia and Europe and are gaining acceptance in high performance homes in the United States.

Other models of central air conditioners are produced as single package systems. In package systems, all the components that comprise a split system, including the air circulation equipment, are placed in a single cabinet. The packaged system resides outdoors and conditioned air is conveyed to the conditioned space via ducts. There are other central air conditioner and heat pump types that are referred to as “space constrained” products, including small-duct, high-velocity (SDHV) systems and through-the-wall (TTW) condensing units.

There are several technologies that could increase the efficiency of central air conditioners and heat pumps, including enhancements to the heat exchanger coils (e.g., enhanced fin surfaces and rifled or grooved refrigerant tubing), efficiency improvements to the compressor, and the use of two-speed or variable-speed blowers, fans, and compressors. The most energy-efficient central air conditioners and heat pumps on the market achieve efficiencies exceeding 20 SEER (seasonal energy efficiency ratio) by utilizing variable-speed indoor blowers and outdoor fans coupled with two-speed compressors.

Recent Rulemaking Activities: DOE issued a direct final rule amending the energy conservation standards for residential furnaces and central air conditioners and heat pumps on June 27, 2011. The standards established in the final rule will be applied starting May 1, 2013 for non-weatherized furnaces and January 1, 2015, for weatherized furnaces and central air conditioners and heat pumps.

For central air conditioners and heat pumps, there are national standards, as well as separate standards for southeastern and southwestern regions. Standards for split system and package central air conditioners vary based on region. In the southeastern region the standard is 14 SEER, in the northern region it is 13 SEER, and in the southwestern region standards are 14 SEER, with additional EER requirements for certain products split systems.  Standards for space-constrained equipment is 12 SEER.  For heat pumps, standards for all regions range from HSPFs (heating seasonal performance factors) of 7.4 to 8.2. There are also new standards for standby and off modes.

Efficiency Rating: Central air conditioners and the cooling portion of heat pumps are rated with a SEER rating. The heating portion of heat pumps is rated with a HSPF rating. As their names imply, both the SEER and HSPF are efficiency descriptors that account for the equipment's performance over an entire cooling or heating season. The descriptors represent the ratio of the cooling or heating provided in a typical season (in Btus) over the energy input to the equipment (in watt-hours) during the same season.

The Air-Conditioning, Heating and Refrigeration Institute (AHRI), the trade association representing most manufacturers of central air conditioners and heat pumps, provides both on-line and downloadable versions of directories of certified central air conditioning and heat pump equipment. These directories provide the cooling capacity and efficiency of all certified models.

Product Classes: For statutory purposes, residential central air conditioners and heat pumps are divided into several product classes that may be subject to different efficiency standards. For the new efficiency standards, DOE considered the following product classes:

• Split-system air conditioners
• Split-system heat pumps
• Single-package air conditioners
• Single-package heat pumps
• Small-duct, high-velocity systems
• Space-constrained products—air conditioners
• Space-constrained products—furnaces

Analyses: Under contract to DOE, LBNL's Energy Efficiency Standards Group (EES) conducted analyses for the recently completed rulemaking for the preliminary analysis and the direct final rule, including:

• Shipments Analysis
• National Impact Analysis (National Energy Savings and Net Present Value)
• Utility Impact Analysis
• Environmental Assessment
• Regulatory Impact Analysis

Efficiency Standards History: Previously, the minimum efficiency standards for split and single package central air conditioners and heat pumps were 13 SEER for central air conditioners, and 13 SEER and 7.7 HSPF for heat pumps, as of January 23rd, 2006.  EES staff examined the life-cycle costs to consumers of moving from 10 SEER to 11 - 13 SEER in a paper published by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) in 2002.

Earlier standards, which took effect on January 1st, 1992, for split and single package air conditioners were 10 SEER and 9.7 SEER, respectively. The standards for the heating side of split and single package heat pumps were 6.8 HSPF and 6.6 HSPF, respectively.