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Mini-duct Air Conditioning

I want to install a central air-conditioning system that is quiet and controls the humidity level well. How effective and energy efficient are easy-to-install mini-duct and high-velocity systems?—Barb S.

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 have no existing central sheet-metal ductwork. It can be difficult to install standard large sheet-metal ducts in an existing house.

The design concept of using insulated mini-ducts (2-inch inside diameter) makes much more sense than running large, noisy sheet-metal ducts throughout your home. Most sheet-metal ducts are leaky and much of the cooled or heated air is lost 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. With the many outlets 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 and then the mini-ducts run from them to each room.

With the air at a higher pressure, a more compact and dense set of cooling coils can be used without impeding the airflow as it would 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 can sweats, the moisture condenses out of the air on the cold coils and the water is piped to a drain or outdoors.

The high-pressure, dry, cool air (heated air during the winter) exits the tiny room outlets at a very high velocity near the ceiling. This high-velocity air, coupled with its sudden pressure drop, creates an aspiration effect and gentle circular air currents. This causes all the 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 in the summer for big savings without sacrificing comfort.

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.

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