How Often Should You Change Your Air Filter?

Changing the air filter is the cheapest, easiest thing you can do for your heating and cooling system, and it's the one most often forgotten. A clogged filter is behind a surprising number of service calls, from frozen AC coils to furnaces that shut themselves off. Here's how to get it right.

How often to change it

• Standard 1-inch filters: every 1 to 3 months.
• Homes with pets, allergies, smokers or recent construction: every month.
• Thicker 4- to 5-inch media filters: usually every 6 to 12 months, depending on the manufacturer.
• Heavy-use seasons: check monthly in the peak of summer and winter, when the system runs the most.

The simplest test: hold it up to a light. If you can't see light through it, it's time.

Where to find your filter

Most homes have the filter in one of two places: behind a return air grille in a wall or ceiling, or in a slot at the indoor unit (furnace or air handler) in a closet, attic, garage or crawlspace. Some homes have more than one return, and each one needs its own filter.

Getting the right size

The size is printed on the edge of the old filter, such as 16x25x1. Use that exact size. A filter that's too small lets dirty air slip around it and coat the indoor coil, which is far harder to clean than a filter is to replace.

Which way it goes in

Every filter has an arrow on the frame. The arrow points in the direction of airflow: toward the furnace or air handler, away from the return grille.

Understanding MERV ratings

MERV ratings tell you how fine the filter is. Higher catches smaller particles but also restricts more air.

• MERV 1–4: basic fiberglass. Protects the equipment, does little for air quality.
• MERV 8–11: a good choice for most homes, catching dust, pollen and pet dander without choking airflow.
• MERV 13: better for allergies, but in some systems with undersized returns it can restrict airflow. Ask us if your system can handle it.

Thicker media filters can use higher ratings with less restriction because they have more surface area.

What a dirty filter actually does

• In summer: low airflow can freeze the indoor coil, leading to water leaks and no cooling.
• In winter: a furnace can overheat and shut down on its safety limit, and a heat pump has to work harder and lean on expensive backup heat.
• All year: higher power bills, more dust in the house, and extra wear on the blower motor.

Make it easy to remember

Buy filters in multi-packs, write the install date on the frame, and set a recurring reminder on your phone. If your filter is hard to reach, in an attic or crawlspace for example, mention it during your next tune-up. We can talk about adding a filter grille somewhere easier to reach.