1. Visible dust around your supply vents within days of cleaning them.
2. Musty smell when the system first kicks on.
3. Allergy symptoms that get worse indoors, not better.
4. Uneven airflow — some rooms hot, some cold, even with a balanced system.
5. Rising energy bills without a rate change.
6. Recent remodel, new floors, or drywall work.
7. Pet hair visible at the return grille.
8. You just moved in.
9. Cobwebs at the register.
10. The last cleaning was more than 5 years ago — or you don't know when it was last done.