Dear Jim: With the new higher efficiency requirements, I think I should install a new central air conditioner. I want to get the most efficient and quiet model which provides the best comfort. What should I get? – Paul T.

Dear Paul: As of early this year, central air conditioner manufacturers are no longer allowed to produce models with a SEER (seasonal energy efficiency ratio) less than 13. You may still find inexpensive, less efficient models in inventory, but you will be better off with one of the newer designs.

The most efficient models available have a SEER of up to 21. When you compare this to a typical older central air conditioner with a SEER of less than 10, installing a new one can reduce your electric bills by more than 50 percent. The new ones also are much quieter, indoors and outdoors.

If you really want the highest efficiency, least noise and best comfort, you should install a two-stage central air conditioner with a variable-speed indoor blower motor. In addition to replacing the outdoor condenser unit, you also must replace the indoor air handler unit to get the variable speeds. This will also improve furnace efficiency and comfort during winter.

A two-stage system operates similarly to having two separate cooling systems: one low-output and one high-output. On mild days and at night, the cooling needs of your home are less than during peak afternoon heat.

During these times, the compressor runs at the low-output level. This allows the air conditioner to run in longer cycles for better dehumidification, steadier indoor air temperatures and less noise. Even though it is running for longer cycles, it uses less electricity overall because the compressor and all the motors are running slower.

When the outdoor temperature gets very high and the sun is beating on the roof, the thermostat senses more cooling is needed and the compressor automatically switches to the high-output level. Depending upon the specific compressor design, this may almost double the cooling output.

The majority of two-stage models use a new two-stage scroll compressor. This compressor design creates a continuous compression cycle instead of pulses from reciprocating pistons in an older-style standard compressor.

Another two-stage design uses two small standard compressors inside one outdoor unit. Still another uses a reversing compressor to vary the output.

A variable-speed (ECM) indoor blower motor is required for the highest efficiency and comfort. It runs at a much lower speed when the air conditioner is running in the low-output stage and slowly ramps up to speed each cycle to minimize the start-up noise. I prefer a model that uses ozone-friendly R410A refrigerant instead of older R-22 refrigerant.

Write for or instantly download from www.dulley.com Update Bulletin No. 921, a buyer’s guide of the 18 most efficient central air conditioners listing stages, outputs, SEERs, refrigerant, warranties, and a savings chart. Please include $3 and a business-size self-addressed stamped envelope.

Dear Jim: My mother always told me a “watched pot never boils.” The bubbles seem to come from the bottom, so I do not understand this. If I watch when it boils, I can turn it off sooner and save some energy. – Kenner B.

Dear Kenner: Your mother is correct. It does take longer for an uncovered pot of water to boil. It also takes more energy to keep an uncovered pot boiling. One reason is heat is lost from the top surface of the water.

A second reason is, as the water boils and becomes water vapor, it escapes into the air. If the pot is covered, this vapor condenses back into water and drips into the pot. When vapor condenses, it gives off (latent) heat.

Send inquiries to James Dulley, Bangor Daily News, 6906 Royalgreen Dr., Cincinnati, Ohio 45244 or visit www.dulley.com.