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Even though my old central air conditioner still works, does it make economic sense to replace it with a new one? It does not cool enough on the hottest days. What is new for 1997, and what features are best? -- D. W.

A -- New central air-conditioners have been peaking at Seasonal Energy Efficiency Ratios (SEER) of 15 to 16 for the past couple of years. Your old one probably had a SEER of 8 when it was new. Installing a new one can cut your cooling bills in half.

The most significant improvements in the past year have been in comfort, improved indoor air quality (especially for allergy sufferers) and reduced noise at super-high efficiency levels.

A more stable indoor temperature and better air quality allow you to set the thermostat higher without sacrificing comfort. Each degree that you raise the thermostat can cut your electric bills 2 to 3 percent.

For the greatest efficiency and comfort, select a three-cooling-stage central air conditioner. This provides three cooling output levels to match the varying cooling needs of your house throughout the day and night. There is also a new natural gas-powered model, Triathlon, with 17 output levels.

Some new units have two compressors in the outdoor unit. When the outdoor air temperature is below 88 degrees, it starts on the small compressor. During this mild weather, it runs longer on the small compressor for better dehumidification, air cleaning and air circulation. When the outdoor temperature is above 88, it switches to the large compressor. On very hot days, they both run for maximum cooling.

Other multistage designs use a two-speed compressor instead of two separate compressors. These are effective in high humidity climates with varying summer weather conditions. You can expect to pay about 25 to 30 percent more for these units than for a standard single-stage unit.

Lower-cost single-stage units also have SEERs up to 15 for significant electricity savings. They provide satisfactory comfort, especially in dry, hot climates. In humid climates, consider installing a dehumidification kit. It has a humidistat to control the blower speed for more moisture removal. Single-stage units with scroll compressors are quiet and reliable.

Write for Update Bulletin No. 624, a list of the 20 most efficient three, two and single-stage central air conditioners, cooling outputs, SEERs, blower speeds, comfort features and pay-back chart. Please include $2 and a business-size, addressed, stamped envelope and send your request to James Dulley, The Buffalo News, 6906 Royalgreen Drive, Cincinnati, Ohio 45244. Or instant download at

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