How to change your HVAC filter (size, arrow, MERV)

If your heating or cooling seems weaker than usual, or the house feels dustier, start with the air filter. A dirty filter makes your system work harder and can lead to bigger problems, like a frozen AC coil.

Changing it takes about five minutes and no tools. The only tricky parts are finding where the filter lives, buying the right size and putting it in facing the right way. We'll cover all three.

What you'll need

A new filter in the same size as your old one Shop for A new filter in the same size as your old one (opens in a new tab)
Flashlight
Marker for writing the date on the filter
Trash bag for the old filter

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It's smart to keep a spare filter or two on hand so you're never tempted to skip a change.

Find your filter

Filters sit on the return side of the system, where air is pulled back in to be heated or cooled. Look in these places:

In or next to the furnace or air handler. Trane says this is the most common spot. Look for a narrow slot with a cover, either where the big return duct meets the unit or behind the blower compartment door.
Behind a return grille. Some homes have the filter behind a large vent on a wall or ceiling. The grille usually has clips or thumb screws.
In more than one place. Larger homes may have several return grilles with filters, or more than one system. Check them all.

If you rent, ask your landlord where the filters are and who's responsible for changing them.

Find the right size

Pull out the old filter and look at the cardboard frame. Trane says the size is printed on one or more edges. It will look like three numbers, such as 16 x 25 x 1. That's width, height and thickness in inches.

Buy the exact size printed on the frame. A filter that's too small lets dirty air slip around the edges. One that's too big won't fit the slot. Write the size somewhere handy, like inside a cabinet door or in your phone.

No size printed? Measure the slot or the old filter, and take those numbers to the store. An HVAC technician can also tell you at your next service visit.

Choose a MERV rating

MERV stands for Minimum Efficiency Reporting Value. The EPA explains that it rates how well a filter catches small particles, on a scale from 1 to 16. Higher numbers catch smaller particles, like fine dust, pollen and smoke.

The EPA suggests choosing at least MERV 13, or as high a rating as your system fan and filter slot can accommodate. Denser filters can slow airflow in some systems, so check your system's manual or ask an HVAC technician which rating works for your equipment.

Filter thickness	Typical change interval (Carrier)
1 inch	About every 30 to 60 days
4 to 5 inches	Longer, often around 6 months or more

Always follow what the filter package and your system maker say for your setup.

Change the filter, step by step

Turn the system off. Set the thermostat to Off.
Open the filter door or grille. Slide off the cover or open the return grille.
Note the arrow on the old filter. Before you pull it out, look at which way the arrow points. Carrier suggests noting this so you can match it.
Slide out the old filter. Do it slowly so dust doesn't fall into the duct. Put it straight into a trash bag.
Write the date on the new filter. Use a marker on the frame.
Insert the new filter with the arrow pointing toward the unit. The arrow shows airflow. It should point toward the furnace or air handler, away from the return duct. Carrier says the arrows should point toward the blower motor. On a ceiling or wall grille, that usually means pointing up into the ceiling or into the wall.
Close the cover and turn the system back on. Make sure the filter door is snug so air can't leak around it.

How often to change it

There's no single answer. Trane suggests planning on every 30 to 90 days. The Department of Energy recommends checking and changing AC filters every month or two during the cooling season. Change more often if you have:

Pets that shed
Allergies or asthma in the home
Smokers in the home
Construction, remodeling or a lot of dust
Heavy use during very hot or cold months

The easiest test: check it monthly. If it looks gray and clogged, change it, even if the calendar says you have more time.

When to call a pro

A filter change is a DIY job. Call an HVAC technician if:

You can't find the filter, or the filter slot is damaged and air leaks around it.
The filter gets very dirty within days of a change.
You see mold, water or rust inside the filter area.
Airflow is still weak with a clean filter.
You want to switch to a higher MERV or thicker filter and aren't sure your system can handle it.

Read how to choose an HVAC technician. If cooling is the problem, see our guide to an AC that's not cooling.

Make it a habit

Set a monthly reminder to check the filter. Our maintenance calendar can add it to your phone.
Buy filters in multi-packs so you always have one ready.
Write the size and the install date on every filter.
Keep the area around return grilles clear of furniture and drapes.
Add filter checks to your monthly home maintenance checklist.

Common questions

Which way does the arrow on an air filter go?

The arrow shows the direction of airflow, so it should point toward the furnace or air handler and away from the return duct. Carrier puts it simply: the arrows should point toward the blower motor.

How often should I change my furnace filter?

It depends on the filter and your home. Trane suggests every 30 to 90 days, and Carrier notes that thicker 4- to 5-inch filters can last longer. Check the filter monthly and follow the filter and system maker's guidance, more often with pets or allergies.

What MERV rating should I buy?

The EPA suggests at least MERV 13, or as high a rating as your system fan and filter slot can handle. Some systems aren't designed for denser filters, so check your manual or ask an HVAC technician.

Can I wash and reuse my furnace filter?

Only if it's sold as washable. Trane notes that most central air filters are disposable and aren't meant to be cleaned.