How often should you change your AC filter?

The air filter is the cheapest part of your heating and cooling system and the one that gets ignored the most. Most people think of it as an "air quality" item. Its first job is actually to protect the equipment: it keeps dust off the blower and the indoor coil so the system can move air the way it was designed to.

What happens when a filter clogs

• Airflow drops, so rooms take longer to cool or heat and the system runs longer cycles.
• Energy use goes up. The U.S. Department of Energy says replacing a dirty, clogged filter with a clean one can lower an air conditioner’s energy use by 5% to 15%.
• The indoor coil can get too cold and freeze over. You may see ice on the copper lines or water around the indoor unit once it thaws.
• In heating mode, low airflow can make a furnace overheat and shut itself off on its safety limit switch.
• Dust that gets past a collapsed or missing filter coats the coil and blower, which costs far more to clean than a filter costs to replace.

How often to change it

There is no single number that fits every home. The Department of Energy recommends cleaning or replacing filters every month or two during the cooling season, and more often if the system runs constantly, you live somewhere dusty, or you have pets that shed. A good starting point:

• Standard 1-inch filters: check monthly and replace every 1 to 3 months.
• Thicker 4- to 5-inch media filters: usually every 6 to 12 months, but check them too.
• Replace sooner during peak summer or winter, wildfire smoke, remodeling or construction dust, or if anyone in the home has allergies.
• In desert cities like Las Vegas and Phoenix, fine dust loads filters quickly. Monthly checks during summer are a good habit.

Picking the right filter

• Size: use the exact size printed on the frame of your current filter (for example 20x25x1). A filter that is too small lets air sneak around the edges, unfiltered.
• Direction: the arrow on the frame points toward the blower, in the direction the air flows.
• MERV rating: higher MERV catches smaller particles. The EPA recommends a filter rated at least MERV 13, or as high as your system can accommodate.
• That last part matters. Some systems, especially older or undersized ductwork, can’t move enough air through a dense filter. If a higher-MERV filter makes the system louder, weakens airflow or causes the coil to freeze, ask a technician to measure the system’s static pressure and recommend the right filter.
• Never run the system without a filter, even for a few days.

Where to find it

Filters are usually behind a return-air grille on a wall or ceiling, or in a slot at the furnace or air handler. Some homes have more than one return and more than one filter, so check every large grille that pulls air in. If you can’t find a filter at all, have a technician show you where it goes.

When to call a pro

• Ice on the refrigerant lines or indoor unit, even with a clean filter.
• Weak airflow from the vents after a filter change.
• Filters that get unusually dirty, fast. That can point to a leak in the return ductwork pulling dust from the attic or garage.
• A musty smell when the system runs.