Q: Dear Jim: I want to install a central air-conditioning system that is quiet and can control the humidity level in my home. How effective and energy efficient are easy-to-install mini-duct and high-velocity systems! - Barb S .

A: Dear Barb: Actually "high-velocity" and "mini-duct" systems are different names for the same type of super-efficient and comfortable central air-conditioning and heating unit. These systems are quiet, distribute the cool air evenly throughout your house, and dehumidify very effectively.

Although these systems are becoming more popular in new homes for both heating and cooling, they are most often installed in existing homes for central air-conditioning. Since the flexible, insulated mini-ducts are easy to install with little remodeling, they are particularly popular in houses with hot water heat or baseboard electric heat that don't have existing central sheet-metal ductwork.

The concept of using insulated mini-ducts (2-inch inside diameter) makes much more sense than running large, noisy, sheet-metal ducts throughout your home. Too many sheet-metal ducts are leaky, allowing much of the cooled or heated air to escape before it ever gets to the rooms. The return air ducts, which often use the studded wall cavities, leak even more and often draw heated air down from the attic.

With a mini-duct system, several tiny 2-inch openings are located in the ceiling, or high on the walls, in each of your rooms. You can barely notice the outlets when they are painted the same color as the room. For a decorative look, natural wood outlets are available. The cool air blows out of these small openings and when properly located, there are no drafts.

The key to the high efficiency and excellent dehumidification is the higher-pressure blower that is used with the mini-duct systems as compared to a regular ducted system. This blower unit is often mounted centrally in the attic to reduce noise and the overall length of the mini-ducts. Insulated, flexible main ducts extend from the compressor. From there the mini-ducts run to each room.

With the air at a higher pressure, a more compact and dense set of cooling coils can be used without the airflow being impeding as it would be in a standard ducted system. This allows the dense air to be in contact with the cooling coils longer, so it is cooled to a lower temperature. Just like an ice-cold soft drink glass can sweat, the moisture condenses out of the air onto the cold coils. The condensation is piped to a drain or outdoors.

The high-pressure, dry, cool air (heated air during the winter) exits the tiny room outlets near the ceiling at a very high velocity. This high-velocity air, coupled with its sudden pressure drop, creates an aspiration effect and gentle circular air currents.

This causes all air in the room to circulate and mix. With these even room temperatures and lower humidity, you can usually set the thermostat several degrees higher for big savings without sacrificing comfort.

When you hear the words "high-velocity and pressure," you would naturally think these systems are noisy. Actually, you can barely hear the insulated blower unit running and there is no airflow sound from the tiny outlets.

When a mini-duct system is installed in an existing two-story house, the mini-ducts are run down from the attic, through corners of closets or inside walls, to the first floor.

Several methods to balance a mini-duct system are available to get the proper amount of cooled air to each room. Reducer outlets, with smaller openings, can be installed to reduce the air flow. Adjustable outlets are also available to let you fine tune and change the air flow from summer to winter if you are also using it for heating.

Mini-duct systems use standard high-efficiency outdoor air-conditioning compressor units like a standard ducted system. A unit with a single-speed scroll compressor is an ideal choice for most homes. Your contractor can recommend a compressor model that is compatible with the mini-duct blower.

Write for (instantly download -www.dulley.com) Utility Bills Update No. 432 • buyer's guide of mini-duct, high-velocity air-conditioning systems listing sizes and features. Include $3.00 and a business-size SASE. James Dulley, Illinois Country Living, 6906 Royalgreen Dr., Cincinnati, OH 45244.

James Dulley is a mechanical engineer who writes on a wide variety of energy and utility topics. His column appears in a large number of daily newspapers.

18 ILLINOIS COUNTRY LIVING www.aiec.org