Carbon monoxide is the one furnace topic where both extremes are dangerous: ignoring it entirely, and being scared into a purchase by it. Here's what's actually true — how the risk works, how real testing proves it either way, and how to protect your household.
A gas furnace burns fuel inside a sealed metal chamber called the heat exchanger. Your household air passes over the outside of that chamber, picks up heat, and never touches the combustion gases inside — which vent safely out the flue. Carbon monoxide risk appears when that separation fails: a cracked or corroded heat exchanger can let combustion byproducts mix into the air your blower is pushing through every room. Age, metal fatigue from thousands of heat cycles, and airflow problems that overheat the chamber (a clogged filter is the classic cause) are what create cracks. This is why filter changes are genuinely a safety habit, not just an efficiency one — and why furnaces past 15-20 years deserve annual inspection even when they seem to run fine.
A proper inspection uses evidence you can see: camera inspection of the exchanger surfaces, combustion analysis measuring what's actually in the flue gases, and CO measurement in the supply air — the air reaching your rooms. When we test, we show you the readings and the images either way. That matters because "cracked heat exchanger" is also the industry's most abused scare tactic — a diagnosis delivered verbally, without evidence, minutes before a replacement quote. Our standard is simple: if a technician condemns your furnace, you should be looking at the crack. If you can't see evidence, get a second opinion before spending thousands. We'll say that even knowing it costs us the occasional sale, because it's true.
CO detectors on every level and outside every sleeping area, tested at the start of furnace season, and replaced by manufacture date (5-10 years, printed on the back — the test button confirms the alarm works, not the sensor). Fresh filters through the heating months. Clearance around the furnace, because combustion equipment needs combustion air. And a real annual inspection on aging equipment. If a detector ever alarms: get everyone out, then call 911 or the gas company from outside. If you smell gas — before anything else, before calling us — leave and call the gas utility.
Headache, dizziness, nausea, fatigue, and confusion — easily mistaken for the flu, except it improves when you leave the house and returns when you come back. If multiple household members feel ill at the same time in the same rooms, take it seriously: get everyone outside and call 911 or your gas utility.
Not automatically — it means the barrier between combustion gases and your breathing air is compromised, which is a real risk that warrants action. What it doesn't justify is panic-buying a furnace on the spot. A proper diagnosis includes showing you the evidence: camera images of the crack, CO measurements, the actual condition. Anyone who condemns a furnace without showing evidence is selling, not diagnosing.
Every 5-10 years depending on the model — the sensor degrades even if the test button still beeps. Check the manufacture date printed on the back. Every sleeping area and every level of the home should have one, and furnace season is the time to verify.
That depends entirely on what testing shows — which is exactly why the testing matters. A furnace with a verified sound heat exchanger and passing CO readings can run safely while you plan. One with a confirmed breach shouldn't. We'll tell you honestly which situation you're in, with the evidence to back it.
Getting ready for the season? See the Heating Season Readiness checklist. Furnace acting up now? Start with Furnace Not Turning On or book a furnace diagnostic — CO and safety testing included where warranted.