• Time | Simply put, ductwork gets dirty over time and so the most common reason to clean your ducts is the passage of time. If the environment in your home remains unchanged, you should have your HVAC system cleaned every 3 to 4 years. If a member of your household has sensitivities, allergies, or asthma that timeline is shortened to 1 to 2 years, depending on the severity.
• Renovations | When you update your home, you should plan to clean your ductwork once the work is completed. The bigger the renovation, the more sawdust, drywall dust, and building materials get drawn into your HVAC system. These contaminants find their way into your furnace, AC evaporator coil, and ducts and should be removed upon completion of the project.
• New Builds | Building a home is similar to an extensive renovation, so having your ductwork cleaned before you move in is a must. Typically the ductwork is put in place after the framing, and so the drywalling, flooring, and finishing work is all completed while your HVAC system is moving air through your new home. Include a duct cleaning as part of your building plan, you will probably be shocked how dirty it is! 
• Pets | As lovable as our 4 legged friends are, they contribute a tremendous amount of fur and dander to your HVAC system. The more shedding pets you have in your home, the more often you should have your ducts cleaned.

Dryer Takes Longer to Dry Clothes
Often overlooked, dryer vents should be cleaned periodically. Length of vent, number of bends, and amount of use dictate how often it is required, but if your dryer is taking longer to dryer your clothes it’s time to consider getting it cleaned. A plugged dryer vent can have disastrous consequences.

Length and location of chimney, frequency of use, and the type of wood burned are all factors to consider when deciding how often to clean your chimney. If you use your fireplace or wood stove regularly, you should have the chimney cleaned annually.