Internally lined ductwork can become quite dirty after many years of steady use. This increases the resistance to air flowing through the ducts, and reduces the volume of air that the supply-air fan can move through the system. This is evidenced by a typical fan curve, which shows a decrease in volume as static pressure increases.
Another problem caused by dirty ductwork is the possibility of spores and mold in the insulated lining. This may contribute to the deterioration of the building environment and the comfort of the occupants. Internally line ductwork, after it’s cleaned, is usually coated to reduce the static friction of the airflow.
Replacing the ductwork is an expensive process that involves removing the ceiling throughout the building. Cleaning the existing ducts is an economical alternative. The dirt and grime are brushed off and vacuumed up by specialized equipment. Then the insulation is chemically treated and sealed, to counter any future threat to air quality.
If the entire duct system cannot be refurbished in this manner, the main trunk line and the branch line most distant from the supply-air fan can be cleaned. This has the most impact on reducing the static pressure at the fan to within the original design specifications. It also optimizes the efficiency of the entire air-distribution system.
In any case, some shrewd remedial practices help to keep the moisture level low in the future, to minimize mold problems. First, keep the fan running for 5 or 10 minutes at the end of every day—withthe cooling coils disabled—so all the water in the unit and its drain pan is evaporated. Second, an inexpensive ultraviolet device at the supply duct can cycle on periodically and eliminate any spores and molds in the airstream.
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Excerpt from Energy Conservation Projects.