Tapes and mastic. Metal-reinforced tapes and mastic approved by Underwriters Laboratories are the preferred, conventional choice for sealing ducts. Mastic is rubbery, fiber-reinforced goo that is applied with a brush. Large holes are generally patched with sheet metal and then sealed with mastic. Ducts are still often sealed with standard duct tape, but, despite its name, duct tape is a poor material for that purpose.
Aerosol sealants. A technology sold under the trade name Aeroseal by Carrier Corp. has the potential to dramatically mitigate leaks and the problems they cause. It works by blowing sticky particles into ducts. The particles attach themselves to the edges of leaks and effectively seal them. Sold as part of a franchised service that includes diagnostics, repair, safety testing, sealing, and reporting, Aeroseal is the only technology on the market that can seal leaks in ducts made inaccessible by walls and insulation.
How Aeroseal works. When the computerized Aeroseal process begins, all supply registers are removed and foam plugs are installed in their place. An access hole is cut into the supply- or return-air plenum and a temporary collar is attached. The air-conditioning coil, fan, and furnace are temporarily blocked with a foam plug so that sealant particles produced by the machine will not find their way into the equipment. The Aeroseal injection machine is then connected to the duct system using a long, flexible plastic tube (see Figure 1). Larger commercial buildings will likely require the simultaneous use of multiple Aeroseal injection machines to effectively seal the ducts.
Figure 1: Duct connection from plenum to Aeroseal machine
An inflatable plastic tube is installed from the Aeroseal machine to the collar at the new plenum opening.
The Aeroseal software allows the technician to accurately measure duct system leakage. A fan blows small, dry adhesive particles into the temporarily blocked duct system, depositing these particles directly onto the edges of holes to create seals. Because all primary registers and the furnace are blocked, the suspended adhesive particles forced into the ducts have only one place to go. As the airstream exits the duct through leaks, the adhesive particles stick to the edge of the leak and build a seal up to five-eighths of an inch wide without leaving excess deposits on interior duct surfaces. The Aeroseal software allows the technician and the customer to view the sealing process in real time. In addition, while the sealing process is under way, the technician will manually seal leaks found at register boots (the connections from the duct to the register), at return platforms, and in HVAC equipment. Once the aerosol sealing is complete, the technician measures the duct system leakage using the Aeroseal machine. The computer measures duct leakage amounts before and after sealing and shows the improvement in overall system capacity.
To complete the Aeroseal process, the technician removes all register and equipment plugs from the system, patches the injection hole, and replaces the room registers so that everything is returned to its original state.
In 2014, the Western Cooling Efficiency Center published the report Speed Program Demonstration For Sealing Duct Leaks Using Aeroseal, (PDF) which reviewed the sealing process of University of California, Davis art building. The report found total energy savings at 19 percent, with electric savings of 42,085 kilowatt-hours per year and 3,651 therms per year in heating. The sealing process cost $78,175, and expected savings per year was $5,653, which leads to a simple payback period of 14 years. The study notes that actual savings should be higher than modeled savings due to reduced infiltration after rebalancing and additional exhaust fan power savings of an estimated 10 to 15 percent. Over time, feedback for actual savings will accumulate.