HVAC Air Filters: MERV Ratings, Sizes and How Often to Change Them

The air filter is the cheapest part of your heating and cooling system, and one of the most important. A clean, correctly chosen filter protects your equipment, keeps airflow strong and improves indoor air quality. A clogged or wrong filter can cause frozen coils, overheated furnaces, higher bills and service calls. Here's everything you need to know.

What the air filter actually does

Your HVAC filter's first job is protecting the equipment. It keeps dust, hair and debris off the blower motor and the indoor coil, where buildup reduces efficiency and can cause failures. Its second job is improving indoor air quality by capturing particles from the air circulating through your home.

Where to find your filter

Filters are usually in one of these places:

• In a slot at the furnace or air handler, typically where the return duct connects
• Behind a return air grille on a wall or ceiling, held by clips or thumb screws
• In a media cabinet next to the furnace (for thick 4–5-inch filters)

Some homes have more than one return grille with a filter in each. Make sure you know about all of them.

How to find the right size

Look at the frame of your existing filter. You'll see a nominal size like 16x25x1 or 20x20x1 (width x height x depth in inches). Buy that size. The actual dimensions are slightly smaller so it fits the slot, which is normal.

Tips:

• Write the size on the furnace or in your phone.
• If a previous owner used the wrong size, measure the slot. Gaps around a filter let dirty air bypass it.
• Shore house owners: leave spares labeled with the size for family or renters.

MERV ratings explained

MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles of different sizes, on a scale from 1 to 16 for most residential and light commercial filters.

Brand-specific rating systems (like MPR or FPR) are used by some retailers; look for the MERV equivalent on the package.

Higher isn't always better

Higher-MERV filters capture more but can also restrict airflow more, especially in the thin 1-inch format. Too much restriction can:

• Reduce heating and cooling capacity
• Freeze the AC coil in summer
• Overheat the furnace and trigger safety shutdowns
• Make the blower work harder and use more electricity

For most homes, MERV 8–11 in a 1-inch filter is a safe balance. If you want MERV 13 for allergies or air quality, a 4–5-inch media filter cabinet gives much more surface area with less airflow restriction. We can add one during service or installation.

How often to change your filter

Change more often if you have:

• Pets, especially ones that shed
• Allergies or asthma in the household
• Ongoing renovation or construction
• Heavy system use during heat waves or cold snaps
• A shore home, where sand and salt dust get tracked in
• Lots of trees nearby, or homes in sandy Pine Barrens areas

The best rule: check monthly and change when it looks dirty or gray.

How to change a filter

1. Turn the system off at the thermostat.
2. Remove the old filter and note the airflow arrow direction.
3. Insert the new filter with the arrow pointing toward the furnace or air handler, in the direction of airflow.
4. Make sure it sits snugly with no gaps, and close any access door or grille.
5. Turn the system back on and write the date on the filter frame.

Common filter mistakes

• Installing it backward. The arrow matters.
• Forgetting a second return grille that also has a filter.
• Running without a filter "just for a few days." Dirt goes straight into the coil and blower.
• Using the wrong size and leaving gaps around the edges.
• Choosing the highest MERV available for a system that can't handle the restriction.
• Leaving it for a year. We see this on many service calls, and it's often the cause of frozen coils and furnace lockouts.

When the filter isn't the whole answer

If you change filters regularly and still have dust, allergy symptoms or rooms with weak airflow, the cause may be leaky return ducts pulling in dust from attics or crawl spaces, a dirty blower or coil, or an undersized return. These are things we check during a tune-up or service visit.

Related: AC running but not cooling, furnace not working checklist and our seasonal maintenance checklist.

We'll check it every visit

Every tune-up includes a filter check, and we'll show you where your filter is and what size to buy. Message us on WhatsApp to book in Lakewood, Toms River, Mount Laurel, Eatontown or anywhere we serve.