Dirty duct systems are a major source of indoor air pollution. Airborne contaminants are pulled into the ducts every time the heating and cooling system runs. Over time, these contaminants build up inside the ductwork, creating an ideal breeding ground for mold, bacteria, fungi and other microbes. The removal of such contaminants from the HVAC system should be considered as one component in an overall plan to improve indoor air quality (IAQ).

There are several factors that help determine if the duct system should be cleaned. One major factor is visible mold growth inside hard surface ducts or on other components of your system. Be aware that although a substance may look like mold, it may not be. Only a laboratory analysis can tell you whether a sample is mold or simply a substance that resembles it. If the air duct insulation is moldy it should be removed and replaced and the cause of the growth corrected before cleaning occurs. Other factors that may indicate cleaning is needed include rodent or insect infestation and a clogged system that spreads contaminants through the building.

Ducts can be cleaned at any time of the year, but for the most noticeable reduction in allergy irritation, it should be done at the start of a new heating season. Molds that flourished in the summer humidity dry out, die and release their spores when the heat goes on. They are then blown throughout the building, often causing the "first heat of the season sneezes".

The most effective way to clean air ducts and ventilation systems requires a contractor to place the system under negative pressure, through the use of a specialized, powerful vacuum. While the vacuum draws air through the system, devices are inserted into the ducts to dislodge any debris that might be stuck to interior surfaces. The debris can then travel down the ducts to the vacuum, which removes it from the system and the home. There are two main types of vacuum collection devices: (1) those mounted on trucks and trailers, and (2) portable units. Truck/trailer mounted equipment is generally more powerful than portable equipment. However, portable equipment can often be brought directly into a facility, allowing the vacuum source to be located closer to the ductwork. All vacuum units should be attached to a collection device for safe containment prior to disposal. Any vacuum collection device which exhausts indoors must be HEPA (high efficiency particle arrestor) filtered.

A vacuum collection device alone will not get an HVAC system clean. The use of methods and tools designed to agitate debris adhered to the surfaces within the system, in conjunction with the use of the vacuum collection device(s), is required to clean HVAC systems. (For example: brushes, air whips, and “skipper balls.”)

Sanitizers are anti-microbial chemicals applied to the interior surface of the air ducts, designed to control microbial contamination. Before any sanitizers are used, the system should be thoroughly cleaned. It is critical that any anti-microbial treatment used in your system be EPA registered for the intended use in HVAC systems. Ask to see the chemical’s Material Safety Data Sheet (MSDS). If you are still concerned, call the EPA at 1-800-438-4318.

The amount of time it takes to clean a HVAC system depends on many variables such as the size of the building, the number of systems, the extent of the contamination and the number of HVAC cleaners performing the job. Ask at least two contractors to inspect your system and give you a time estimate for your particular system. This will give you a general idea of how long the job should take as well as an idea of how thoroughly the contractor plans to do the job.

Once initial problems are addressed, you need to schedule a maintenance program. A written preventive maintenance plan is an effective tool for managing IAQ. It should include monitoring, inspecting and cleaning HVAC components such as outside air intakes, outside air dampers, air filters, drain pans, heating and cooling coils, the interior of air handling units, fan motors and belts, air humidification, controls and cooling towers.