How Often to Change a Furnace Filter (By Filter Size, Pets and Season)

Change a 1-inch furnace filter every 1 to 3 months, and a 4-inch filter every 6 to 12 months. Change it sooner, about every 30 to 45 days, if you have pets, allergies or a dusty home. The best rule is to check it monthly and replace it when it looks gray and clogged.

A dirty filter is one of the most common reasons a furnace fails in winter. It is also the cheapest thing to fix. This guide gives you a schedule that fits your home and shows you how to check a filter in 30 seconds.

How often should you change a furnace filter?

The right interval depends on the filter thickness and your home. These are typical ranges from filter and HVAC sources. They are estimates, so your furnace manual and the filter box come first.

In winter your furnace runs more, so the filter fills faster. Many people check monthly during the heating season and every couple of months otherwise.

Things that make a filter clog faster

• Pets. Hair and dander fill filters quickly.
• Allergies or asthma. You may want to change sooner for cleaner air.
• A dusty home, renovations or smokers.
• A big house or a furnace that runs a lot.
• High-MERV filters. Denser filters catch finer dust and can clog sooner.
• Open windows, construction or wildfire smoke.

How to check your filter

1. Turn off the furnace at the thermostat.
2. Find the filter. It is usually in a slot at the blower, or in a return air grille in a wall or ceiling.
3. Slide it out. Note the arrows on the frame, which show the airflow direction.
4. Hold it up to a light. If you cannot see light through it, or it looks gray and furry, replace it.
5. Write the size (for example 16x25x1) on the frame or your phone so you can buy the right one.

How to change a furnace filter

1. Turn the furnace off.
2. Slide the old filter out.
3. Slide the new one in with the arrows pointing toward the furnace (the direction of airflow).
4. Close the cover or grille securely.
5. Write the date on the frame with a marker.
6. Turn the furnace back on.

A filter installed backwards works poorly and can fall out. A loose cover lets dust bypass the filter.

What MERV rating should you choose?

MERV measures how fine a particle the filter catches. A higher number catches smaller particles, but also restricts airflow more. Some HVAC sources suggest MERV 8 to 11 for most homes. A very high MERV filter in a system not built for it can strain the blower.

If you want a high-MERV filter, check your furnace manual or ask an HVAC technician. See the furnace shutoff guide for what happens when airflow is restricted.

What happens if you never change it?

• Weak heat. Airflow drops and rooms warm slowly.
• Overheating. The furnace's safety switch shuts off the burners, so it cycles on and off.
• Higher energy use. The system works harder.
• More dust in the house and on the vents. See dust control tips.
• Stress on the blower motor and a shorter system life.
• Frozen coil if you also run air conditioning with the same filter.

If your furnace is blowing cold air, check the filter first. See furnace blowing cold air.

Reusable vs disposable filters

• Disposable: cheap, easy. Replace on schedule.
• Washable or reusable: need regular washing and must be completely dry before you reinstall. They can be less effective at fine dust.

Do not mix a washable filter with a dirty habit. If you will not wash it on schedule, a disposable is safer.

How to remember

• Set a repeating phone reminder for the first of each month in winter.
• Buy filters in packs and store one next to the furnace.
• Write the size on the unit.
• Change at the start of the heating season and again in midwinter.

When to call a professional

If the filter is new and the furnace still shuts off, blows cold air or does not start, the cause is elsewhere. See which pro to call for no heat.

Bottom line

Check monthly, replace every 1 to 3 months for a 1-inch filter, and sooner with pets or dust. Use the right size, point the arrows toward the furnace, and write the date on the frame.