Cleaning HVAC Systems

The HVAC system is a crucial aspect of indoor air quality. For detailed information about HVAC operation, refer to Module 5, Hygienic Operation of Air Handling Systems.

The HVAC system requires adequate preventive maintenance (PM) and timely repairs to operate correctly and provide a comfortable environment with acceptable indoor air quality. HVAC system opera- tors must have an adequate understanding of the overall system design, its intended function, operational requirements, and its limitations. The PM program must be properly budgeted and implemented, not merely planned on paper.

A well-implemented PM plan will improve the functioning of the mechanical systems and usually save money when evaluated on a life-cycle basis. However, in some buildings, because of budgetary constraints, lack of knowledge, or poor management, maintenance is delayed until breakdowns occur or complaints arise. Such practice often increases operational costs and could result in IAQ problems.

HVAC maintenance practices vary depending on the type of equipment, building types, and existing envelope measures, as well as building location, size, use pattern, and purpose. This makes it difficult to establish a single set of maintenance practices.

A clean mechanical room, free of tracked-in dirt and stored chemicals, is an important element in the prevention of indoor air quality problems. The state of the mechanical room is often an indicator of the care and commitment of the building operator. Airborne contaminants in the mechanical room can be drawn into ductwork through return air openings or unsealed seams in return ducts and be circulated throughout a building.


Any duct cleaning should be scheduled during periods when the building is unoccupied to prevent occupant exposure to chemicals and loosened particles. The air handling unit should not be used during the cleaning or as an air movement device for the cleaning process. The National Air Duct Cleaning Association recommends that the system should be run to allow at least eight air changes in the occupied space after duct cleaning has been completed.

Negative air pressure that will draw pollutants to a vacuum collection system should be maintained at all times in the duct cleaning area to prevent migration of dust, dirt, and contaminants into occupied areas. Where possible, use vacuum equipment or fans
during cleaning and sanitizing to make sure that cleaning vapours are exhausted to the outside and do not enter the occupied space.

If it is determined that ductwork should be cleaned, careful attention must be given to protecting it and its interior surfaces (especially insulation and sound attenuating devices).


When gaining access to sheet metal ducts for cleaning purposes, it is essential to seal access holes properly in order to maintain the integrity of the HVAC system. Use existing duct system openings where possible because it is difficult to repair the damage caused by cutting new access entries into the ductwork. Access doors are recommended if the system is to be cleaned periodically, and all access locations should be identified on the building’s mechanical plans.

Duct cleaning performed with high velocity airflow should include gentle, well-controlled brushing of duct surfaces or other methods to dislodge dust and other particles. Duct cleaning that relies only on a high velocity airflow through the ducts is not likely to achieve satisfactory results because the flow rate at the duct surface remains too low to remove many particles. Vacuum equipment should be used with care because high negative pressure can collapse ducts.

Since duct cleaning requires high volumes of air, seasonal timing is crucial. In cold or hot weather when high amounts of space conditioning are required, HEPA filtration and interior discharge of dust cleaning air will reduce make-up air requirements and energy costs.

Only HEPA filtered vacuuming equipment should be used if the vacuum collection unit is inside the occupied space or discharges to the space. Conventional vacuuming equipment may discharge extremely fine particulate matter back into the atmosphere, rather than collecting it. Duct cleaning equipment that draws dust and dirt into a collection unit outside the building is also available. People should not be allowed to remain in the immediate vicinity of these collection units.

If biocides are to be used, a written rationale and application plan should be prepared in advance. Select only approved products, use the products according to the manufacturer’s directions, and pay careful attention to the method of application.

Air duct after cleaning


Careful cleaning and sanitizing of any parts of coils and drip pans can reduce microbiological pollutants. Prior to using sanitizers, deodorizers, or any cleansing agents, carefully read the directions on the pro- duct label. Once cleaned, these components should be thoroughly rinsed and dried to prevent exposure of building occupants to the cleaning chemicals.

Water-damaged or contaminated porous materials in the ductwork or other air handling system components should be removed and replaced. Even when such materials are thoroughly dried, there is no way to guarantee that all microbial growth has been eliminated.

After the duct system has been cleaned and restored to use, a preventive maintenance program will prevent the recurrence of problems. Such a program should include particular attention to the use and maintenance of adequate filters, control of moisture in the HVAC system, and periodic inspection and cleaning of HVAC system components.

The National Research Council (NRC) has conducted an evaluation of the effectiveness of duct cleaning. A report entitled “Testing Effectiveness of Duct Cleaning and Its Impact on Airborne Particles, Mold and Biocide Levels in Commercial Office Buildings” is available at by clicking here.

Filters that are not changed when required (based on their design loading and maximum allowable pressure drop) can become a bed for fungal growth, sometimes allowing particles or microorganisms to be distributed within the building. As filters are increasingly loaded, the fans use more energy to operate and move less air.

Clean air filters increase airflow through the system, resulting in improved system efficiency, indoor air- quality, and better occupant satisfaction. One of the simplest and most effective methods of increasing an HVAC system’s airflow and efficiency is to inspect and replace system air filters according to manufacturers’ recommendations.