In my 36 years fixing heating and air equipment, I’ve seen just about every way a furnace can fail. Most late-night service calls on the first freezing night of the year come down to small, preventable issues. Taking ten minutes to run through a quick visual checklist before flipping that thermostat to “Heat” will save you money, protect your system, and keep your home safe.
1. Inspect and Replace Your Air Filter
A clogged filter restricts airflow, which forces your blower motor to work twice as hard and can cause the high-limit switch to trip—shutting your furnace down to prevent overheating.
Slide out your filter and hold it up to a light source. If you can’t see light passing through, throw it away.
Install a fresh filter, making sure the directional arrow printed on the frame points toward the furnace blower (the direction of airflow).
2. Clear the Perimeter Around the Unit
Furnaces often sit in basements, utility closets, or garages—places that accumulate storage boxes, paint cans, and seasonal gear over the summer.
Maintain a 3-foot clearance: Never store combustible materials, chemicals, or cardboard near the unit.
Check the intake and exhaust pipes: For modern high-efficiency units PVC-vented through the sidewall, clear away leaves, rodent nests, or debris blocking the outside pipes.
3. Open and Unblock All Supply and Return Vents
A common misconception is that closing vents in unused rooms saves heating costs. In reality, modern forced-air systems are balanced for specific static pressure; blocking vents causes pressure build-up, heat heat-exchanger stress, and uneven heating.
Walk room to room and make sure all supply registers are fully open.
Verify that area rugs, drapes, or furniture aren’t covering main return air grilles.
4. Test Your Thermostat and Battery Health
Before relying on the furnace control board, test the wall unit giving the commands.
Replace the thermostat batteries annually—a dying battery can cause erratic cycling or false error codes.
Switch the mode to Heat and set the target temperature 3 to 5 degrees above the current room reading. Listen for a soft click signaling the system to initiate its start sequence.
5. Check Safety Detectors and Test Run
Gas furnaces produce carbon monoxide (CO) during combustion, which is safely vented outdoors under normal operating conditions. However, cracked heat exchangers or blocked flues present serious safety hazards.
Test all smoke alarms and carbon monoxide detectors in the home, replacing backup batteries.
Run the furnace for 15 minutes. A faint, dusty smell is normal during the initial startup as heat burns off summer dust. However, if you smell sulfur, burning plastic, or continuous heavy smoke, shut the system off at the main breaker immediately and call a technician.
Know When to Call a Pro
Running through these five steps prevents the vast majority of early-season breakdowns. However, internal components like the flame sensor, igniter, gas valve, and heat exchanger require specialized tools and training to test safely. Schedule a professional fall tune-up to keep your system operating efficiently all winter long.