How to Change a Furnace Filter

A clean furnace filter helps your heating and cooling system move air efficiently and keeps dust from circulating through your home. Swapping one takes about five minutes once you know where it lives and which size to buy.

Find the Filter Slot

Filters usually sit in one of a few places:

• In a slot on the side, bottom, or top of the furnace or air handler, where the return duct meets the unit.
• Behind a large return air grille on a wall or ceiling, held closed by clips or screws.
• In both places on some systems, so check your owner’s manual if you’re unsure.

Get the Right Size and Type

The dimensions are printed on the edge of the current filter, listed as length, width, and depth in inches, such as 16x25x1. Write them down or take a photo before you shop.

Filters are commonly rated on the MERV scale. Higher numbers trap smaller particles but can also restrict airflow on some systems. Many homes do well with a mid-range filter. Check your furnace manual, or ask an HVAC technician, if you’re not sure what your system can handle.

Swap the Filter Step by Step

1. Turn the system off at the thermostat so the blower doesn’t pull dust through while the slot is open.
2. Open the filter cover or grille and slide out the old filter. Note the arrow on its frame showing the airflow direction.
3. Check the slot for dust buildup and vacuum it if needed.
4. Slide the new filter in with the arrow pointing toward the furnace, the direction air travels from the return duct into the blower.
5. Close the cover securely so there are no gaps for unfiltered air to slip around.
6. Turn the system back on and listen for normal airflow. A whistling sound can mean the cover isn’t closed or the filter is the wrong size.

How Often to Replace It

The right interval depends on the filter’s depth and your household. As a general guide:

• Thin 1-inch filters often need replacing every one to three months.
• Thicker media filters, around 4 to 5 inches deep, can last six months or more.
• Homes with pets, allergies, smokers, or ongoing renovation dust may need more frequent changes.

The simplest test is to hold the filter up to a light. If you can barely see light through it, or it looks gray and caked with dust, it’s time for a new one. Don’t wait for a scheduled date if the filter is obviously dirty, and don’t run the system with no filter at all, since dust can then build up on internal parts.

Quick Tips

• Write the install date on the filter frame with a marker.
• Buy several filters at once so you always have a spare on hand.
• Set a recurring reminder on your phone.
• If your filter is the washable type, let it dry completely before reinstalling it.
• If the furnace makes unusual noises or cycles on and off frequently even with a clean filter, schedule a professional inspection.