High-Velocity Mini-Air-Duct Distribution Systems Can Bring Quick Relief to Homes Without HVAC Systems.

For homeowners without forced-air HVAC systems, adding central air conditioning and/or heating ducts can be a daunting task. Installation can require adding unsightly box chases or removing large sections of walls, floors, or ceilings, making it expensive and difficult. In historic homes, traditional forced-air systems may also mar original architectural features, such as plaster moldings.

For these homes, high-velocity heating and cooling systems can offer an easier solution. These systems use small-diameter ducts that can be more easily threaded through floor, ceiling, and wall cavities and around common obstructions. This minimizes necessary alterations, thereby reducing the installed cost of the system.

High-velocity systems operate by using a special fan coil and air-handling unit that generates high-pressure air that is forced through the small-diameter ducts. The main supply trunk is composed of either a rectangular or a round duct that supplies air to flexible, insulated, 2-inch-diameter plastic feeder ducts. Installation of thermostats and automated climate-control systems is the same as with more traditional ductwork.

System Benefits

Currently, the primary benefit of mini-duct systems is their suitability for retrofits. However, system manufacturers—including Unico, Inc., which produces the Unico System, and Mestek, Inc., which produces Spacepak—assert that high-velocity systems also improve dehumidification, energy efficiency, and room air mixing over standard air-delivery duct systems. By delivering air at higher velocity through smaller supply lines, the systems mix room air more effectively. This reduces temperature stratification, and eliminates the need for duct outlet placement at windows or exterior walls to prevent hot and cold spots.

According to manufacturers, the systems also offer better dehumidification than conventional forced-air systems. Dehumidification can reduce energy costs in the cooling season by making occupants feel more comfortable with the thermostat set at a higher temperature. Both the Unico and Spacepak systems claim to reduce moisture by 30 percent versus conventional systems.

System Limitations

One drawback of high-velocity systems is their increased noise compared with conventional systems. To address this issue, manufacturers have added various sound-reducing features. The Unico System uses an air handler to isolate noise and vibration, and uniquely-designed supply tubes lined with sound-dampening, spun-bond nylon. The Spacepak system’s dynamic sound attenuators are installed at each outlet, and continually test system units to guarantee quiet performance.

Although high-velocity systems are often the most cost-effective option for existing homes, they can be more expensive than conventional ductwork for new construction. The systems generally require more outlets than traditional systems—about five duct outlets per ton of cooling, or one duct outlet for every 2,400 BTUs of heating—which adds to material costs and installation time.

Installation is relatively easy, yet, as with any new technology, ease of use and installation is dependent upon the contractor’s familiarity with the product or system. Both the Unico System (www.unicosystem.com) and Spacepak (www.spacepak.com) Web sites provide technical specifications and information for contractors.

In general, high-velocity systems have lower efficiency in terms of SEER (Seasonal Energy Efficiency Ratio) and HSPF (Heating Seasonal Performance Factor) than standard ducted systems. This is mostly due to the fan energy needed to push the conditioned air through the ducts. However, manufacturers claim that, when taking into account tighter ducts, higher delivery temperatures, and lower humidity levels, the high-velocity systems become less expensive to operate.

For More Information
To find out more about high-velocity mini-air-duct distribution systems, visit the PATH Technology Inventory at www.toolbase.org/techninventory.

S. Craig Drumheller is a senior energy engineer with the NAHB Research Center in Upper Marlboro, Md. He has a range of experience with HVAC systems, including equipment design, sizing, lab and field testing.