Some duct cleaning methods rely solely on vacuuming. This spinning brush device provides for more thorough cleaning.

One contractor used foam blocks in an attempt to seal the registers before cleaning the ducts.

With the brush, the second duct-cleaning contractor managed to rid the ducts of items that remained after the first cleaning, which consisted only of vacuuming.

We hired two sets of professional duct cleaners to clean a sheet-metal duct system that had been sealed with the aerosol sealant about four years earlier. The professional cleaners were two local HVAC contractors. They cleaned the ducts as they would for a regular service call. No attempt was made to change the tasks they performed in order to make the duct cleaning more effective.

At first, the pressure difference across the duct (measured at the furthest part of the duct system from where the vacuum hose was connected) was 35 Pa. The relatively low vacuum pressure resulted from a big leak that was created when the cardboard that was used to isolate the furnace was misplaced. After this leak was fixed, the pressure difference across the duct increased to 254 Pa.

Next, the return registers were taped, and the return duct was also vacuumed for about 15 minutes. The pressure difference across the return duct was 980 Pa, falling to 540 Pa at the end of the vacuum cleaning. Although we don't know why the pressure dropped, we assume that the poor method of connecting the vacuum hose to the ducts or a shifting of the cardboard blocking panel was responsible.

For the final cleaning, the supply and return ducts were vacuumed simultaneously. An 8-inch-diameter vacuum hose was connected to the furnace burner access panel. Only a rudimentary attempt was made to seal around this connection: The contractor wrapped a rag around the hose. All of the registers were simply covered with pieces of paper rather than being completely sealed. The pressure difference across the ducts was about 220 Pa and stayed at the same level during the vacuuming process. 

A second HVAC contractor then cleaned the same duct system by inserting a spinning brush into the ducts at the registers--brushing all the way to the plenum, if there was no damper to block it-- and then vacuuming. The supply and return duct systems were isolated, and each grille was removed before he inserted the spinning brush. Then the register was plugged with a piece of foam. The supply duct was depressurized to about 330 Pa.

The combined vacuuming and brushing cost about four times as much as a simple vacuuming alone--about $400 compared to $100. Since the brushing method requires inserting brushes into each register, it takes about four times longer than the vacuuming alone. Also, brushing increases the possibility of damaging the system and then having to fix it, and this average repair cost is included in the cost.

After cleaning by vacuuming only, duct leakage measurement revealed that both the supply and return leakage were unchanged--staying within 0.1 CFM at 25 Pa of the precleaning leakage. The brush cleaning was a much more severe test of the aerosol sealant, because the sealant could have been abraded by the brushes. In fact, combining brushing and vacuuming did result in a very small increase in leakage. However, the final measured leakage was still within 2 CFM at 25 Pa of the precleaning leakage. This increase is very small in absolute terms and is close to the measurement uncertainty. We do not consider this increase to be significant.

Our no-major-effects results are generally consistent with the conclusions of a 1994 study of the effects of duct cleaning in 33 homes in Canada. Canadian Mortgage and Housing Corporation researchers took before-and after-duct cleaning measurements of air flow, dust concentrations in the air, and the amount of electricity used by the furnace fan. They found that the duct cleanings had not changed the energy use by the furnace fan, nor had the cleanings affected air flow rates. Finally, very dirty return air vents did get cleaner, but no changes in dust levels in the house air were found.

For a more detailed discussion of the effectiveness of duct cleaning, see the U.S. Environmental Protection Agency's report titled "Should You Have the Air Ducts in Your Home Cleaned?" It is available at www.epa.gov/iaq/pubs/airduct.html.

Home Energy can be reached at: contact@homeenergy.org
Home Energy magazine -- Please read our Copyright Notice