In many cases, the two terms "air duct cleaning" and "indoor air pollution" are inextricably connected. In fact, air duct cleaning has evolved in response to the increasing concerns over indoor air pollution. Although most indoor air quality problems can be traced to problems outside the physical air conveyance system, a significant portion of indoor air quality issues can be traced, at least in part, to conditions within the air conveyance system itself. Most indoor air quality (IAQ) problems are a combination of many factors, the physical condition of the heating, ventilation, and air conditioning (HVAC) system being one of them.

The goal of HVAC or air duct cleaning is to remove visible contaminants such as dust, debris and moisture from the entire air conveyance system so that the air passes over clean surfaces. Microbial contaminants such as bread molds, mildews and soil fungi, which can be invisible to the naked eye, can also be removed from the HVAC system with the proper cleaning and sanitizing procedures. Presence of microbial is not uncommon in an HVAC system. However, the presence of active microbial growth is of concern, since it may affect indoor air quality and pose a health risk for some people.

The benefits of air conveyance system cleaning include lessening the chances of indoor air pollution, health and comfort complaints by the occupants, and it may decrease energy costs by allowing the mechanical components to operate more efficiently and last longer.

The duct cleaning industry is currently an unregulated one. In 1989, a group of ventilation cleaning contractors and equipment manufacturers formed the National Air Duct Cleaners Association (NADCA).

NADCA has, since its inception, developed industry standards for its members, as well as an Air Systems Cleaning Specialist (ASCS) certification program which includes continuing education and training programs.

NADCA has not only developed cleaning standards, but has worked closely with the Environmental Protection Agency (EPA) on testing the effectiveness of air duct cleaning, resulting with the EPA's suggestion that an air duct cleaning service provider follow NADCA standards.

Duct cleaning equipment has evolved on the principal of 'Source Removal,' i.e., removing visible contaminants from within the air conveyance system. Vacuum collection systems are the center of duct cleaning. All vacuums which are exhausted inside the building must be HEPA-filtered with a 99.97% collection efficiency for 0.3 micron size particles. The cleaning process typically involves connecting a large HEPA-filtered vacuum to the air conveyance system, or a portion of the system, to create a negative pressure so no contaminants are allowed to escape into the occupied space.

As evidence of the industry's technological advancement within the last 10 years, one simply has to examine the vast extent of equipment that is now available and used during air duct cleaning.

Inspection devices are used to inspect the inside of the AHU and/or ductwork for debris, contamination, and serviceability. The optical borescope is one such device that is used for direct viewing. The borescope is placed through an existing opening, such as a vent opening, or a service opening is created and the scope is placed into the area to be inspected. Scopes normally have at least a 115 watt light source, a standard 40 degree field of view and can accept a 35mm camera or a digital camera for documentation purposes.

Fiber optic scopes and video scopes operate on the same principal as the optical borescope. In addition to the above, they can be pushed into the ductwork and can actually be manipulated around obstacles and maneuvered to specific sections of the ductwork. A closed circuit television (CCTV) camera system is used to remotely direct the head of the system and a video recorder, with voice, can also be attached for documentation purposes. Remote controlled robotic equipment can be driven through the ductwork and has all of the advantages as the previously stated inspection equipment. Robotic equipment comes in all shapes and sizes and looks similar to toy army tanks with their treaded wheels. Robots can have two cameras, one forward, for entry in to the ductwork, and one reverse, for exiting. Robots can also be used for cleaning, spraying and sampling operations.

There are basically two types of vacuum collection devices: High-efficiency Particle Air (HEPA) -filtered (which filter the air via HEPA filters, where the air enters an occupied space) and Non-HEPA-filtered (which filter the air into unoccupied spaces, normally outside the building).

HEPA-filtered vacuums are very efficient vacuums - in fact, HEPA filters are used in clean rooms and hospital operating rooms. They filter 99.97% of particles which are 0.3 microns (about one-hundredth of a pencil dot) and larger. Large HEPA-filtered vacuums, commonly called negative air collectors or negative air machines, are used to place the portion of the system under a negative pressure. This ensures that any dust or debris in the system is drawn into the vacuum and not allowed to enter the occupied space. Negative air collectors have a staged filter system with the final stage being the HEPA filter. Portable HEPA-filtered vacuums, which look and operate similar to shop-vacs, are used to hand vacuum, or direct contact vacuum, the AHU and/or the ductwork. There are also HEPA-filtered wet vacuums for removing and containing contaminated water commonly found in drain pans and in ductwork that has been flooded.

Non-HEPA-filtered vacuums are commonly used to create a negative pressure and move materials. Their static pressure capabilities are extremely high because they are not very well filtered and, therefore, are placed outside of occupied spaces, normally outside the building. The basic concept of their use in the cleaning process is the same as described above and they can be used to vacuum bulk materials such as blown-in insulation.

Remote controlled robotic equipment has the attributes noted above in the inspection section and can be fitted with spinning brushes, directional air nozzles and air whips, sample collection devices, and spraying attachments for spraying sanitizing solutions or various coatings. Robots are invaluable in difficult to access areas.

They may preempt a situation in which access through a plaster ceiling is considered or the use of scaffolding or lifts. Electric and pneumatic tools are operated by their respective power sources, electricity and compressed air. Both categories include spinning brushes that remove debris from the surface of the duct. The debris is then drawn into a negative air collector or it is hand vacuumed. One combination cleaning device has an electrically power brush at the end of the HEPA-filtered vacuum hose.

As the debris is loosened, it is simultaneously vacuumed. Pneumatic tools include blowguns, directional air skippers and air whips which agitate, loosen and push (with compressed air) the debris toward the negative air collector which is pulling the debris into it. (This is called the push - pull technique.)

Spraying devices are used to apply sanitizers and coatings. Airless sprayers are commonly used for this purpose. A technician can physically spray the solution from a service access or a spray cart or robot can be used for spraying. CCTV and video recorders can be attached to the spray cart to view remote spraying or the spray attachments can be connected directly to a robot.

As early as 1983, the World Health Organization identified a new disease called Sick Building Syndrome (SBS). The condition refers to types of physical discomfort associated with people who work indoors for a long period. Specific symptoms include headaches, dizziness, eye irritation, dry itchy throat, allergies, chest tightness, dysphoria and difficulties in concentrating. While most people who spend lengthy periods working indoors almost always experience such discomfort, they usually attribute it to the lack of oxygen caused by inadequate indoor fresh air.

In actuality, the fresh air in most modern buildings has to undergo strict scientific calculation allowing a large margin of error to ensure its quality. As a result, the real problem is not the quality of the fresh air, but the quality of the air supplied indoors by leading causes of SBS such as central air conditioning and ventilation systems. If we were to prevent and reduce SBS’s incidence, improving indoor air quality by taking better care of these systems is a must.

Since the SARS outbreak in 2003, health conditions arising from central air conditioning systems have come under the scrutiny of relevant authorities and users. The large quantity of dust accumulating in duct systems is one of their biggest concerns. This is because viruses such as SARS attach themselves to the dust and are transmitted throughout buildings by central air conditioning duct systems. As this generates cross-infection in groups or individuals, regular cleaning of central air conditioning duct systems is vital.

As properly cleaned air duct systems are also very good at cutting energy consumption, they can lower power bills as much as 25-40%. Such duct systems also improve the heat exchange efficiency of fan coils while increasing the volume of air pumped out by the air conditioning compartment. When the ducts and filters of an air conditioning system are clean, the system’s engines do not have to work so hard and comfortable temperatures, energy reduction and improvements in health can all be achieved. Since clean air ducts have so many benefits, why not start taking better care of your systems today?