What you're probably noticing
Maybe the furnace is still running fine, but you noticed a faint smell when it kicks on, something between formaldehyde and a wet match. Maybe someone in the house has had a string of headaches this winter that seem to clear up when they leave for work. Maybe the furnace just seems to be running longer than it used to, cycling on and off more often to keep the house at the same temperature. Any one of these on its own could be nothing. Together, especially in a furnace that's been through ten or fifteen Ozark winters, they're worth paying attention to.
A cracked heat exchanger is one of the few furnace problems that moves past a repair and into replace territory fast. It's also one that a lot of homeowners around Nixa don't think about until something forces the issue, usually a technician finding it during a routine check or a carbon monoxide detector going off at 2 a.m.
Why Nixa furnaces see this more than you'd expect
Southwest Missouri winters are a strange mix. You get stretches of mild, almost spring-like weeks, then a hard cold snap drops in off the plains and the furnace that's been loafing along suddenly has to work full tilt for three or four days straight. That kind of on-off, mild-to-brutal cycling puts more stress on a heat exchanger than steady, even cold would. The metal heats up, expands, cools, contracts, over and over, and over the years that's exactly the kind of movement that opens a crack along a seam or a weld.
A lot of the housing stock in and around Nixa, Ozark, and the rest of Christian County dates back to building booms in the 80s, 90s, and early 2000s. Furnaces installed in those homes are now sitting right in the range where heat exchangers start to fail, usually somewhere past the fifteen-year mark, sometimes earlier if the system was undersized for the house or has run dirty for a long stretch.
More on this from Why A Cracked Heat Exchanger Ends A Furnace.
What you can actually check yourself
Start with the furnace filter. A clogged filter restricts airflow, which makes the heat exchanger run hotter than it's designed to, and heat stress is the main thing that cracks these things open over time. If you can't remember the last time you changed it, that's your answer. Check it now, and get in the habit of looking every month during the heating season, not just once in the fall.
Look at the flame through the furnace's viewing window if it has one. A steady blue flame is normal. A flame that flickers, dances, or turns yellow when the blower kicks on can be a sign that air is leaking into the burner chamber somewhere it shouldn't, which sometimes points toward a cracked exchanger.
Pay attention to soot around the furnace cabinet, any rust-colored streaking near the burners, or a sharp metallic smell specifically when the blower starts up. None of these confirm a crack by themselves, but they're the kind of thing worth mentioning to a technician rather than ignoring.
And check your carbon monoxide detector. Not just that it's there, but that the batteries are fresh and it's less than seven or eight years old, since the sensors inside them wear out. This is the cheapest insurance you have against the actual danger here.
Where this stops being a homeowner job
You cannot safely inspect a heat exchanger yourself. It sits inside the furnace cabinet, wrapped around the burners, and a hairline crack is often invisible without the furnace running and without specific tools, usually a camera scope or a chemical smoke test that shows whether combustion gases are escaping into the air stream instead of going up the flue.
If you've got any combination of the signs above, an older furnace, a strange smell, headaches that come and go with time spent in the house, that's the point to call for furnace repair in Nixa rather than keep watching it. A technician can run a combustion analysis and a proper inspection in under an hour, and that's the only way to actually know rather than guess.
What happens after a crack is confirmed
If a heat exchanger is cracked, the furnace gets shut off on the spot. There's no patch or seal for this part. It's not like a duct leak you can tape or a belt you can tighten. The exchanger is a sealed chamber separating the flame and exhaust gases from the air that blows through your vents, and once it's breached, carbon monoxide has a path into that air stream.
Replacing just the heat exchanger is possible on paper, but on a furnace old enough to have cracked in the first place, most people are better off putting that money toward a new unit instead of a part that costs a large fraction of one. That's the real reason this particular failure tends to end a furnace rather than just cost you a repair bill. It's less about the part itself and more about what kind of furnace is usually attached to it by the time the crack shows up.