A clogged filter is behind more heating and cooling problems than any single failed part. It is the cheapest thing in the system to keep right.
What changes how quickly it fills
- Thin fiberglass filters fill faster than thicker pleated or media filters
- Pets, smoking and nearby construction add a lot of dust and hair
- The system runs more during hot summers and cold winters
- The filter manufacturer’s guidance is the best starting point
What a dirty filter does
Air has to get through the filter before it reaches the equipment. When the filter clogs, airflow drops. An air conditioner’s coil can freeze, a furnace can overheat and shut itself off, and the system works harder and costs more to run.
Choosing a filter
Filters are rated for how fine a particle they catch, often on the MERV scale. A higher rating catches more, but a very fine filter can restrict airflow on a system not designed for it. The right filter is the finest one the system handles without losing airflow.
Match the size printed on the old filter’s frame, and install the new one with the airflow arrow pointing toward the equipment.
Where to find it
Filters sit either in a slot at the furnace or air handler, or behind a return grille in a wall or ceiling. Some houses have more than one. Switch the system off at the thermostat before changing it.