New Technologies for Residential HVAC Ducts

There are many problems with residential duct systems as they are currently installed. It has been shown that they lose significant amounts of energy through leakage and conduction to their surroundings (Cummings et al. 1990, Davis 1993, Modera et al., 1991, Modera 1993, Modera and Jump 1995, Parker 1989, Parker 1993, Proctor et al. 1992, Treidler and Modera 1994). For electrical utilities this is of particular concern because the effects of this leakage and conduction are more pronounced during periods of peak electrical demand.

Unfortunately, the problems with duct systems are not widely recognized within the construction industry and there are no strong economic incentives to solve them. Duct system performance is not evaluated and HVAC contractors overcome duct system shortcomings by installing oversized equipment. Currently, most duct systems are installed with minimal insulation and by methods that give little thought to insuring proper sealing.

The lack of incentives for improved duct system performance has repressed innovation. With the exception of insulated plastic wireflex duct, residential duct systems are essentially unchanged since the 1920's. ucts with higher levels of insulation have recently become available bu duct fittings have seen no change. Fittings are uninsulated, have a large potential for leakage, and are difficult to install in a manner which will insure no leaks.

This report summarizes the potential for new technologies for ducts, duct fittings, and insulation. It begins with a review of what technology is currently in use or available and found that the only inexpensive ducts in production are insulated wireflex ducts, sheet metal ducts, fiberglass board ducts, and uninsulated plastic ducts. For duct fittings, the market was found to be dominated by sheet metal fittings with some use of ductboard. Fittings that snap together were found for use with steel ducts but are too expensive for a residential setting. An uninsulated sheet metal duct which uses a rubber gasket was also found. Two companies are trying to develop plastic fittings, but their designs don't consider improving the method of attachment to wireflex duct.

A survey was conducted of California HVAC contractors to determine what methods they currently use, how concerned they are with sealing duct connections, and what fractions of their expenses go to duct materials and installation. It was found that insulated wireflex ducts and sheet metal fittings are used in almost all residential installations in California. Fiber board and sheet metal are used equally for plenums. It was found that there are basic misunderstandings about sealing duct connections. For example, gaskets were placed on registers in such a way that they would not prevent the register leaking air into the wall cavity. The use of duct tape to attach flexible duct was also a common practice even though it is common for duct tape to fall off after some time. The cost portion of the survey showed that HVAC equipment, duct materials, and duct installation are approximately equal parts of the contractors' costs.

Ideas are presented for new duct technologies that are foolproof to install, sufficiently insulated, and not prone to leakage. For ducts, the Gas Filled Panel (GFP) technology of Griffith et al. (1992) was FP ducts were developed and their qualities for manufacturing, ease of installation, compressibility, etc. were evaluated. A model was then made of the most promising design. Ideas for plenum fittings, attaching registers to boots, and attaching ducts to fittings were created and evaluated for their potential advantages. Several of the ideas for fittings are applicable to existing wireflex ducts and sheet metal fittings.

In order to be accepted, new designs would have to pass code requirements. The most difficult code requirement to pass economically is fire spread and smoke generation tests. The plastics currently used in buildings for windows and wall panels will not pass the stricter smoke generation criteria for duct materials.

Finally, we evaluated the major hurdle for acceptance of new technologies, economics. Analysis of initial costs for the GFP technology shows that it is more expensive than existing technology even before considering steps necessary to pass fire regulations. However, new fitting designs appear to be competitive with other options for improving ducts and offer simple payback times of no more than 6 years.