Question - Does it make sense to replace my 10-year-old central air conditioner - even though it still runs - with a new super-efficient one? What is new with super-efficient air conditioners for 1994? - R.L.

Answer - Replacing your old central air conditioner with a new efficient unit can lower your cooling costs by 30 percent to 40 percent. The newest designs for 1994 are also quieter and more reliable. The long-term payback often justifies replacing an older, inefficient unit that still runs.

The most recent innovations in central air conditioners are two-speed and totally-variable-speed compressors and blowers. These continually fine-tune the cooling output to match the varying cooling needs of your house throughout the day and night.

On all but the hottest days, the compressor runs at a slow efficient speed. This reduces the amount of electricity consumed. When the cooling needs increase on a very hot day, it automatically shifts to higher speeds.

By coupling a variable-speed compressor with a new variable-speed indoor blower, comfort is improved. The compressor runs longer at the efficient slower speed and cycles on and off less often.

This maintains a steady indoor temperature. If you have allergies and use a central air cleaner, it will be more effective.

The most efficient single-speed air conditioners use scroll compressors. Scroll compressors have fewer moving parts than standard reciprocating piston compressors. Without the pistons and hardware, scroll compressors are much quieter.

As scroll compressors wear over years of operation, they seal better and operate smoother than when they were new. The basic design is reliable and they should continue to operate at high efficiency levels for many years.

One new efficient model, made by Aqua Cal, uses a triangular plastic outdoor housing with smooth contoured edges. There are no dangerous sharp corners for children to crash into. The plastic housing never rusts.

Use SEER (seasonal energy efficiency ratio) ratings to compare the overall operating efficiency of various models. Also consider the type of compressor and number of speeds, which affect comfort and noise level.

Always do a payback analysis before selecting an air conditioner. Consider both SEER ratings and comfort factors. For most climates, a variable-speed model provides a good compromise. Its special blower is also needed if you plan to install a new, highly efficient variable-speed gas furnace as well.

Write for Utility Bills Update No. 937 showing a buyer's guide of 20 super-efficient central air conditioners, SEER efficiency ratings, cooling output capacities, compressor types and speeds, and an annual savings/payback chart. Please include $2.00 and a self-addressed envelope. Write to James Dulley, Deseret News, 6906 Royalgreen Dr., Cincinnati, OH 45244.

Question - I just bought an oscillating pedestal fan with three speeds. Does it use more electricity when it oscillates? - W. B.

Answer - An oscillating fan is an energy-efficient choice among various fan designs. A small oscillating fan produces the same cooling effect as a larger non-oscillating fan. With a smaller motor, a small fan uses less electricity.

The oscillating feature does not require a separate motor. The slow rotation is driven by the same motor that spins the fan blades. The additional amount of electricity used to oscillate the fan is negligible.