I started with simple checks before buying anything expensive. First, I looked at the HVAC filter, checked whether vents were blocked by furniture, and noticed which rooms felt stale more often. Then I used a small indoor air quality monitor for humidity and basic readings, because guessing was not helping much. This page, https://dryerventcleaningtoday.ca/testing-air-quality-how-to-check-air-quality-in-home/, gave me a clearer list of things to watch at home. It made me connect stale air with dust, ventilation, humidity, and possible buildup in ducts. After that, I changed filters more consistently and checked airflow room by room. Testing does not have to be complicated, but it should be more than just smelling the air.
