How to Find and Replace Your Furnace Filter: Complete Step-by-Step Guide

Replacing your furnace filter is the single most important maintenance task a homeowner can do — and one of the easiest. A clogged filter restricts airflow, overworks your furnace, and reduces heating/cooling efficiency. Yet millions of furnace filters go unchanged for years. Here's exactly how to do it.

Step 1: Locate Your Furnace Filter

Furnace filters can be in two locations:

Location A: In the Furnace Itself

Open the lower access panel on your furnace (usually clips off or has a couple of screws). The filter is typically in a slot on the return air side of the blower compartment. Look for a cardboard-framed panel with a mesh or pleated material.

Location B: In the Return Air Register

Many homes have the filter in a large ceiling or wall register — the "return air" vent that pulls air back to the furnace. These are typically larger than supply registers (where air blows out), often 16"×25" or 20"×25", and located in a central hallway or main living area. Some have a built-in filter grille that swings open.

If You Can't Find It

Run your hand near the furnace's intake side — you'll feel suction when the blower is running. The filter is in the path of that suction. If your home has a filter grille in the ceiling, that's the location.

Step 2: Identify Your Filter Size

Filter size is printed on the side of the existing filter: width × height × depth (e.g., 16×25×1 or 20×20×4). Write this down before heading to the store. Common residential sizes:

• 16×20×1, 16×25×1, 20×20×1, 20×25×1 (1" standard)
• 16×25×4, 20×25×4, 20×20×4 (4" media filters)

If there's no existing filter or the size is unclear, measure the filter slot (width × height) and note the depth of the housing. Filters are usually ½" smaller than their labeled size (a "16×25×1" filter actually measures about 15.5"×24.5"×0.75").

Step 3: Choose the Right Replacement Filter

For most Minnesota homes:

• Single pet or no allergies: MERV-8 pleated filter
• Multiple pets or allergies: MERV-11 pleated filter
• Concerned about air quality (smoke, PM2.5): MERV-11 to MERV-13 (verify your furnace can handle it — see our MERV filter guide)

Avoid the cheapest fiberglass filters (blue mesh panel type) — they protect the equipment but do almost nothing for air quality.

Step 4: Replace the Filter

1. Turn off your furnace at the thermostat (set to OFF, not just up) or at the furnace power switch. This prevents the blower from running while the filter is removed and pulling unfiltered air into the system.
2. Remove the old filter — slide it out carefully. A very dirty filter may shed debris when moved; hold it level and slide it into a trash bag immediately.
3. Note the airflow direction arrow on the new filter's frame. The arrow points toward the furnace/blower — away from the return air stream. Installing it backwards significantly reduces effectiveness.
4. Slide in the new filter — ensure it fits snugly with no gaps around the edges. Air bypassing around the filter does nothing for air quality.
5. Close the access panel securely — furnaces have a door safety switch that won't allow operation with the panel open.
6. Turn the furnace back on and verify normal operation.

Setting a Reminder

The most common reason filters go unchanged is simply forgetting. Set a recurring reminder in your phone calendar:

• Every 90 days for standard 1" filters
• Every 30 days if you have multiple pets or a dusty environment
• Every 12 months for 4" media filters (set it for the same date as your annual furnace tune-up)

Buy 3–4 filters at a time so you always have one on hand when the reminder fires. The extra storage cost is negligible; the convenience is high.

What a Clogged Filter Does to Your Furnace

Visual proof helps: hold a clogged filter next to a new one. Even a moderately dirty filter dramatically restricts airflow. In practical terms, a severely clogged filter:

• Forces the blower motor to work 2–3× harder to move the same air volume
• Reduces heating/cooling output by 15–25%
• Can trip the high-limit switch, causing the furnace to short-cycle and throw error codes
• May cause the evaporator coil to freeze in summer
• Shortens blower motor lifespan significantly

A $15 filter replacement is the highest-ROI maintenance task in your entire home. No other single action does more to protect a $3,000–$5,000 piece of equipment at lower cost.

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