Step 2 · Replace or Clean the Air Filter

The air filter is usually located in the return air duct where it enters the air handler (furnace or fan coil). Slide out the old filter. If it's a disposable filter, note its size and MERV rating and insert a new one of the same specifications, ensuring the airflow arrow on the filter points in the correct direction (usually towards the furnace/fan coil). If you have a washable filter, clean it according to the manufacturer's instructions (typically by vacuuming or washing with water) and allow it to dry completely before reinstalling.