How often should you replace your furnace filter?

A dirty furnace filter restricts airflow and can crack a heat exchanger.

A restricted filter makes the heat exchanger run hotter than designed. On a gas furnace that trips the high-limit switch, so the burner shuts down and relights repeatedly. That short-cycling is hard on the ignitor and, over years, on the heat exchanger itself, which is the one part whose failure matters for carbon monoxide.

1. Set the thermostat to Off. Not just to a lower temperature. You want the blower still while the slot is open.
2. Open the filter access. Usually a slot beside the blower compartment, sometimes behind the removable front panel of the furnace.
3. Check the arrow before removing. Note which way it points relative to the furnace. Fitting the replacement backwards is the most common error.
4. Inspect what came out. Ordinary grey dust is normal. Soot, or a burnt smell, is worth a technician visit.
5. Fit and date the new one. Arrow toward the blower, size matched exactly, date written on the edge.

• The furnace starts and stops every few minutes instead of running a full cycle
• Weak airflow from registers with the blower clearly running
• A burning dust smell that does not clear after the first cold snap

• Assuming the summer schedule carries over: a heating season in a cold climate loads a filter faster than a cooling season does
• Leaving the old filter in through a mild autumn and starting the heating season already restricted
• Ignoring a filter that comes out sooty rather than dusty, which is a combustion symptom and not a filter problem

• A long, cold heating season shortens the interval considerably
• Basement installations in dusty or unfinished spaces load faster
• A whole-home humidifier on the system adds mineral dust

Soot on the filter, a yellow rather than blue burner flame, or repeated high-limit shutdowns all need a heating technician before the next cold night. Do not keep resetting it.