How Often to Change Your Furnace Filter (and How)

The furnace filter protects the blower and heat exchanger from dust, and it is the single most common cause of furnace trouble. A clogged filter starves the furnace of air, so it overheats, trips its high-limit safety switch, and shuts off. The result is short cycling, cold rooms, higher gas bills and extra wear on the blower motor. Changing the filter takes five minutes and costs less than any service call.

Check a standard 1-inch filter every month during the heating and cooling seasons and replace it every 1 to 3 months. Replace it more often if you have pets, allergies, ongoing renovation dust, or during long cold snaps when the furnace runs almost constantly. Thicker 4- to 5-inch media filters usually last 6 to 12 months; follow the manufacturer's schedule. A simple test: hold the filter up to a light. If you can barely see light through it, replace it.

The size is printed on the frame of the existing filter, for example 16x25x1. Buy the same size. For most homes a MERV 8 to 11 pleated filter is a good balance between catching dust and allowing enough airflow. Very high ratings (MERV 13 and up) in a thin 1-inch filter can restrict airflow on some furnaces, so check your furnace manual before upgrading. Basic 1-inch filters cost about CAD $8–$18, higher-rated pleated filters CAD $15–$30, and media filters CAD $30–$80.

1. Set the thermostat to OFF, or turn off the furnace service switch, so the blower is not running.
2. Find the filter slot. It is usually between the return air duct and the furnace, on the side or bottom of the blower compartment, behind a small cover. Some homes have the filter behind a return air grille in a wall or ceiling instead.
3. Note the airflow arrow printed on the old filter, then slide it out.
4. Check the size on the frame and get a matching filter.
5. Slide the new filter in with the arrow pointing toward the furnace, which is the direction the air flows (away from the return duct).
6. Close the cover fully and turn the system back on.
7. Write the date on the new filter's frame or set a phone reminder for next month's check.

The furnace runs in short bursts and shuts off, airflow from the vents is weak, the filter is grey and matted, there is more dust around the house than usual, or you hear whistling from the return side. On furnaces with a diagnostic light, a high-limit or overheating code is often caused by a dirty filter.

Call an HVAC technician if problems continue with a fresh filter, if a new filter gets very dirty within a couple of weeks (which can point to leaky return ducts), or if you see water around the furnace. A yearly fall tune-up checks the burners, flame sensor, heat exchanger and venting that a filter change cannot, and some manufacturer warranties require proof of regular maintenance.