How to Inspect Your Furnace Before Heating Season
Safety Warning
CRITICAL SAFETY HAZARD: Furnaces produce carbon monoxide (CO), an odorless, colorless poison that can be fatal. Improper furnace operation, cracked heat exchangers, or blocked ventilation can leak CO into your home. Never attempt to repair furnace internals, disassemble the burner assembly, or diagnose combustion issues yourself. If you smell gas, suspect a leak, or find visible cracks in the furnace cabinet, evacuate immediately and call your gas utility's emergency line (usually 911 in the US). A licensed HVAC technician MUST perform annual professional inspections including: combustion analysis, heat exchanger inspection, flue gas temperature testing, and safety limit checks. Do not delay this professional service—it costs $125-200 and can save your life.
A thorough furnace inspection prevents dangerous carbon monoxide leaks, ensures efficient heating, and helps you avoid costly mid-winter breakdowns when temperatures drop. While some basic checks can be performed by homeowners, a professional inspection is strongly recommended annually to safely evaluate combustion, ventilation, and electrical components. This guide covers inspection steps you can take, but we'll explain when to call a professional for the critical safety checks.
What You'll Need
Supplies
- •Furnace Air Filter (size: 16x25x1, 20x25x1, or your furnace size)($15-30)
- •Duct tape or foil tape (for sealing ductwork gaps)(optional)($5-10)
- •Furnace vacuum filter bags (if using shop vacuum)(optional)($10-15)
- •Pipe leak detector solution (soapy water alternative)(optional)(null)
Tools
- •Flashlight or headlamp (for inspecting dark areas)
- •Screwdriver set (Phillips and flathead for blower cover screws)
- •Shop vacuum with soft brush attachment (for blower cleaning)
- •Non-contact voltage tester (to confirm power is OFF before opening furnace)
- •Wrench set or adjustable wrench (for gas connection inspection)(optional)
- •Soap and small spray bottle (for leak testing gas connections)(optional)
Step-by-Step Instructions
Turn Off Power and Let the Furnace Cool
Switch off the furnace at the thermostat and the electrical disconnect switch (usually a switch on or near the furnace unit). Wait at least 30 minutes for all components to cool completely before touching any part of the furnace. This prevents electrical shock and burns from hot surfaces.
Pro Tip: Take a photo of the electrical disconnect switch location for future reference; it should be clearly labeled and easily accessible.
Visually Inspect the Furnace Cabinet and Connections
Examine the outside of the furnace cabinet for any visible cracks, rust, holes, or separation at seams. Check all ductwork connections and gas line connections for gaps, loose fittings, or signs of corrosion. Look for any debris, dust buildup, or obstructions around the furnace unit, especially near the air intake and exhaust vents.
Pro Tip: Use a flashlight to inspect hard-to-see areas like the bottom and back of the furnace; document any damage with photos.
Replace or Clean the Air Filter
Locate the air filter (typically a 1-2 inch thick pleated filter in a slot on the furnace cabinet or return air duct). If it appears gray, brown, or heavily soiled, replace it with a new filter of the same size and MERV rating (usually MERV 8-11 for most homes). If only lightly dusty, you can vacuum it gently using a soft brush attachment, but replacement is preferred.
Pro Tip: Check the filter every month during heating season; a clogged filter reduces efficiency and can cause the furnace to overheat and shut down.
Inspect the Blower Compartment and Motor
Remove the blower compartment cover (typically held by a few screws) and visually inspect for dust, debris, and lint accumulation on the blower wheel and motor. Use a vacuum with a soft brush attachment to carefully remove dust from the blower wheel and motor exterior—do not touch the motor bearings or windings. Ensure the blower wheel spins freely by gently rotating it by hand (only after confirming power is OFF).
Pro Tip: Excessive dust buildup restricts airflow and reduces heating efficiency; clean this area annually before heating season.
Check the Vent System and Exhaust
Locate where the furnace exhaust pipe (flue) exits your home—typically through the roof, wall, or basement ceiling. Inspect the pipe from inside for any visible cracks, gaps, corrosion, or signs of moisture/condensation (which indicates incomplete combustion). Look outside where the exhaust exits for blockages, animal nests, or debris accumulation that could restrict airflow.
Pro Tip: A blocked or leaking exhaust pipe is a serious CO risk; if you find any obstruction or damage, contact an HVAC professional immediately.
Test the Thermostat and Safety Controls
Set your thermostat to 'Heat' mode and raise the temperature setting 5 degrees above the current room temperature. The furnace should ignite within 30 seconds and the blower should start within 2-3 minutes of flame ignition. Listen for any unusual sounds (loud banging, squealing, or grinding) and note any burning or chemical smells, which could indicate a safety issue.
Pro Tip: If the furnace doesn't ignite, the blower doesn't start, or you hear/smell anything abnormal, turn off the furnace and call a professional—do not attempt troubleshooting.
Inspect Gas Connections and Pilot Light
If your furnace has a pilot light (older models), locate the sight glass and confirm the pilot flame is present and blue (not yellow or orange, which indicates incomplete combustion). Examine all visible gas line connections for signs of leaking (soap bubble test: apply soapy water to connections; bubbles indicate a leak). Do NOT attempt to adjust the pilot or gas valves—this requires professional tools and training.
Pro Tip: A yellow or flickering pilot light, or any gas smell, requires immediate professional inspection; evacuate if you smell strong gas.
Schedule a Professional Inspection and Document Findings
Contact a licensed HVAC technician to schedule your annual professional inspection before heating season (September-October in most regions). Provide your technician with notes on any visual issues, unusual sounds, or smells you observed. Professional inspection includes combustion analysis, heat exchanger cracks detection, electrical safety checks, and flue gas temperature verification—services homeowners cannot safely perform.
Pro Tip: Keep records of all inspections and maintenance; this protects your home warranty and helps technicians track furnace performance over time.
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