Is duct cleaning worth it, or it it just hot air?

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. But even if you have no special health concerns, duct cleaning may appeal to you at an intuitive level. After all, if your ducts are clean, all that air flowing out of your vents should come out clean too, right?

Well, actually, no.

Companies that perform duct cleaning would love you to believe you need their services. Some might even 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 ..."; but no data back up these claims. Even if your ducts are very dirty, cleaning them likely won't provide any measurable benefits. In fact, the little independent research performed on duct cleaning indicates that the process stirs up so much dust that it creates a bigger problem than it solves.

Here's how duct cleaners work: Companies connect a powerful vacuum to one or more openings in the ductwork to suck out loose dust and other debris. Because lots of dust can cake on the inside of ducts, firms use a variety of methods — a rotary brush or compressed air nozzles, for example — to shake it loose.

Many duct-cleaning companies also offer services you can get from conventional heating-and-cooling firms such as cleaning heat exchangers and cooling coils.

Here's the thing: Dust that settles in your ventilation ducts typically stays there unless disturbed and is usually harmless. 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."

Having your ducts cleaned may actually create a dust problem or make an existing problem worse. In the 1990s the Canada Mortgage and Housing Corporation (CMHC) tested 33 homes in Montreal before and after duct cleaning, finding 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.

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.

Want to control dust? Regularly change your filter. Frequently changing air filters is the best way to keep dust, allergens, and other particles out of your home. Most should be replaced every two or three months.

Another claim made by most duct-cleaning operations and their trade association is that dirty ducts and equipment overburden heating and cooling equipment, which wastes energy. Again, it intuitively makes sense that a cleaner system will run smoother and last longer — after all, that's why we and HVAC equipment manufacturers and repair services recommend that you regularly change your filters. But while much of the energy used to power heating and cooling equipment is indeed wasted, that waste is because of inefficient equipment, lousy insulation, leaks around doors and windows, and unsealed ductwork. While there's some benefit to cleaning and maintaining HVAC equipment, that benefit is relatively small, and very little energy waste is attributable to dirty ducts or equipment.

The EPA suggests having air ducts cleaned only if there is visible evidence of specific problems:

• Infestation of insects or rodents

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

For specific health concerns, such as allergies or asthma, consult your physician first. The doctor may suggest other courses of action than duct cleaning. Bottom line: Your ducts are probably not the problem.