The most reliable sign is what you can see inside the duct. Many other symptoms have more than one cause.
Signs worth acting on
- Thick dust or debris when you look past a vent cover
- Particles puffing from a supply vent when the system starts
- Droppings, nesting material or insects
- Visible growth near a vent, especially with a musty smell
Signs that may be something else
- Dusty furniture, which often has other sources
- A dusty return grille, which is normal since it pulls in air
- Weak airflow, which may be a filter, a closed damper or a damaged duct
- Higher energy bills, which can have many causes
Check it yourself
Turn the system off, remove one supply vent cover and one return grille, and look inside with a flashlight. Take a picture if you are not sure. You are welcome to describe what you see when you call us.
Can dirty ducts make my home dustier?
They can if particles are actually blowing out of the vents. More often, household dust comes from other sources or from leaks in return ducts. An inspection can help sort that out.