How often should you change your furnace filter?

The short answer

ENERGY STAR says to check the filter every month, especially in the months the system works hardest (winter and summer), and to change it at least every 3 months. Its maintenance checklist puts it the same way: inspect, clean or change the air filter in your furnace, central air conditioner or heat pump once a month. A clogged filter makes the blower work harder for the same air and lets more dust reach the equipment.

The same filter usually serves both heating and cooling, because the furnace or air handler's blower moves the air for both. So the schedule doesn't stop when the heat goes off: the air conditioner pulls air through that same filter all summer.

When to change it sooner

• PetsFur and dander load a filter fast. Look at it every few weeks.
• Dust and projectsSanding, drywall work, a renovation or a new build can clog a filter in days. Change it after the work is done, too.
• Heavy-use monthsIn the Chicago area and much of the Midwest, the furnace runs hard from about November to March and the AC through the hottest summer weeks. Those are the months to check most often.
• Allergies or smoke outsideA cleaner filter matters more when the air does. EPA's guidance on filter ratings is below.

When you can go longer

Thicker filters, the 4- and 5-inch "media" filters that sit in a cabinet beside the furnace, hold far more dust than a 1-inch filter and are made to last longer. Their makers print the interval on the filter or in the cabinet's manual, often several months or more. Use that number, but keep the monthly look: a filter that's gray and caked is done, whatever the calendar says.

How to tell it needs changing

Slide it out and hold it up to a light. If you can't see light through it, or the side facing the return air is gray and furry, change it. A new filter is white or a light color on that side. Other signs: more dust on furniture, rooms that heat or cool unevenly, or a system that runs longer than it used to.

Which filter to buy

The size is printed on the edge of the filter you have now, as width by height by depth (for example 16 x 25 x 1). Buy that size. The numbers on the label are the "nominal" size, rounded up, so don't measure the slot and buy to the fraction.

Filters are rated by MERV, a scale from 1 to 16: the higher the number, the more small particles it catches. EPA suggests choosing a filter rated at least MERV 13, or as high a rating as your system's fan and filter slot can handle, and asking an HVAC technician which is the highest that works for your system. A denser filter that your blower can't push air through well isn't an upgrade, so if you step up a lot, ask first.

How to change a furnace filter

1. 1. Turn the system offSet the thermostat to off so the blower doesn't start while the filter is out.
2. 2. Find the filterIt's usually in a slot where the big return duct meets the furnace, behind a door on the furnace itself, or behind a return grille in a wall or ceiling. Some houses have more than one.
3. 3. Note the arrow and slide it outEvery filter has an arrow on its edge showing which way the air flows. Look at which way it points before you pull the old one.
4. 4. Put the new filter in, arrow toward the furnaceThe arrow points in the direction the air moves: away from the return duct and toward the blower. Make sure it sits flat with no gaps around it.
5. 5. Close it up and turn the system back onPut back any door or grille, set the thermostat, and write down the date (or let an app do it).

Washable and electronic filters

Some systems have a permanent filter you wash, or an electronic air cleaner with cells you clean. They still need the monthly look. Clean them on the schedule in their manual, and let a washed filter dry completely before it goes back in.

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