How often should I change my HVAC filter?

Change your HVAC filter every 1-3 months depending on filter type and household conditions. 1-inch fiberglass filters: every 30 days. 1-inch pleated filters: every 60-90 days. 4-inch media filters: every 6-12 months. HEPA filters: every 12 months. Homes with pets, smokers, or residents with allergies should change filters more frequently — every 30-45 days for 1-inch pleated filters. Homes with one person and no pets can extend to 90 days.

What happens if you don't change your HVAC filter?

A clogged HVAC filter causes several problems: the blower motor works harder and uses more electricity (a dirty filter can increase energy costs by 15-20%), airflow is restricted causing uneven heating or cooling, the evaporator coil can freeze in summer from insufficient airflow, indoor air quality drops as the filter can no longer trap particles, and the extra strain can shorten the lifespan of the blower motor and compressor. A severely restricted filter can cause the system to shut down on high-limit protection.

How do I change my HVAC filter?

To change your HVAC filter: (1) Turn the system off at the thermostat. (2) Locate the filter — it is usually in the return air vent (a large grille on a wall or ceiling) or at the air handler/furnace in the filter slot. (3) Note the airflow arrow direction on the old filter or on the filter slot. (4) Remove the old filter and slide it into a garbage bag to avoid spreading dust. (5) Insert the new filter with the arrow pointing toward the blower motor (away from the return air, toward the system). (6) Turn the system back on. (7) Write the change date on the filter edge in marker.

What HVAC filter should I buy?

Choose your HVAC filter by MERV rating (Minimum Efficiency Reporting Value). MERV 1-4: fiberglass flat filters, very low cost, minimal filtration — not recommended. MERV 8: basic pleated filter, captures dust, pollen, and pet dander — good for most homes. MERV 11: captures smaller particles including mold spores and pet allergens — recommended for allergy sufferers. MERV 13: near-hospital-grade filtration, captures bacteria and smoke — may restrict airflow on older systems. Check your HVAC manual for the maximum MERV rating your system supports before buying a high-efficiency filter.

Where is my HVAC filter located?

HVAC filters are located in one of two places: (1) In the return air grille — a large wall or ceiling vent, usually 14x25 inches or larger, that has a grille you can open. Open the grille and the filter slides in behind it. (2) At the air handler or furnace — look for a slot on the side of the unit where a flat filter slides in. Some systems have filters in both locations. If you cannot find the filter, set the thermostat to fan-only mode and feel along the ductwork near the air handler for the strongest suction — the filter is on the suction side of the blower.

Can a dirty HVAC filter cause health problems?

Yes — a dirty or clogged HVAC filter contributes to poor indoor air quality. When a filter is full, it can start releasing trapped particles back into the airstream, worsening allergy and asthma symptoms. Restricted airflow can also promote mold growth on the evaporator coil, which then circulates spores throughout the house. In Southern Indiana and Louisville, where pollen counts are high in spring and fall, running your system with a clean filter is particularly important for allergy sufferers.

What size HVAC filter do I need?

The filter size is printed on the side of your old filter in the format LxWxD (length x width x depth), such as 16x25x1 or 20x20x4. The most common residential sizes are 16x25x1, 20x20x1, 20x25x1, and 16x25x4 for media filters. If you cannot read the old filter, measure the filter slot with a tape measure. Note that nominal sizes (what is printed) are typically 1/2 inch smaller than the actual slot dimensions — always buy the size printed on the old filter or in your system manual.

Is it OK to run the AC without a filter?

Running your AC or furnace without a filter is not OK for more than a day or two. Without a filter, dust and debris accumulate on the evaporator coil, blower wheel, and ductwork. Even a short period can allow enough buildup to cause the evaporator coil to ice up in summer or reduce airflow significantly. Running without a filter also voids most manufacturer warranties. If you are out of filters, run the system on fan-only mode (not cooling or heating) while you get a replacement — never run heating or cooling without a filter for more than 8 hours.

HVAC 7 min read Beginner Mar 4, 2026

How to Change Your HVAC Filter (And Why It Matters)

A dirty HVAC filter is one of the most common causes of high energy bills, poor air quality, and premature system failure. Here's everything you need to know to pick the right filter and change it correctly.

The Most Neglected 5-Minute Maintenance Task in Your Home

Your HVAC system moves hundreds of cubic feet of air through your home every single hour. Every bit of that air passes through your filter — and if that filter is clogged, your system has to work harder, your energy bill climbs, and the air your family breathes gets worse. The good news: changing your filter takes about five minutes and costs less than a cup of coffee. It's one of the highest-return maintenance habits you can build.

Why Your Filter Actually Matters

Most people think air filters are just about air quality. They're half right. Your filter does two important jobs:

Protects the equipment. The filter's primary job is to keep dust, pet hair, and debris out of your blower motor, evaporator coil, and ductwork. A clogged filter starves your system of airflow, causing the evaporator coil to freeze and the motor to overheat. That turns a $5 filter into a $1,500 repair.
Improves air quality. A clean, high-quality filter traps pollen, mold spores, pet dander, and fine dust before they circulate through your home. This matters especially if anyone in your household has allergies or asthma.

