Refrigerant laws are changing. January 1st, 2010 the old chlorine based R-22 refrigerant is totally going out of production. The only good news for all that are using it is, recycled refrigerant will soon be available to consumers, hopefully this will help to ease some of the price gouging out there. If you are in the market for a new system you will want to make very sure that your new unit will use the new "Puron" R-410a refrigerant. Carrier has been selling "Puron" systems for many years now, and currently leads the industry in R-410a equipment sales. Many competitors have just recently changed their product line to "Puron". I would use much caution when choosing the manufacturer of my new unit, because you definately do not want to choose a company that has just recently began manufacturing the R410a systems. The base line efficiency for any equipment manufactured today is 13 SEER (Seasonal Energy Efficiency Ratio). Buy as much efficiency as you are able to afford. Electricity just keeps going up in cost. The two-speed units are the most efficient. Two-speed systems start at 16 SEER and top out at 21 SEER for a conventional system. Geothermal units will go much higher in efficiency. Geothermal units are measured in EER (Energy Efficiency Rating).