How to Change Your Furnace Filter (Indiana Homeowner's Guide)

Published June 9, 2026 - Cavinder Plumbing, Heating and Cooling - Granger, IN

Quick answer: To change your furnace filter, turn the system off at the thermostat, find the filter slot in the return-air grille or the blower cabinet, note the size printed on the old filter's cardboard frame (for example 16x25x1) and the direction of the airflow arrow, slide the old filter out, then slide the new filter in with the arrow pointing toward the furnace or blower. Turn the system back on. In Michiana, plan to check the filter monthly and replace it every one to three months. Below is how to find your exact size, what MERV rating to pick, and the signs your filter is overdue.

If your furnace is short-cycling, blowing weak air, or you'd rather have a technician handle it during a tune-up, call (574) 633-4557. Cavinder serves Granger, South Bend, Mishawaka, Elkhart, and the surrounding Michiana area.

Step-by-Step: Changing the Filter

The whole job takes a couple of minutes once you know where the filter lives and which way it faces.

Turn the system off. Set the thermostat to "off" so the blower isn't pulling air while the slot is open. This keeps dust and loose debris from getting sucked into the system.
Locate the filter. It sits in one of two places: behind a large return-air grille on a wall or ceiling, or in a slot on the furnace cabinet right where the large return duct meets the blower.
Note the size and the arrow. Before you pull the old filter, read the size printed on the cardboard frame and look for the airflow arrow stamped on the edge.
Remove the old filter. Slide it straight out. If it's gray, matted, or you can't see light through it, it was past due.
Insert the new filter. Slide the new filter into the same slot with the airflow arrow pointing toward the furnace or blower (the direction air is traveling, away from the room and into the system).
Turn the system back on. Restore the thermostat setting. You're done.

How to Find Your Filter Size

You don't need to measure anything. The size is printed right on the cardboard frame of your current filter, usually along one edge, in a "length x width x thickness" format such as 16x25x1, 20x25x1, or 16x20x4. That nominal size is what you buy.

A few notes for Michiana homes:

Nominal vs. actual: The printed (nominal) size is rounded. A "16x25x1" filter actually measures closer to 15.75 x 24.75 inches so it slides in and out without binding. Buy by the nominal number on the frame.
If the old frame is unreadable, measure the filter itself (length, width, then thickness) and round up to the nearest inch.
Thickness matters. A 1-inch slot takes a 1-inch filter; a 4-inch or 5-inch media cabinet takes a thicker filter. Don't try to force a 1-inch filter into a 4-inch cabinet or stack two thin filters.

Where the Filter Lives: Return Grille vs. Blower Cabinet

Older and ranch-style homes around South Bend and Mishawaka often have the filter behind a hinged or pop-out return-air grille on a hallway wall or ceiling. Newer high-efficiency furnaces usually hold the filter in a dedicated slot on the side or bottom of the blower cabinet, sometimes behind a removable cover panel.

Some homes have a filter in both places, or a thick media cabinet bolted to the side of the furnace. If you're not sure you've found every filter in the system, that's worth confirming during a maintenance visit so one doesn't get missed for years.

MERV Ratings Explained: 8 vs. 11 vs. 13

MERV (Minimum Efficiency Reporting Value) measures how well a filter captures particles. Higher MERV captures smaller particles, but it also restricts more airflow, and that tradeoff matters on residential equipment.

MERV 8: Captures household dust, lint, and pollen. A solid baseline for most homes and the easiest on airflow.
MERV 11: Adds finer dust, pet dander, and some mold spores. A good step up for homes with pets or mild allergies.
MERV 13: Captures very fine particles, including some smoke and bacteria-sized particles. Helpful for serious allergy or air-quality concerns.

Here's the part homeowners often miss: higher MERV is not always better. A dense MERV 13 filter in a 1-inch slot can choke airflow on a system that wasn't designed for it, raising static pressure, reducing comfort, and making the blower work harder. As a rule, the thicker the filter media (a 4-inch or 5-inch cabinet), the higher the MERV you can run safely. If you want MERV 13-level filtration on a system with only a 1-inch slot, the better path is usually a dedicated media cabinet rather than cramming a high-MERV 1-inch filter into the existing slot. When in doubt, ask which MERV your specific furnace can handle.

