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I am ready for a new, quieter, and more efficient central air conditioner. My old one doesn’t always keep the house comfortably cool. What should I look for?—Ronnie L.

You picked a good year to replace your central air conditioner because there have been significant changes and improvements.

If you have electric resistance heating, before you decide on a new central air conditioner (cool only) model, consider installing a heat pump model instead. All of the central air-conditioning units that I will discuss have corresponding heat pump models for both heating and cooling your house year-round. A heat pump model will cost about $350 to $500 more initially.

(Editor’s note: before installation it’s also a good idea to call your local electric cooperative about an energy audit, to make sure the new air conditioner is the best way to improve energy use in your home. For example, if your house isn’t cool enough, the problem could be leaky ducts rather than the air conditioner.)

This year, the cooling efficiencies (SEER—Seasonal Energy Efficiency Ratio) for central air conditioners have skyrocketed to more than 19 and the comfort level is much improved. The SEER relates the amount of cooling (Btu) to the amount of electricity consumed (watts). The highest-efficiency models can cost substantially more initially, so have your cooling contractor do payback analyses on models in several efficiency ranges.

If your existing air conditioner is older than 10 years, installing a new one will significantly cut your cooling electric bills. A higher-efficiency model will also reduce the peak electricity demand on the hottest afternoons. Reducing peak demand can help control long-term electric rates.

The highest-efficiency models use a two-stage compressor design to provide the best comfort, particularly in very humid climates and during the milder weather of early and late summer. These are the times when you often sense a clammy feeling. Your old standard air conditioner can adequately cool the indoors during the mild weather, but it does not run long enough to also adequately dehumidify the indoor air.

Two-stage compressor models vary the cooling output to the instantaneous cooling needs of your home. During mild weather, they operate in the super-efficient stage with reduced cooling output. In this longer running low-stage cycle, the indoor air is better dehumidified as it slowly passes over the cooling coils.

There are several effective designs of efficient, reciprocating piston two-stage compressors. Some vary the cooling output depending on which way the motor is rotating. Another design uses a two-speed compressor while others use two small compressors.

The newest and one of the best designs is a two-stage scroll compressor using very few moving parts and operating quietly. The two scrolls rock together to produce a continuous compression process. This scroll compressor uses an ozone-friendly R410A refrigerant.

The R410A requires higher operating pressures, so tubing and materials must be made stronger, lowering the sound frequency and making it less noisy. Other soundproofing features and materials are incorporated into the new models. Old compressors using Freon will gradually be phased out of production by law by 2010.

Whether you choose a super-efficient two-stage or a lower-priced one-stage model, consider replacing the indoor coil and blower unit with a variable-speed blower. This will increase the efficiency and comfort level with much less indoor noise year-round even if you use an oil or gas furnace during the winter. Installing one of the new thermostats (compatible with variable-speed blowers) can also control the blower speed to enhance dehumidification and comfort.

Write for Utility Bills Update 533—buyer’s guide of 19 air conditioners listing cooling outputs. Include $3.00, a business-size SASE, and Update number. Mail requests and questions to James Dulley, Kentucky Living, 6906 Royalgreen Drive, Cincinnati, OH 45244. Go to www.dulley.com to instantly download.

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