The humble furnace filter is the cheapest, most overlooked part of your heating and cooling system — and getting it wrong quietly costs you comfort, efficiency, and even equipment life. Here's what you need to know.
What a Filter Actually Does
Your furnace filter protects the equipment first and improves air quality second. It catches dust and debris before they coat the blower and coil, where buildup restricts airflow and makes the system work harder. A clean filter keeps airflow healthy; a clogged one strains the furnace and AC alike.
MERV Ratings Explained
MERV (Minimum Efficiency Reporting Value) measures how much a filter captures — higher numbers catch smaller particles:
- MERV 1–4: basic, cheap, protects equipment but captures little fine dust
- MERV 8–11: the sweet spot for most homes — good dust, pollen, and pet-dander capture with healthy airflow
- MERV 13: captures finer particles (smoke, some bacteria); good for allergies, but confirm your system handles the airflow restriction
- MERV 14+: usually for specialised or hospital settings — can choke residential blowers
How Often to Change It
| Filter type | Replace every |
|---|---|
| 1-inch fiberglass/pleated | 1–3 months |
| 1-inch with pets or allergies | 1–2 months |
| 4–5-inch media filter | 6–12 months |
Check a 1-inch filter monthly — especially during heavy heating and cooling season — and replace it when it looks grey and loaded. Homes with pets, renovations, or allergy sufferers need more frequent changes.
Signs Your Filter Needs Changing
- The filter looks grey and packed with dust
- Weaker airflow from the vents
- Higher heating or cooling bills
- More dust settling on surfaces
- The furnace or AC runs longer than usual
If you've kept up with filters and still have dust or allergy issues, a whole-home solution may help — see our indoor air quality page. And a neglected filter is a common cause of the airflow problems behind no-heat and frozen-AC calls.