How to Inspect Your Furnace for Cracks and Carbon Monoxide Leaks

Your furnace's heat exchanger is the barrier between combustion gases and the air that circulates through your home. When it cracks, carbon monoxide—a colorless, odorless poison—can seep into your living spaces. Most homeowners never look inside their furnace, which is exactly why cracks go undetected until someone gets sick. This inspection takes an hour and requires nothing more than a flashlight, a mirror, and a battery-powered CO detector. Done right, it gives you real confidence that your heating system isn't a silent threat. The goal here isn't to become an HVAC technician. It's to spot the obvious red flags: visible fractures in the metal, rust bloom patterns that suggest heat stress, or a CO detector that registers anything above zero near the furnace seams. You'll learn where to look, what to watch for, and exactly when to stop and call a pro.

  1. Turn off the furnace and let it cool completely. Flip the furnace power switch to OFF at the unit itself, then switch off the breaker at the electrical panel. Wait at least 30 minutes for the heat exchanger to cool enough to touch safely. A hot furnace can cause burns and will also make cracks harder to see because thermal stress can close them temporarily.
  2. Remove the furnace access panels. Most furnaces have one or two removable panels held by sheet-metal screws or clips. Locate them on the front or side of the unit. Unscrew or unclip them carefully and set them aside. You're looking for access to the combustion chamber and heat exchanger—the large metal drum or tube bundle inside.
  3. Inspect the heat exchanger visually under bright light. Use a flashlight and a small mirror to look at every surface of the heat exchanger—the top, bottom, sides, and seams. Look for hairline cracks, corrosion pitting, or visible gaps where two pieces of metal meet. Pay special attention to the sides and bottom where condensation collects and corrosion starts. Soot buildup or discoloration in a line pattern often indicates a crack that's been leaking gases.
  4. Check the combustion chamber and burners for cracks. The combustion chamber is the enclosed space where gas ignites. Look at the chamber walls, the floor, and especially where the burners attach. Cracks here are less common than in the heat exchanger, but they happen. Use your mirror to see the back wall if it's not fully visible from the front. Any visible break in the metal, no matter how small, is a red flag.
  5. Test around seams and connections with a CO detector. Turn the furnace back on and let it run for 5 minutes until it reaches operating temperature. Then, with your CO detector in hand, slowly move it around all visible seams, connections, and the flue pipe exit. Keep the detector within 2 inches of each joint. Any reading above 35 ppm (parts per million) at a seam indicates a leak. Most modern CO detectors will alarm at 70 ppm or higher.
  6. Check the draft and flue pipe for backdrafting. While the furnace is running, light a match or use a smoke pencil near the flue pipe opening and the draft hood (the metal collar where exhaust exits the furnace). The smoke should be drawn up and out consistently. If it hesitates, wavers, or blows back into the furnace area, you have a drafting problem—a sign the flue may be blocked or the furnace shell may have cracks allowing pressure imbalances.
  7. Document findings and shut down if needed. If you found visible cracks, soot patterns around seams, or CO readings above 35 ppm, turn off the furnace immediately and do not use it. If the draft test showed backdrafting or hesitation, also shut it down. Call an HVAC technician the same day. If everything looks clean and CO is zero, reassemble the panels and document the date of your inspection.