A small-duct high-velocity system was used in this Johns Island, SC house. The ducts vents can … [+]
Photo courtesy of Margaret Rambo for Prefabulous Small Houses, published by Taunton Press
Energy Star estimates that 20% to 30% of the air that moves through residential ductwork is lost through “leaks, holes, and poorly connected ducts,” accounting for a major loss of energy.
Small- or mini-duct systems are a whole lot more efficient. Because the ducts in these systems are significantly smaller than traditional systems, there is less surface area for energy loss. Unlike most sheet-metal ducts, mini-ducts are insulated, and their joints are much tighter, often connected with gaskets. Depending on the manufacturer, high velocity ducts leak no more than 5% of the air forced through them, if they leak at all. Many can have a 0% leakage if they are installed professionally and properly following manufacturer’s installation specifications.
Most duct systems use outlets that are 6 inches tall and about 12 inches wide. SDHV systems use smaller and less noticeable outlets, often several per room, so that air is evenly distributed. These flexible, insulated feeder ducts are only 2 inches to 3 inches in inner diameter. In diameter, and air moves through them at about 1,500 feet per minute (fpm) compared with 500 fpm to 600 fpm in standard ductwork. High pressure also eliminates dust buildup within the ducts, which improves indoor air quality.
This more efficient system includes an air handler that works with a home’s boiler, central air-conditioning system, geothermal system, or heat pump to reduce humidity by about a third when cooling, compared to conventional duct systems, requiring less energy. SDHV systems also lower relative humidity when in the heating mode, reducing the need for higher thermostat settings.
This is a cutaway of how a small-duct high-velocity system works.
Diagram courtesy of Unico
Mini-ducts eliminate unsightly registers and radiators and more evenly circulate air. The systems are installed in new homes and can easily be retrofitted into older homes during renovation. The Unico System, one of the manufacturers of these systems, has been endorsed by the National Trust for Historic Preservation. One of the advantages of using this system in a retrofit is that the small ducts will most often be able to be fit more easily into floors, walls and ceilings than the traditional duct systems.
As with all HVHC related systems it is important to have qualified professionals install these systems so they function as they were designed.
Initial installation costs can be high, but homeowners save over the life of the system and their indoor environments are more comfortable. For further information about this type of system, visit the websites of two of the manufacturers who produce these systems Unico or Spacepak.