The Airflow Arrow: Get the Direction Right

Every filter has an arrow printed on the cardboard edge. The arrow shows the direction air flows through it, which is from the room, into the return duct, toward the furnace and blower. Install the filter with the arrow pointing toward the furnace.

A backwards filter still moves air, but it filters less effectively and can collapse or bow over time, which is why the arrow matters. If you forget which way it went in, remember: the arrow always points the way the air is going, into the equipment.

How Often to Change It in Michiana

For most homes here, check the filter monthly and replace a standard 1-inch filter every one to three months. Thicker 4-inch and 5-inch media filters last longer, often six months to a year. Replace more often when:

It's pollen season - spring and fall in Northern Indiana load filters faster.
You have pets - dander and hair clog filters quickly.
You're running heat or AC hard - more runtime means more air pulled through the filter.
Anyone in the home has allergies or asthma - a cleaner filter helps, but only if you actually swap it on schedule.

The simplest habit: check it the first of every month and replace it whenever you can't easily see light through the media.

Signs of a Clogged Filter

A neglected filter is one of the most common causes of furnace and AC service calls. Watch for:

Weak airflow from the registers, even with the blower running
Rooms that won't reach the set temperature
The furnace short-cycling (turning on and off repeatedly)
A frozen evaporator coil in summer (restricted airflow over the AC coil)
More dust around the house and on vents
A musty or stale smell when the system runs

A clogged filter starves the blower of air, which can overheat a furnace's heat exchanger or ice up an AC coil. A two-dollar habit prevents a service call. If you've replaced the filter and still have weak airflow or short-cycling, something else is going on - see our furnace repair page or call us.

Frequently Asked Questions

What MERV rating should I use?

For most Michiana homes, MERV 8 to MERV 11 is the sweet spot for good filtration without choking airflow on a standard 1-inch system. Go to MERV 11 if you have pets or mild allergies. Reserve MERV 13 for systems with a thick 4-inch or 5-inch media cabinet, or have us add one, rather than forcing a dense high-MERV filter into a 1-inch slot it wasn't designed for.

Can I run my furnace without a filter?

No, not even briefly if you can avoid it. Without a filter, dust and debris coat the blower, the evaporator coil, and the heat exchanger, which reduces efficiency and shortens equipment life. If you're out of filters, it's better to run the system briefly with a clean used filter than with no filter at all, and replace it as soon as you can.

Why does my filter get dirty so fast?

Common reasons in our area: pets, lots of system runtime during cold snaps or summer humidity, pollen season, recent remodeling or drywall dust, or a high-MERV filter capturing more (which is the filter doing its job). If a filter is filthy in under a month every month, it's worth checking for duct leaks pulling in attic or crawlspace dust.

Reusable vs. disposable filters: which is better?

Disposable pleated filters are the right choice for most homes - they filter better than washable ones and you just swap them out. Reusable (washable) filters save on buying replacements but typically carry a low MERV rating, need thorough drying before reinstalling, and filter less effectively. For allergy or air-quality goals, disposable pleated or a media cabinet is the better path.

Does a thicker 4-inch filter last longer?

Yes. A 4-inch or 5-inch media filter has far more surface area than a 1-inch filter, so it loads up more slowly and typically lasts six months to a year. It also lets you run a higher MERV without choking airflow. The catch is your furnace needs the deeper cabinet to hold it - you can't just drop a 4-inch filter into a 1-inch slot. Ask us about adding a media cabinet, covered on our air filtration page.

Need Help With Your Furnace?

Call (574) 633-4557 or book online. Licensed in Indiana (CO19900013 HVAC / PC19700254 plumbing). We service furnaces, change filters during tune-ups, and install media cabinets and air filtration in Granger, South Bend, Mishawaka, Elkhart, and across St. Joseph and Elkhart County. 24/7 emergency service available.

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