How Often to Change a Furnace Filter and Which MERV Rating to Buy

A standard 1-inch furnace filter should be changed every 1 to 3 months, a 2-inch filter every 3 to 6 months, and a 4- or 5-inch media filter every 6 to 12 months, following the filter maker’s label. Homes with pets, allergies, smoke or ongoing renovation need changes at the short end of each range. For most homes a MERV 8 to MERV 13 filter is the right choice, as long as the furnace is rated to handle the airflow resistance of the higher numbers.

How often should I change my furnace filter?

Filter thickness sets the starting point, and how dusty the house is sets the actual schedule.

• 1-inch filters: every 1 to 3 months.
• 2-inch filters: every 3 to 6 months.
• 4- and 5-inch media filters: every 6 to 12 months.

Change the filter sooner if any of these apply:

• You have dogs or cats, especially ones that shed heavily.
• Someone in the house has allergies or asthma.
• The system runs hard in a very cold winter or hot summer (most furnaces share their blower and filter with central air conditioning).
• There is construction, sanding or drywall work in the house.
• Wildfire smoke reaches your area, or someone smokes indoors.

The simplest check is to look at the filter every month. If the surface is gray and coated, or you cannot see light through a thin pleated filter when you hold it up, replace it.

What happens if I do not change the furnace filter?

A clogged filter restricts airflow, so the blower works harder and the system runs longer to heat or cool the house. That raises energy use and can cause uneven temperatures between rooms. On a furnace, poor airflow can let the heat exchanger overheat until a safety switch shuts the burners off, which is one reason a dirty filter is among the first things to check when the furnace will not turn on. On air conditioning, low airflow can let the indoor coil freeze. Dust that gets past a worn filter also builds up on the blower and coil. The U.S. Department of Energy lists regular filter changes as basic furnace maintenance.

What does MERV mean?

MERV stands for Minimum Efficiency Reporting Value, a rating from an ASHRAE test standard that measures how well a filter catches particles of different sizes. Residential filters usually range from MERV 1 to MERV 16, and a higher number means the filter traps more and smaller particles. Some retailers and brands use their own scales, such as MPR or FPR, so compare the MERV number when the package lists it. The EPA explains the rating and its use in homes.

• MERV 1 to 4: basic fiberglass or flat filters that mainly protect the equipment from large dust and lint.
• MERV 5 to 8: pleated filters that also catch mold spores, pet dander and finer dust.
• MERV 9 to 12: better capture of fine dust and some smaller particles.
• MERV 13 to 16: capture a large share of very fine particles, including many in smoke.

Which MERV rating should I buy?

Pick the highest rating your system handles without choking airflow, and match it to what your household needs.

• MERV 8 is a sensible default for most homes without special air concerns.
• MERV 11 suits homes with pets or mild allergies.
• MERV 13 is the level the EPA suggests for reducing fine particles, including during wildfire smoke, if the system can take it.
• MERV 14 to 16 filters are usually meant for systems designed for them, often with thicker filter cabinets.

Higher-MERV filters have denser media, which adds resistance. Older or smaller blowers, and some systems with a thin 1-inch slot, may lose airflow with a MERV 13 filter. Check the furnace manual or the manufacturer’s support site for the recommended filter, or ask an HVAC technician. A thicker 4- or 5-inch media cabinet gives high-MERV media more surface area, so it filters well with less restriction.

Watch for whistling at the filter slot, weak airflow at the registers, or a filter that bows inward after a change. Any of these means the filter may be too restrictive, and stepping down a level or switching to a thicker cabinet will help.

How do I change a furnace filter?

1. Turn the thermostat off, or switch off the furnace, so the blower does not pull dust in while the slot is open.
2. Find the filter. It sits in a slot between the return duct and the blower, in a cabinet beside the furnace, or behind a return air grille on a wall or ceiling.
3. Slide out the old filter and read the size printed on its frame. Nominal sizes like 16 x 25 x 1 are rounded, so buy the same printed size.
4. Find the airflow arrow on the new filter. Point it toward the furnace and blower, away from the return duct.
5. Slide the new filter in so it sits flat with no gaps around the edges, then close any cover.
6. Write the date on the frame or set a phone reminder, then turn the system back on.

Do not run the furnace without a filter, even for a day or two, because dust goes straight onto the blower and coil. Dust on equipment is a common cause of wasted energy around the house, and the same logic applies to cleaning refrigerator coils.

Are washable furnace filters worth it?

Washable filters can be rinsed and reused, which saves buying replacements. They usually have low MERV ratings, and they must dry completely before going back in, because a damp filter can grow mold. For most homes, a disposable pleated filter in the MERV 8 to 13 range traps more fine particles.

Quick answers

Does a higher MERV filter save energy?

A higher MERV filter does not save energy by itself, and one that is too restrictive for the system can raise energy use. Changing filters on schedule has a bigger effect on efficiency.

Can I use an air conditioner filter in my furnace?

In most homes the furnace and central air share one filter, so the same filter does both jobs. Use the size and rating the system calls for.

What size furnace filter do I need?

Read the size printed on the edge of the current filter, or measure the slot. Write the size down so you buy the right one next time.