During the past five years, research has quantified the impacts of residential duct system leakage on HVAC energy consumption and peak electricity demand. A typical house with ducts located in the attic or crawlspace wastes approximately 20% of heating and cooling energy through duct leaks and draws approximately 0.5 KW more electricity during peak cooling periods. A 1991 study indicated that sealing leaks could save close to one Quadrillion Btus per year.

(see also Commercializing a New Technology)

Because the major cost of sealing leaks in existing air distribution systems is the labor for the location and sealing process, reducing the labor could greatly improve the cost-effectiveness of such a retrofit. Field studies of duct sealing programs performed by HVAC contractors show that labor costs vary between three and six times material costs. Another conclusion of these studies is that in many instances it is virtually impossible to get to the leaky ductwork. Between 1992 and 1994, we obtained laboratory proof-of-concept of a technique to seal duct systems remotely using an internally injected aerosol. Our tests have shown that holes in the ducts, as well as leaks between duct joints, can be sealed remotely and that even those leaks beyond bends and junctions in the ductwork can be sealed.

Figure 1. Predicted (solid line) and measured slot width over the course of the sealing process for a 3 mm by 40 mm leak.

Aerosol Sealing
To develop a successful technology for remotely sealing leaks with an aerosol, we needed to solve two fundamental problems: how to deposit aerosol particles preferentially at the leaks to be sealed rather than on the walls of the duct, and how to have the particles span and ultimately seal the leaks completely. The solution to the first problem involved two steps: first, choosing an aerosol that is small enough to reach the leaks before settling out of the flow stream but large enough to leave the airflow streamlines at the leaks; and second, choosing and controlling the flow rate and pressure in the duct system so as to expand the range of usable aerosol sizes that can seal the leaks capable of achieving the sealing. The solution was to use turbulent flow to minimize particle transit time through the duct system and to use particles of 2 to 20 µm in diameter. (Turbulent air generates higher velocities and therefore a shorter time in the ductwork.) We solved the problem of spanning the leaks by ensuring that the particles were essentially in solid phase; therefore, they did not deform when they were deposited at the leak boundaries. Controlling the flow rate and relative humidity of the pressurizing airstream evaporates all the carrier liquid within a short distance of the aerosol injection point, preventing large wet particles from hitting the walls of the injection chamber.

A simplified model of the rate of particle deposition on the leak boundaries helped us understand and predict the efficiency of the sealing process. This model predicts the fraction of aerosol passing through the leak that will be deposited on the leak's boundaries as a function of particle size, pressure and flow conditions, the size of the leak, and the thickness of the leak boundaries (see Figure 1). Videotaping the sealing process verified the model--it was shown to work remarkably well over the range of our experiments (see Figure 2).

Figure 2. Top view of a 3 mm by 40 mm slot after 20 minutes of injection.

In-Situ Sealing Apparatus
In 1994, we designed, built and field-tested an in situ aerosol sealing apparatus (see Figure 3). Designed for portability and ease of use. Besides performing the sealing process, the field apparatus also measures the leakage of the duct system before and after sealing, eliminating the need for additional hardware.

Figure 3. Video-frame capture of the first prototype of in situ sealing apparatus. Back to Commercializing a New Technology

Field tests in 1994 in a single-family house in Berkeley, California, demonstrated that the sealing apparatus performed well on its first outing. The device was found to seal approximately 60% of the leakage in the duct system within 15 minutes using about $6 worth of sealing material. The cost for tape to seal the registers temporarily during the sealing process was higher than the cost for sealing material, and the setup time far exceeded the sealing time. The field test included measurements of particle and volatile organic compound concentrations in the house before and after sealing. Total suspended particles were found to decrease after the sealing process, and there was no change in VOC concentrations detected after the sealing process. We are exploring the longevity of the seals by tracking the airtightness of the duct system. The degree of sealing has remained stable during the many months of nonoperation since the original sealing process. However, in a more difficult test during the winter months of 1994-95, the system will be made to cycle continuously in heating mode, creating larger stresses on the seals.

Figure 4. Video-frame capture of the apparatus used to test aerosol sealant effectiveness.

In 1995, our efforts are focused on accelerated laboratory testing of the seals produced by the aerosol, designing reusable, quick-installation seals for the registers and HVAC heat exchangers, larger-scale field testing, construction of a second prototype sealing apparatus, and the related activities required for the technique's commercialization.

--Mark Modera and Francois Remi Carrie

Mark Modera
Indoor Environment Program
(510) 486-4678; (510) 486-6658 fax