Residential space conditioning equipment is responsible for about 9% of total U.S. end-use energy consumption. Through the combustion of fossil fuels, both in the home and at the power plant, space conditioning accounts for 423 million metric tons (MMT) of CO2 emissions annually. It also results in 1.2 MMT of sulfur dioxide (S02) and 830,000 metric tons of nitrogen oxides (NOx)' as well as significant emissions of carbon monoxide, particulates, volatile organic compounds and lead