How to Change Your Furnace Filter (And How Often)

One of the few HVAC tasks anyone can do. Costs $15–$40 in parts and 5 minutes, and a clogged filter is responsible for 30% of "my furnace stopped working" calls in winter.

How often

• **MERV 8 1-inch:** every 2–3 months
• **MERV 11 1-inch:** every 3 months
• **MERV 13 1-inch:** every 2 months (more restrictive, clogs faster)
• **MERV 11–13 4-inch or 5-inch:** every 6–9 months
• **In a home with pets or someone with allergies:** halve those intervals

If you can't see light through the filter when held up to a window, it's overdue.

Which MERV

• **MERV 8:** standard. Catches dust and lint. Good for most households.
• **MERV 11:** noticeably cleaner air. Good if anyone in the house has mild allergies.
• **MERV 13:** hospital-grade for residential. Catches sub-1-micron particles. Worth it if you're in a high-pollen area or have asthma.

Higher MERV = more restrictive. Don't go above MERV 13 in a residential furnace without asking us — older blowers sometimes can't move enough air through high-MERV filters and the heat exchanger overheats.

How

1. **Find the filter.** Usually slides into a slot on the side of the furnace cabinet, near the return duct. Often on the bottom side. Some homes have it in a slot in the return ductwork instead.
2. **Note the orientation.** There's an arrow on the filter showing airflow direction. It points TOWARD the furnace (the direction air flows on the return side).
3. **Slide the old one out.** Have a garbage bag ready — they get dusty.
4. **Slide the new one in with the arrow pointing the same direction.**
5. **Mark your calendar** for the next change.

Common mistakes

• Installing the filter backwards. Reduces effectiveness; can damage the blower over time.
• Using a thicker filter than the cabinet was designed for. Forces air around it = no filtering.
• Cheap fiberglass filters. They protect the equipment but not your air — most particles pass right through.

That's it. If you can't find the filter slot or the unit looks unfamiliar, send us a photo and we'll point at where it is. (416) 258-2460.