Q. I must replace my old central air conditioner soon. I want one that is super-high-efficiency, reliable, and quiet. What are types of units are available to meet my needs? — R.M.

A. There have been many recent significant improvements in residential-sized central air conditioners. Some of the new super-high-efficiency central units have efficiencies (called SEER) as high as 17. Installing one of these air conditioners can cut your cooling bills by more than 50 percent.

The most energy-efficient central air conditioner uses a totally-variable speed compressor and fan. This allows the unit to operate at maximum efficiency for varying cooling needs and weather conditions. On very hot days, it operates at a higher speed than on cooler days. An electronic microprocessor determines the most efficient operating speeds.

Since it doesn’t cycle on and off as often, the more steady-state operation improves the efficiency and your comfort. The cooling output and air circulation are more constant throughout your home. It also operates quieter at the lower speeds.

Central air conditioners with two-speed compressors are also available with SEERs up to 15. Another unit has a single-speed compressor, but a two-speed condenser fan to boost efficiency to more than 12.

The scroll compressor design is now used in many of the super-high-efficiency air conditioners. Its advantages are its quiet operation, high-efficiency, and simplicity. The noise levels from a scroll compressor can be as low as 7.0 dB compared to 8.2 dB for a standard reciprocating compressor. This is a significant noise difference.

With only a few moving parts, a scroll compressor is less prone to mechanical and lubrication breakdown.

You can write to me for Utility Bills Update 394 showing a buyer’s guide for 21 manufacturers of super-high-efficiency central air conditioners listing models, cooling capacities, rated efficiencies, sound levels, types of compressors and numbers of speeds. Please include $1 and a self-addressed business-sized envelope. Send to James Dulley, 6906 Royalgreen Drive, Cincinnati, Ohio, 45244.

James Dulley studied energy management at the doctoral level at Harvard.