Pro Tip: A dirty filter can increase your energy bill by 5–15%. If you haven't changed yours in three months, your HVAC system is probably working harder than it needs to right now.

Step 1: Find Your Filter and Note the Size

Your filter is almost always located in one of two places: in the air handler cabinet itself (usually in a utility closet or attic), or in a return air vent on your wall or ceiling. The return vent is typically the large louvered vent that pulls air in rather than blowing it out — it's bigger than your supply vents and doesn't blow air when you hold your hand near it.

Pull the old filter out and look at the cardboard frame. You'll see three numbers printed on the side — something like 16x25x1 or 20x20x4. That's width × height × depth in inches. Write this down or snap a photo before you go shopping. Filters must fit snugly — a loose filter lets air bypass it entirely and defeats the whole purpose.

Pro Tip: If the numbers on your old filter are worn off, measure the slot opening with a tape measure. The actual filter will be about 1/2 inch smaller than the listed size — that's normal and intentional.

Step 2: Understand Filter Types

Walk down the filter aisle at any hardware store and you'll find four main types. Here's what you're actually looking at:

Filter Type	Cost	Lifespan	Best For
Fiberglass (flat)	$1–3	30 days	Basic equipment protection only
Pleated (standard)	$5–15	60–90 days	Most households — good balance
Pleated (high-MERV)	$15–30	60–90 days	Allergy/asthma households, pets
Washable/Permanent	$25–60 one-time	Years	Those who want to skip buying filters

Step 3: Decode MERV Ratings

MERV stands for Minimum Efficiency Reporting Value — it's a standardized scale from 1 to 16 that tells you how fine a particle the filter can trap. Higher is not always better for residential use:

MERV 1–4: Fiberglass filters. Catches lint and large dust. Essentially useless for air quality.
MERV 8: Standard pleated filter. Catches dust mites, pollen, mold spores. Fine for most homes with no pets or allergies.
MERV 11: Catches pet dander, fine dust, some smoke particles. Good for pet owners.
MERV 13: The sweet spot for allergy and asthma sufferers. Catches bacteria, some viruses, smoke. This is the residential sweet spot if air quality is a priority.
MERV 14–16: Hospital-grade. Restricts airflow significantly in most residential systems — can actually harm your equipment unless your system is designed for it.

Warning: Don't assume the highest MERV filter is always the best choice. Check your HVAC manual or call your HVAC tech before jumping to MERV 13+ — some older systems don't have the blower capacity to push air through a dense filter, which strains the motor.

Step 4: Replace the Filter — Step by Step

Turn off your HVAC system at the thermostat. You don't want it kicking on while the filter slot is open and unfiltered air is getting pulled through.
Open the filter compartment. Return vents usually have two latches or screws. Air handler cabinets have a panel door or a dedicated filter slot.
Slide out the old filter. Hold it flat — a clogged filter can drop a significant amount of dust if tilted. Slide it into a garbage bag immediately.
Check the slot for debris. Shine a flashlight inside and wipe any obvious dust buildup around the edges with a damp cloth.
Install the new filter. Look for the airflow arrow printed on the filter frame — it should point toward the air handler (away from the return air, toward the blower). Installing it backward reduces effectiveness by 50%.
Close up and turn the system back on. Done.

Pro Tip: Use a marker to write the installation date on the filter frame the moment you install it. Next time you check it, you'll know exactly how old it is without having to guess.

How Often Should You Change Your Filter?

The one-size-fits-all answer of "every 3 months" is a starting point, not a rule. Your household situation changes the math significantly:

Single occupant, no pets, mild allergies: Every 6–12 months for a MERV 8.
Average family of 3–4, no pets: Every 90 days.
One pet (cat or dog): Every 60 days.
Multiple pets or someone with allergies: Every 30–45 days.
During heavy pollen season (spring/fall): Check monthly regardless of household size.

The real answer is simple: pull the filter out and look at it. If it's gray and clogged, change it. If it still looks relatively clean, give it another 30 days. You'll quickly learn your home's rhythm.

Set a Reminder So You Don't Forget

The biggest reason filters go unchanged for a year is not laziness — it's out of sight, out of mind. Set a recurring reminder in your phone right now. If you use a smart home assistant, ask it to remind you monthly to check the filter. Some HVAC apps and smart thermostats (like Ecobee and Nest) can send filter reminders automatically. You can also sign up for a filter subscription service that ships filters to your door on a schedule — Filtrete, Nordic Pure, and FilterBuy all offer this, and the cost is comparable to buying at the store.

Tools You'll Need

That's it. Five minutes, a $10 filter, and you've just protected a $5,000–$15,000 piece of equipment and improved the air quality in your home. Do it today.

Need a Pro For This Job?

If this project feels too big to tackle yourself, find a licensed professional in your area.

Find a Local Pro

Find HVAC Service Near You

Browse licensed local contractors serving Kentuckiana.

Browse All HVAC Service Pros in Kentuckiana