How to Change an AC Filter (Step by Step)
Changing your AC filter takes 5 minutes and is the single most impactful thing you can do for your HVAC system. Here's exactly how to do it right.
By Askento Editorial Team · 4 min read · Apr 28, 2026
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Changing the AC filter is the easiest, cheapest thing you can do for your home's HVAC system — and the most commonly skipped. A clogged filter restricts airflow, reduces cooling efficiency, increases energy use, and can eventually cause system damage.
The whole job takes about 5 minutes once you have the right filter on hand.
Find Your Filter Size
Before buying a replacement, you need the filter dimensions: length × width × depth (thickness). The size is printed on the cardboard frame of your current filter. Common sizes include 16×20×1, 20×25×1, and 16×25×1 — but there are dozens of sizes, so measure rather than guess.
Where to find the filter:
- At the return air vent (large louvered grille on a wall or ceiling) — the grille opens or the filter slides in from the side
- Inside the air handler cabinet (the indoor unit — usually in a closet, basement, or attic)
Some homes have multiple return vents with individual filters. Check them all.
Choose the Right Filter
MERV rating guide:
| MERV | Catches | Best for |
|---|---|---|
| 1–4 | Large dust, debris | Basic protection (not recommended) |
| 8 | Dust, pollen, pet dander | Most homes |
| 11 | Mold spores, fine particles | Allergy sufferers, homes with pets |
| 13+ | Bacteria, smoke | Hospitals — too restrictive for most residential HVAC |
For most homes, MERV 8 or MERV 11 is the right choice. Go higher only if your HVAC tech specifically recommends it for your system.
Recommended filters:
- Filtrete 20x25x1 MERV 11 Filter — great all-around filter, available in multi-packs
- Nordic Pure MERV 8 Filter — budget-friendly, good airflow
- Honeywell FC100A1029 Media Filter — thick 4-inch media filter, lasts up to 12 months
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Step-by-Step: Changing the Filter
1. Turn Off the System
Switch the thermostat to OFF before changing the filter. This prevents the system from pulling unfiltered air (and loose debris) through the system while the filter is out.
2. Open the Return Vent or Air Handler Panel
For a return vent: the grille is usually hinged or held by clips. Open it and the filter slides out.
For an air handler: locate the access panel (usually held by a screw or snap clip), open it, and the filter is visible in a slot.
3. Note the Airflow Direction
Before removing the old filter, look for the arrow printed on its cardboard frame. This indicates airflow direction — the new filter must go in the same way. Airflow runs from the room toward the air handler (not the other way around).
4. Remove the Old Filter
Slide it out carefully — it will be covered in dust. Slide it into a garbage bag immediately to avoid spreading dust back into the room.
5. Install the New Filter
Check the arrow on the new filter and align it with airflow direction. Slide it in firmly so there are no gaps around the edges — air bypassing the filter defeats the purpose.
6. Close Up and Restore Power
Close the grille or panel, turn the thermostat back to your desired setting, and you're done.
Set a Reminder
The number one reason filters go unchanged too long is forgetting. Pick a recurring reminder:
- Monthly — take 30 seconds to check it visually
- Quarterly — change it regardless of how it looks (for 1-inch filters)
- Seasonally — mark it on your calendar with filter changes in spring and fall
Buying a 6-pack or 12-pack of the same filter when you find one that fits is the easiest way to ensure you always have one on hand.
Washable Filters
Washable/reusable filters exist and seem economical, but they typically max out at MERV 4–5, which is lower filtration than most disposable options. They also require complete drying before reinstallation (wet filters grow mold inside the system). For most homes, quality disposable filters are the better choice.
Related guides
Frequently Asked Questions
- How often should you change an AC filter?
- Every 1–3 months for standard 1-inch filters. Every 6–12 months for thicker 4–5 inch media filters. If you have pets, allergies, or live in a dusty area, change more frequently. The easiest rule: check monthly, replace when you can no longer see light through it.
- What MERV rating should I use for my home AC filter?
- MERV 8–11 is the sweet spot for most homes. MERV 8 catches dust and pollen. MERV 11 also catches mold spores and fine particles. Avoid MERV 13+ for residential systems unless recommended by your HVAC tech — too-high MERV restricts airflow and stresses the system.
- What happens if you don't change your AC filter?
- The filter clogs, restricting airflow. This makes the system work harder, increases energy bills, reduces cooling effectiveness, and can cause the evaporator coil to freeze. Long-term, it shortens the life of the compressor — the most expensive part to replace.
- Where is the AC filter located?
- Usually at the return air vent — a large grille on a wall or ceiling that pulls air in. Or inside the air handler cabinet itself, typically in the basement, attic, or utility closet. Some systems have one filter at each return vent.
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