How Often Should You Change Your Furnace Filter? (And Which MERV Rating to Use)

The furnace filter is the cheapest part of your heating and cooling system and the one that causes the most trouble when it's ignored. A clogged filter is behind a lot of the repair calls we take every winter: furnaces overheating and shutting off, frozen AC coils in summer, rooms that never get warm.

Here's how often to change it in a Minnesota home, what the MERV number on the package actually means, and how to pick a filter that cleans the air without choking your furnace.

How often to change it

It depends mostly on how thick the filter is. Look at the edge of the filter frame for its size. The third number is the thickness.

• 1-inch filters: every 1 to 3 months. In a Minnesota winter, when the furnace runs for hours a day, plan on the shorter end.
• 2-inch filters: every 2 to 4 months.
• 4- or 5-inch media filters (in a separate cabinet beside the furnace): typically every 6 to 12 months. Follow the manufacturer's guidance for your cabinet.

The simplest habit: check it once a month from October through April, and again when the AC is running hard in summer. Pull it out and hold it up to a light. If you can't see light through it, or it's gray and caked, change it.

When to change it more often

• You have pets, especially ones that shed.
• You're remodeling, sanding drywall or refinishing floors.
• Someone in the house has allergies or asthma.
• You run the furnace fan continuously.
• There's wildfire smoke in the air, like the Canadian smoke that has drifted over the Twin Cities in recent summers.
• The house is new or was recently renovated. Construction dust loads filters quickly for months afterward.

What MERV ratings mean

MERV stands for Minimum Efficiency Reporting Value. It's a scale, mostly from 1 to 16 for home filters, that measures how well a filter catches particles of different sizes. According to the EPA's explainer on MERV ratings, the higher the number, the better the filter is at trapping small particles.

In plain terms: low-MERV filters (1 to 4), like basic fiberglass, mainly protect the equipment from large dust and lint. Mid-range filters (around 8) catch dust, pollen and pet dander. Higher filters (11 to 13) also catch much finer particles, including a lot of smoke and many bacteria-sized particles.

Is a higher MERV always better?

Not automatically. A higher-MERV filter has a denser material, and in a thin 1-inch filter that can restrict airflow. On a furnace with marginal ductwork or undersized returns, which is common in older Twin Cities homes, a dense 1-inch filter can make the furnace overheat and trip its safety limit, or freeze the AC coil in summer.

The EPA recommends choosing a filter with at least a MERV 13 rating if you're upgrading, or as high a rating as your system's fan and filter slot can handle, and suggests checking with an HVAC technician to find the right fit. That second part matters. The best way to get high filtration without the airflow penalty is a thicker filter. A 4- or 5-inch media filter has far more surface area, so it can filter at a high MERV while letting air move freely, and it lasts much longer between changes.

If you've switched to a high-MERV filter and noticed the furnace shutting off, weaker airflow or a whistling sound at the filter slot, go back to the filter you used before and have the system checked. Our post on furnace short cycling explains why restricted airflow causes problems.

Where is my furnace filter?

If you've never changed it, finding the filter is the first step. In most Twin Cities basements, it's in one of these places:

• A slot in the ductwork right where the large return duct meets the furnace, usually covered by a thin metal or plastic flap.
• Behind the lower front panel of the furnace, in the blower compartment. Turn the furnace off at the service switch before opening the panel.
• In a separate filter cabinet a few inches thick, mounted between the return duct and the furnace. That's where 4- and 5-inch media filters live.
• Behind a hinged return grille in a wall, ceiling or hallway. This is common in some newer homes and townhomes.

Some homes have more than one. And never run the furnace without a filter, even for a day or two while you wait for the right size. Unfiltered air carries dust straight into the blower and the AC coil, and a dirty coil is a far bigger job to clean than a filter is to replace.

Finding the right size

The size is printed on the edge of your current filter, like 16 x 25 x 1. That's the nominal size. The actual size is usually slightly smaller so it fits the slot, and both are often printed on the frame. Buy the nominal size printed on your old filter.

When you put the new one in, find the airflow arrow on the frame and point it toward the furnace, in the direction the air travels. A filter installed backward doesn't filter as well and can collapse. If there's no filter slot at all, or the filter is in a return grille in a wall or ceiling, that's normal in some homes. Just make sure you know where every filter in the system is.

You can order furnace filters in common sizes from our online shop.

What a dirty filter does to your system

• Overheating. The furnace can't move enough air across the heat exchanger, so it trips its high-limit switch and shuts off, over and over.
• Frozen AC coils. In summer, low airflow lets the indoor coil drop below freezing. Our post on frozen AC coils covers that problem.
• Higher energy bills. The blower works harder and runs longer to move the same air.
• Shorter equipment life. Repeated overheating stresses the heat exchanger and wears out motors.
• Dustier air. A filter that's completely loaded can let dust bypass around its edges.

Beyond the filter

If dust, allergies or smoke are a real concern, a filter is only one piece. Whole-house air cleaners, UV lights and fresh-air ventilation can do more than any filter alone. Our indoor air quality page covers those options, and our post on dry air, dust and Minnesota winters explains how they work together.

Filter changes are included in our maintenance plans, and every furnace tune-up includes a filter check. If you'd like help choosing a filter or adding a media filter cabinet, call Heating & Cooling Two at 763-428-3677.