How to Change a Furnace Filter: Find It, Read the Arrow, Get the Size Right

The furnace filter is the one maintenance task every homeowner can do, and the one that gets skipped the most. It takes two minutes, needs no tools, and costs a few dollars a month. Skip it and the system pulls air through a mat of dust: the blower works harder, the coil ices or overheats, the heat exchanger runs hotter than it should, and the air in the house gets worse. HVAC technicians will tell you a clogged filter is behind a large share of their "no heat" and "no cool" calls.

This guide shows you where to find the filter, how to read the airflow arrow so it goes in the right way, how to avoid the size mistake that sends most people back to the store, and what a monthly check looks like. If you have not picked a filter yet, our furnace filter guide explains MERV ratings and names three worth buying. If the filter you pull out is packed solid and the system has been running poorly, that is a sign to book the tune-up on the fall checklist as well.

Before You Start

• A replacement filter in the right size (read the old one first; see step 4).
• A flashlight, and a marker to write the date on the new filter's frame.
• A vacuum with a brush attachment, for the slot and the grille.
• Optional: a trash bag to slide the old filter into, so the dust stays on the filter and not on the floor.

Step-by-Step

Find the filter

1. Start at the furnace or air handler. Look where the big return duct enters the cabinet, usually on the side or the bottom. There is a slot about an inch wide with a metal or plastic cover, sometimes a hinged door, sometimes a strip of foil tape. That is the filter slot, and the filter slides in edgewise.
2. If there is no slot at the furnace, look at the return grilles. Many homes, especially with the air handler in an attic or closet, put the filter behind a large hinged grille in a hallway ceiling or wall. Some homes have several. Open the grille and the filter sits in a frame behind it.
3. Some systems have both. If you find a filter at the furnace and another behind a grille, ask an HVAC tech which one should be in use; running two in series restricts airflow. Usually the answer is one or the other, not both.

Change it

4. Read the old filter before you pull it. Look at the arrow printed on the frame and note which way it points. Then read the size printed on the edge, such as 16x25x1. That number is the nominal size; the actual filter is about a half inch smaller in each direction, which is why a filter labeled 16x25x1 fits a slot that measures closer to 15.5 by 24.5 inches. Buy by the printed nominal size, not by your tape measure.
5. Slide the old filter out and, if you have one, straight into a trash bag. Hold it up to the light: if you cannot see light through the pleats, it was overdue.
6. Vacuum the slot and the grille while the filter is out. Dust that has collected around the slot goes straight into the blower once the new filter is in.
7. Put the new filter in with the arrow pointing toward the furnace. The arrow shows the direction of airflow, and air flows from the return duct into the furnace or air handler and out through the supply ducts. At a furnace slot, the arrow points into the cabinet. Behind a return grille, the arrow points away from the room, into the duct. It is the single most common mistake, and a backwards filter still filters, but the pleats are not supported on the side that takes the load, so it collapses and restricts airflow sooner.
8. Write today's date on the frame with a marker, close the door or grille, and confirm the cover is seated so air cannot bypass the filter around its edges. A filter that rattles in the slot or has daylight around it is letting dirty air past.
9. Turn the system back on and listen. A steady, quiet blower is right. A whistle at the filter means air is being pulled through a gap or the filter is far too restrictive for the system.

Troubleshooting

• The new filter whistles or the airflow at the vents drops. The filter is too restrictive for the blower, which happens with high-MERV filters in a 1-inch slot. Step down to a MERV 8 to 11 filter, or have a tech confirm the system can take a higher rating; our filter guide covers the tradeoff.
• The filter is filthy after a month. Pets, a renovation, a dusty return in the basement, or a duct leak on the return side pulling in attic or crawl-space air. Change it more often for now and have the return checked at the next service.
• The filter is bowed or sucked into the slot. Restricted airflow or a filter too small for the slot. Check the size and the arrow.
• The system short-cycles, the coil ices up, or the furnace shuts off on its high-limit switch. A clogged filter is the first suspect. Replace it; if the problem continues with a clean filter, call for service.
• You cannot find the filter anywhere. Some older systems have it inside the blower compartment, behind the lower furnace panel. That panel is safe to open on most units once the power switch is off, but if you are unsure, ask the tech to show you on the next visit rather than guessing.

How Often

Check it every month and change it every one to three months, the house rule on every page of this site. ENERGY STAR's floor is every three months; the Department of Energy suggests every one to two months in heavy-use seasons, and sooner with pets, allergies, smokers, or construction dust. Thick 4- or 5-inch media filters last longer, typically six months to a year per their manufacturers, but still get a look each month. The maintenance calendar puts the check at the top of every month's list, with a change before heating season in the fall and a check after it in the spring. Buy filters by the case so the change never waits on a store run.

Frequently Asked Questions

Which way does the arrow on a furnace filter go?

Toward the furnace or air handler, in the direction the air travels. At the furnace, into the cabinet. At a return grille in a wall or ceiling, into the duct and away from the room. If you cannot remember which way the old one went, follow the airflow, not the old filter; the previous owner may have had it backwards.

Can I wash and reuse a furnace filter?

Only if it is a washable electrostatic filter with a rigid frame designed for it. Disposable pleated and fiberglass filters cannot be washed; wet media grows mold and the pleats collapse. If you want a washable filter, buy one sold as washable, rinse it per the instructions, and let it dry completely before it goes back in.

Is a more expensive filter better?

Only up to what your system can handle. A high-MERV filter in a 1-inch slot can starve the blower of air and cost more in strain than it saves in dust. For most homes, a MERV 8 to 11 pleated filter changed on time beats a MERV 13 that restricts airflow. The filter guide explains the ratings.

My filter slot has no cover. Is that a problem?

Yes. An open slot lets the blower pull unfiltered air from the basement or closet around the filter, and dust goes straight into the system. A magnetic or foil-taped filter slot cover from the hardware store fixes it for a few dollars.

The Bottom Line

System off, find the slot, read the arrow and the nominal size off the old filter, vacuum the slot, new filter in with the arrow toward the furnace, date on the frame, cover seated, system on. Check it every month and replace it every one to three months. Two minutes a month is the cheapest thing you will ever do for the most expensive appliance in the house.

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