(From CHECKBOOK Update, Fall 2013)

If you or someone in your family suffers from asthma or allergies, you may be considering getting your home’s heating and cooling ducts cleaned. Even if you have no special health concerns, duct cleaning may appeal to you at an intuitive level. After all, if your ducts are cleaner, all that heated or cooled air flowing out of your vents should come out cleaner too, right?

Of course, that’s what companies that perform duct cleaning will tell you. Some duct cleaners advertise health benefits, or suggest that duct cleaning will lower your power bills by improving your system’s efficiency. Some ads even use language like, “Studies have shown…” Is there anything to these claims? In short: no.

What They Do

To clean ductwork, companies place the duct system under negative pressure—essentially connecting a very large, powerful vacuum cleaner to an opening in the ductwork and sucking out loose dust and other debris. More than one opening might be used. And because a vacuum isn’t powerful enough to loosen and remove all particles, duct cleaners must agitate the dust inside the ducts using a rotary brush, compressed air nozzles, and a special tool called a “skipper ball.” Duct-cleaning companies often also clean the heating and cooling equipment (heat exchangers, cooling coils, condensate drain pans, fan motors, fan blades, and fan housings), but cleaning this equipment isn’t necessarily included in a duct-cleaning company’s basic service. You might have to pay extra for equipment cleaning, and you might not need equipment cleaning if your air conditioner and furnace are regularly maintained by a heating and air-conditioning contractor who routinely does such cleaning (some don’t).

Why It’s Unnecessary

While duct cleaners may want you to believe duct cleaning is essential for your health, the evidence does not support their claims.

The dust that settles in your ventilation ducts generally stays where it is, unlikely to become airborne unless disturbed. Under most circumstances, it is inert and harmless.

Government studies and health professionals who have investigated duct cleaning stop short of recommending against it, but neither do they endorse it as a routine measure.

The official advisory of the U.S. Environmental Protection Agency (EPA) concludes:

Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space… Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health.

The Canada Mortgage and Housing Corporation (CMHC) conducted a study in the 1990s to investigate two claims: that duct cleaning makes indoor air healthier and that it reduces energy costs by improving airflow. After testing 33 homes in Montreal before and after duct cleaning, the study found no significant improvement in air quality and that duct cleaning alone did not improve airflow or energy efficiency. In some cases, measured particle levels actually increased immediately after a cleaning. In other cases, particle levels decreased immediately after cleaning, but returned to previous levels within weeks.

The EPA and CMHC research used different methodologies. The CMHC study called on several different duct-cleaning services. The companies were not made aware they were part of a study, and the researchers did not control for time spent or methods used. The EPA study prescribed and controlled methods used on a smaller number of homes. While the duct-cleaning industry argues that both studies have flaws, no other research has challenged the findings. And although the equipment and methods used by duct-cleaning companies has changed since these studies were conducted, the air ducts in homes are the same.

The American Lung Association’s position on duct cleaning affirms the EPA’s statement and recommendations, but adds:

When health problems are believed to be the result of biological contaminants or dust in indoor air, it is important to first determine that contaminated ducts are the cause of the health problems and verify that the ducts are, in fact, contaminated. The source of the problem may lie elsewhere, so cleaning ducts may not permanently solve the problem.

What Can Go Wrong

A thorough duct cleaning removes dust and other particles from your system. Performed improperly, however, duct cleaning can do more harm than good.

If the vacuum pressure isn’t applied carefully, some of the dust that settled in the ducts will be loosened by the agitation and blown into the living space after the cleaning. (This explains the results of the Canadian study in which particle levels actually rose right after a cleaning.)

Running brushes or using compressed air also risks breaking seals in the duct system, which can be especially problematic in the return air portion. Most forced-air systems are designed as closed loops, and leaks in the return-air circuit allow unfiltered air to be sucked from basements or attics, bringing with it dust and moisture. Not every home has sheet-metal ductwork. Flexible coil-style ducts—the kind that looks like a Slinky toy—are more vulnerable to being punctured.

Ductwork fabricated from fiberglass-insulated material, which is less expensive than metal ductwork, has become more common in new homes. These ducts have fiberglass insulation on their interior surfaces. The fiberglass surface is sealed, but if a duct-cleaning company is not careful, the cleaning can damage the insulation, loosening fibers that can become airborne.

And, of course, there are the usual types of problems that can occur when you deal with any contractor— evidenced by the negative comments posted on many air-duct-cleaning companies at Checkbook.org.

If You Have Special Concerns About Mold:

If you suspect a mold problem—either because of visible growth or a musty smell consistently coming from supply vents—cleaning ducts won’t do much good if it doesn’t eliminate the mold. Mold always begins with a moisture problem, and the ducts themselves are unlikely to be the source of the problem. The most likely culprits are the cooling system’s evaporator coils, which your heating and air-conditioning contractor—and most duct-cleaning companies—can inspect and maintain. Leaky return ducts can also introduce moisture, and, again, if you suspect a mold problem, consider having a service company inspect the duct system for leaks. If you suspect—but aren’t sure—that what you see is mold, you might be tempted to have it tested. But experts we spoke with generally recommend against it, reasoning that:

  • Mold is present in all homes; it becomes problematic only when there is a moisture problem.
  • It’s generally not worth the cost and effort to test for mold or to identify the different kinds of mold present. That time and money is better spent tracking down and eliminating moisture problems—whether under a sink or part of a heating and cooling system.

