Air ducts need to be cleaned every three to five years, according to Angie's List. The amount of use affects how often the pipes need cleaning. Annual inspections ensure that the pipes get cleaned when necessary.
More frequent cleaning is necessary when there is mold in the vent pipe. Additionally, when rodents or other pests have infested the home, they may nest in the ducts, requiring additional cleaning. Cleaning is especially important if there is evidence that pests are entering or exiting the pipe. Clear evidence of dust coming through the pipes is also a sign that cleaning them is necessary.
New homeowners should clean the pipes unless they know that the previous owners have cleaned the pipes within the previous three years. Another sign to inspect pipes is when there is a decrease in heating efficiency, because dirty pipes decrease the efficiency of the heating system.
The best time to clean out pipes is in the summer or early fall, right before weather makes it necessary to use the furnace. If the furnace is connected to the chimney, it is important to clean the chimney as well. Cleaning the chimney decreases the build up of flammable materials in the flue.