Dear Jim: Our noisy old central air conditioner is on its last legs. I am considering getting a new one, but all the sales hype is confusing. What is best for 1998, and is installing a new one a wise move?

- Pat M.

Dear Pat: There have been significant changes and improvements in central air conditioners for 1998. Some reduce utility bills, some improve the comfort level and still others affect the environment. Even with an engineering degree, I also find the sales hype confusing at times.

One of the most significant improvements is in the efficiency levels. Some models have SEERs (Seasonal Energy Efficiency Ratios) as high as 18. Compared to your old one at only 8 at best, your electric bills will be cut by more than 55 percent. In most cases, installing a new one is a wise financial decision.

The best new models are actually a step back in complexity but a step forward in reliability and comfort. Gone are the complicated totally variable speed compressors of the mid-'90s. With better controls, simpler two-speed and two-compressor models are as effective.

For the best comfort, indoor air quality and low indoor/out-door noise, a new multilevel cooling output model is a good choice. These models often cost 20 percent to 30 percent more than a typical single-level model, so you must decide if the additional comfort, quiet, etc., justifies the extra cost.

These multilevel output models operate at a super-efficient low-output level most of the time. At this quiet low level, they run longer for better dehumidification and constant indoor temperatures. They only switch to the highest-output level on the hottest afternoons.

For people with allergies, like myself, this is an advantage. Running longer at low output makes the air cleaner more effective. Also, with more constant dehumidification, mold and dust mites can be better controlled.

All of these multilevel models offer extra efficient blower motors, also ideal for continuous air circulation. These motors have soft-start (slowly ramp up to full speed) for quiet, no-draft (for winter heating too) air circulation.

Many of the reasonably priced single-level models use scroll compressors. These are very reliable and quiet with efficiencies up to a SEER of 14. Installing one of these will still cut your electric bills significantly. For climates with steady summer weather conditions, these designs are ideal.

Several models now use ozone-safe R410A (called Puron) refrigerant instead of freon. Current R-22 freon will eventually be phased out of use by law. These new Puron models run at higher pressures, requiring heavy, stronger tubing and compressors. They are durable and operate very quietly.

Write for (or instant download - Update Bulletin No. 747 - buyer's guide of the most efficient multi- and single-level central air conditioners, cooling outputs, SEERs, comfort features, warranties and a payback chart. Please include $3 and a business-size SAE.

James Dulley, Deseret News, 6906 Royalgreen Drive, Cincinnati, OH 45244