Central Air Conditioners and Heat Pumps
Central air conditioners and heat pumps utilize a large compressor unit located outdoors to distribute cooled or heated air through a forced-air system. In a central air conditioning unit, the compressor cycles air from indoors over a coil filled with refrigerant to cool the inside. Heat pumps, on the other hand, are two-way air conditioners. While heat pumps can provide cool air, a reversing valve allows a heat pump system to reverse the air conditioning cycle, where the compressor cycles heat from the outside over a coil for distribution indoors. Central air conditioners and heat pumps can either be “single package” systems, where the evaporator coil and the condensing unit are combined into a single physical unit, or “split systems,” where the condensing unit is typically placed outdoors while the evaporator is indoors. Split system equipment is far more common than single package systems.
The prior national standards for central air conditioners and heat pumps, which raised the minimum Seasonal Energy Efficiency Ratio (SEER) requirement from 10 to 13, became effective in 2006. In January 2010, HVAC manufacturer representatives and efficiency advocates presented a negotiated consensus agreement to DOE to increase efficiency standards for central air conditioners and heat pumps. The consensus agreement included regional standards for three regions: the South, the Southwest, and the North, reflecting varying HVAC needs for each climate. DOE issued a direct final rule (DFR) in June 2011 based on the standard levels in the consensus agreement. These DFR became effective on October 25, 2011 . The new standards increase the minimum cooling efficiency requirement to SEER 14 for split system central air conditioners in the South and the Southwest while maintaining the SEER 13 standard for the North. The new standards also include EER (Energy Efficiency Ratio) requirements for the Southwest region to ensure efficient operation at high outdoor temperatures. For heat pumps, the standards raise the cooling efficiency requirement to SEER 14 for all three regions and also increase the heating efficiency requirements. The standards will become effective on January 1, 2015. DOE estimates that the standards will save about 1 quad (quadrillion Btu) of energy over 30 years and yield a net present value of about $4 billion at a 3 percent discount rate.
In the U.S., about 60% of households have a central cooling system and about 19% of those systems are heat pumps. Virtually all new homes are built with central air conditioning. The efficiency of central air conditioning systems can be augmented by several technologies. A variable speed motor allows more control over air distribution, which can lower energy consumption and increase comfort. In fact, the majority of systems rated over SEER 13 incorporate variable speed motors in order to achieve this efficiency. Advanced compressors and microchannel heat exchangers, which transfer more heat per unit of face area than the typical round tube plate fin heat exchangers, are other options for improving efficiency.
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Timeline reflects state standards from 2001 to present; federal standards from inception to present.