Dear Jim: The heat and humidity are killers this year. I need a central air conditioner that is quiet and controls the humidity level. How effective and efficient are miniduct and high-velocity systems? – Barb S.

Dear Barb: Actually “high-velocity” and “miniduct” systems are just different names for the same type of super-efficient and comfortable central air-conditioning systems. These systems are very quiet, distribute the cool air evenly throughout your house and dehumidify very effectively.

Although these systems are becoming popular in new homes for heating and cooling, they are most often installed in existing homes for central air-conditioning. Since the flexible miniducts are easy to install with little remodeling, they are particularly popular in houses with hot water heat (no existing ductwork).

The design concept of using insulated miniducts (2 inches inside diameter) makes more sense than running large, noisy sheet metal ducts throughout your home. Most older sheet-metal ducts are leaky and much of the cooled or heated air is lost before it ever gets to the rooms.

With a miniduct system, cool air blows out tiny 2-inch openings in the ceiling or high on the walls. You can barely notice the outlets when they are painted the same color as the room. For a decorative look, natural wood ones are available. With the outlets properly located, there are no drafts.

The key to the high efficiency and excellent dehumidification is the higher pressure blower (often mounted in the attic) used with the miniducts. The higher pressure compresses the air and allows it to be in contact with the cooling coils longer. This drops its temperature to remove more humidity.

The high-pressure, dry, cool air (heated air in the winter) exits the tiny room outlets at a high velocity. This, coupled with the sudden pressure drop, creates an aspiration effect that mixes all the air in the room. With even temperatures and low humidity, you can set the thermostat higher for big savings.

When you hear the words “high-velocity and pressure,” you would think that these systems are noisy. Actually, you can barely hear them running and there is no air flow sound at the tiny outlets. Most systems have a short sound-attenuation duct section immediately before each cool air outlet.

In two-story houses, the miniducts are run down from the attic through corners of closets or inside walls to the first floor. In a single-story house, the entire system is located in the attic with outlets in the ceiling below. Miniduct systems use ordinary high-efficiency outdoor air-conditioning compressor units. A unit with a single-speed scroll compressor is ideal.

Write for (instantly download – www.dulley.com) Update Bulletin No. 432 – buyer’s guide of miniduct, high-velocity air-conditioning systems listing sizes, features, installation details and a list of the most efficient outdoor compressor units. Include $3 and a business-size SASE, and send to James Dulley, 6906 Royalgreen Drive, Cincinnati, OH 45244.