When You Might Want It

In general, consider duct cleaning only in response to specific, identifiable problems. For example, the EPA suggests having air ducts cleaned only if there is visible evidence of:

  • Substantial mold growth
  • Infestation of insects or rodents
  • Substantial deposits of dust or debris (if registers were not sealed during a renovation project, for example)

If anyone in your household has specific health concerns, such as allergies or asthma, consult your physician first. It’s important to identify the problem, so your doctor can suggest other courses of action than duct cleaning. Start by identifying whether your ducts are part of the problem (they likely aren’t).

Better Steps You Can Take

Take the following measures before you even consider hiring a duct-cleaning service.

Check and Change or Upgrade Your Air Filter

Frequently changing air filters is the best way to keep dust, allergens, and other particles out of your home. With a newly installed system, or a system in a home you’ve just moved into, check your filter monthly to determine how quickly it gets dirty at different times of the year. You will probably need to replace it two or three times during both the cooling and heating seasons.

Clean and Maintain Key Components of Your Heating and Cooling Equipment

In the CMHC duct-cleaning study, researchers found that when duct cleaners also cleaned the blower fan blades, there was a small reduction in airborne particles. Cleaning the blower fan might also slightly improve your system’s energy efficiency.

The same holds true for the evaporator coils inside your home’s cooling system. Evaporator coils cause condensation, dehumidifying the air before it circulates through your home. Condensed moisture can cause dust and other particles to stick to and build up on the coils. Also, cleaning the collector pan (and the drain spout in the pan) beneath the coils ensures that dirt doesn’t build up and get drawn into the system. It also prevents water, which can cause mold problems, from building up on and beneath the coils.

Also consider having your duct system inspected for leaks, since leaky ducts lower efficiency and introduce air-quality problems.

Some duct-cleaning companies clean heating and cooling equipment (condenser coils, fan blades, etc.) as part of a comprehensive duct cleaning, and some charge less if you skip this step. Your heating and air-conditioning contractor may do this for you, but don’t assume these services are part of regular maintenance; be sure to ask explicitly.

If You Decide to Hire a Duct Cleaner

If you ultimately decide to have your ducts cleaned, here’s how to hire a responsible company.

What to Avoid

Don’t hire a company that makes sweeping claims about health benefits or claims to be EPA-certified for duct cleaning: That agency offers no such certification.

Shop for a good price, but avoid companies that advertise specials under $200, or even under $100. Known in the industry as “blow-and-go” outfits, they will likely just hook up a vacuum to part of your duct system and do a poor job. Or use the low price as a bait-and-switch tactic.

What to Look For

Focus your search on contractors who belong to the National Air Duct Cleaners Association (NADCA), a nonprofit trade association. Ask for written proof of a company’s NADCA membership and certification, or use the NADCA website (www.nadca.com) to locate members in your area. NADCA members must subscribe to the NADCA code of ethics. More important, they must employ at least one NADCA-trained-and-certified technician and employ NADCA-approved methods. Members must also carry at least $500,000 in general liability insurance.

Also consider companies that have received favorable recommendations and comments from CHECKBOOK subscribers in the “Air-Duct Cleaners” section of Checkbook.org.

Get several companies to come to your home to perform inspections and provide estimates. Ask them to perform the inspection while you are present. Inspections might consist of only a flashlight and mirrors or involve inserting a video camera into the ducts. Ask them to show you the contamination that would justify having your ducts cleaned.

Confirm that the cleaning will cover the entire system. A cleaning should include supply ductwork, return ductwork, supply plenum (chamber), return plenum, and all registers and grilles. You may agree, for a reduced price, to exclude the blower fan assembly, heat exchangers, evaporator coils, and collector pans if those are serviced under a maintenance plan with a heating and air-conditioning contractor. But these are the elements most relevant to system efficiency and should be explicitly listed by the duct-cleaning company unless you have agreed to exclude them.

Before agreeing to any work, get written estimates after each inspection.

Get the company to agree in writing that it will perform all of the following (mostly drawn from the EPA’s duct-cleaning guidance document)—

  • Open or create access ports or doors to allow the entire system to be cleaned and inspected.
  • Inspect the system before cleaning to make sure there are no asbestos-containing materials (e.g., insulation, register boots, etc.) in the heating and cooling system. (Asbestos-containing materials require specialized procedures, and should be disturbed or removed only by specially trained and equipped contractors.)
  • Follow NADCA guidelines in attaching some sort of vacuum device to the system during cleaning to remove loosened particles. (NADCA does not endorse one type of equipment or another. Truck-mounted equipment is typically more powerful and ensures that loosened particles are sucked outside the home. Portable equipment can be located nearer the work site.)
  • Use vacuum equipment that exhausts particles outside of the home or, if the vacuum exhausts inside the home, use only vacuuming equipment with high-efficiency particle air (HEPA) filters.
  • Protect carpet and household furnishings during cleaning.
  • Use well-controlled brushing of duct surfaces in conjunction with contact vacuum cleaning to dislodge dust and other particles.
  • Use only soft-bristled brushes for fiberglass duct board and sheet metal ducts lined internally with fiberglass. (Although flex duct can also be cleaned using soft-bristled brushes, it can be more economical to replace accessible flex duct.)
  • Take care to protect the ductwork, including sealing and re-insulating any access holes that have been made or used to make them airtight.
  • Follow NADCA standards for air-duct cleaning and North American Insulation Manufacturers Association recommended practices for ducts containing fiberglass lining or constructed of fiberglass duct board.
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