A --Your reasons for considering replacing your old central air conditioner are all legitimate. With the higher efficiencies, the best new ones can cut electricity usage in half.
For example, if your existing central air conditioner is 12 years old, it probably has a SEER (seasonal energy efficiency ratio) of about 8. The newest, most efficient two-stage (low- and high-cooling outputs) models have SEERs from 16 to 18, more than double your current one. They also will vastly improve your indoor comfort and humidity control.
Central air conditioners that use new earth-friendly R410A refrigerant, instead of ozone-damaging R-22 freon, are becoming very popular. Within 10 years, R-22 freon-type air conditioners will be phased out of production by law. The R-22 freon will still be available for repairs.
There really is no "one best" central air conditioner for every home. The best one for your home depends on your climate, your specific house design and construction, your budget, electric rates, etc.
For example, I recently installed a new R410A central unit in my own home, but I did not select the top-of-the-line two-stage model. I have made my house so efficient that the electricity savings from stepping up to the two-stage model would have taken 30 years to pay back. For another family, it may pay back in just a few years.
If you are looking for the most efficient model with the best comfort, you cannot beat a two-stage model for efficiency and comfort. Plan on spending about 25 percent to 30 percent more for a two-stage outdoor unit as compared to a high-efficiency single-stage unit.
Since the cooling needs of your house change throughout the day and over the cooling season, cooling output varies too. The cooling capacity of a two-stage unit is typically sized for your home so that 80 percent of its running time is in the super-efficient low-output stage. Only on very hot afternoons does it automatically switch to maximum output. There are three basic designs of two-stage models: a single reversing compressor; two smaller separate compressors in one outdoor unit; or a single two-speed compressor. The first two designs provide 50 percent of maximum cooling capacity on the low-output first stage. A single two-speed compressor provides 60 percent on the first stage and 40 percent more at full capacity.
Any of the two-level output models offer many advantages over single-speed models. Since they mostly run at the low-output stage, they do not switch on and off as often. This keeps the air circulating, holds room temperatures more constant, improves air-cleaner effectiveness, reduces noisy start-ups, and dehumidifies better.
Although your old, indoor blower can be used with a two-level model, it is best to install a new one with a variable-speed blower. These special blowers use only 25 percent as much electricity as your old one and have "smart" motors with a soft-start feature to reduce start-up noise and drafts. This also allows a thermidistat to be used to precisely control humidity. If you choose a single-level model for its lower initial cost, select one with a scroll compressor (preferably R410A instead of freon). Scroll compressors use a simple, reliable design and are quiet. Installing a new indoor variable-speed blower along with it will increase the SEER by about one point.

Write for Utility Bills Update No. 824 for a buyer's guide to heat pumps. Include $3.00, a business-size SASE, and Update number. Mail requests and questions to James Dulley, Kentucky Living, P. O. Box 54987, Cincinnati, OH 45254. Go to www.dulley.com to instantly download.