When to clean ducts

In houses that produce voluminous amounts of dust and mold, ducts might need cleaning every few years. But in most homes with reasonably modern furnaces and air conditioners, where filters are replaced regularly, a professional cleaning is rarely, if ever, required.

If no one in the household suffers from allergies or unexplained symptoms or illnesses, and if a check under the registers does not reveal large deposits of dust or mold, cleaning is probably not necessary, according to the EPA. You should consider having air ducts cleaned if there is a noticeable buildup of dirt and dust despite regular filter changes, and if there is substantial and visible mold growth inside the ducts even though the appliances are well maintained.

The cleaning decision also can be affected by the type of ducts you have: traditional sheet metal, nonmetal ducts made of fiberboard, or any type of duct (often flexible ductwork) that has some type of internal liner.

Though there are disagreements, the EPA says current research indicates that all types of ducts support microbial growth to approximately the same degree. But there are some distinctions. One is that moisture can be trapped in some insulated duct systems, which have to be kept reasonably dry to retard mold growth. This is not much of a problem in bare metal ducts that have no liner or porous material to trap moisture.

It's also easier to treat mold contamination on bare sheet metal than on other types of ducts. Contractors can use an EPA-registered biocide (often called a sanitizer) on bare metal.

But once a fiberglass duct liner is contaminated with mold, cleaning is not sufficient to prevent regrowth, according to the EPA. One obvious reason: No products are currently EPA-registered as biocides for use on fiberglass duct board or fiberglass-lined ducts. Both the EPA and the Washington-based National Air Duct Cleaners Association (NADCA) recommend replacing wet or moldy fiberglass duct material.

Professional cleaning services

The services to watch out for are called blow-and-go in the trade. Contractors who provide them might not be licensed, probably are not members of any trade association and often spend less than an hour in your house.

Sales pitches for blow-and-go work often include sweeping but unsubstantiated claims about the health benefits of duct cleaning, claims of EPA certification or approval (both bogus) and recommendations for annual duct cleaning, which is unnecessary, as part of your annual appliance service.

Worse yet, contractors using inadequate vacuum collection systems and short-cut cleaning procedures can cause more problems than they solve. According to the EPA, these substandard jobs can release more dust, dirt and other contaminants into the air than simply leaving the ducts alone. Cost and time estimates vary because of the different sizes of houses and complexities of duct systems. But the EPA says that a professional cleaning should cost from $450 to $1,000 and take four to eight hours in a typical three- or four-bedroom house.

Proper procedure, according to NADCA, is to place the system under negative pressure with a powerful vacuum and insert various brushes and cleaners into the ducts to dislodge debris, which flows to the vacuum. NADCA guidelines allow both vacuum equipment that exhausts particles outside of the house, and inside-exhaust systems fitted with HEPA filters.

Duct surfaces can be brushed and directly vacuumed. Soft-bristled brushes must be used for fiberglass duct board and sheet metal ducts lined with fiberglass. Flexible ducts can also be cleaned using soft brushes, but it might be more economical simply to replace them.

Sealants and sanitizers

Duct sealants are designed to coat duct surfaces and prevent dust and dirt particles from being released into the air. But there are some flaws in the concept.

One is that coatings sprayed into an operating duct system do not completely coat the duct surfaces. Another is that sealants might affect the fire-retarding characteristics of fiberglass-lined ducts and could invalidate the manufacturer's warranty. Yet another is that aging sealants could detach from the ducts and contribute to the amount of particles that need to be cleaned.

Sealants should never be used on wet duct liners or as a stopgap measure to cover actively growing mold or to encase debris in the ducts.

Because of these and other questions about safety and overall effectiveness, most organizations concerned with duct cleaning do not currently recommend the routine use of sealants in any type of duct. The organizations include the EPA, NADCA, the North American Insulation Manufacturers Association (NAIMA) and the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA).

On the other hand, sanitizers containing chemicals that retard microbial growth -- mainly mold -- can be applied to the interior surfaces of ducts. However, most duct systems do not require them. If yours does, the system should be thoroughly cleaned before the sanitizer is applied.

Bear in mind that while there are EPA-registered biocides (sanitizers) recommended for bare metal, there aren't any recommended for duct systems with porous surfaces such as fiberglass.

For more information, contact:

Alan L. Wozniak, CIAQP

(800) 422-7873 ext. 802