Signs of a cracked heat exchanger in Illinois

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A cracked heat exchanger usually shows up as a smell, not a sound. You might notice something like formaldehyde when the furnace kicks on, or soot building up around the burners. Some people notice headaches or nausea in rooms near the furnace, or a carbon monoxide alarm going off. You might also see the flame flicker or lean a certain way when the blower runs, instead of burning straight. The crack forms from years of the metal heating and cooling, and once it's there, exhaust can mix into the air your furnace pushes through the house. With gas furnaces running hard across Illinois winters, this isn't something to wait out. A furnace with a cracked heat exchanger needs to be shut off and looked at, because the risk isn't comfort, it's what's leaking into the air you're breathing.
The exchanger is a sealed metal chamber that keeps combustion gas separate from the air your furnace blows through the house. A crack breaks that seal.
Once it's cracked, the air moving through your ducts can pick up carbon monoxide on its way to your rooms. That's why a working carbon monoxide alarm near the furnace matters as much as any smell you notice.
Furnace runs, nobody's sure
Waiting means more hours of a cracked exchanger pushing combustion gas into the air your ducts carry through the house.
Skip calling around
Checked by someone licensed
A contractor can test the exchanger and the carbon monoxide level directly instead of leaving it to a guess based on smell alone.
Skip calling around
You're down in the basement around six, after dark already, getting the bin of Christmas decorations out of the way so you can get to the water heater. The furnace sits there and doesn't kick on. You notice because the room feels the same temperature as the hallway upstairs, which it shouldn't in November. There's a thin haze hanging near the burner compartment, catching the light from your phone when you crouch down to look. No smell you can place, just the dust. You stand there a second, deciding whether this is something to deal with tonight or something that can wait until morning, and whether it's worth pulling the cover off yourself to look for the crack you read about.
Replace the exchanger alone
A contractor can sometimes swap in a new heat exchanger and leave the rest of the furnace alone. This works best on a newer furnace, one still well inside its expected life, where the cabinet, blower and controls are all in good shape. The part has to match the furnace model, and fitting it means taking the cabinet apart around the burners. It's real labor, but it avoids paying for a whole new furnace when only one part failed. If your furnace is young enough that this is on the table, a contractor can tell you after looking at it directly.
Replace the whole furnace
Once a furnace is old, a cracked exchanger is usually the sign to stop repairing it rather than start. Heat exchangers crack from years of metal heating and cooling with every cycle, and a furnace that's been doing that for a long time has other parts worth the same age. Illinois winters put a lot of cycles on a furnace between October and April, and a new one comes with a warranty covering the exchanger again from zero. A contractor who's looked at the furnace can tell you whether it's closer to the start of that wear or the end of it.
Burners fire and heat the metal walls of the exchanger.
Blower pushes house air across the outside of that metal.
A crack lets combustion gas leak into that air stream.
Your ducts carry the leaked gas through the house.
Old furnace, pushed hard
If your furnace came with the house and it's old enough that nobody's sure of its age, a crack is worth taking seriously the first time you smell something off. An exchanger that's been through years of Illinois winters, heating on and off from October through March, has had a lot of cycles to work the metal until it gives. Once it's cracked, it doesn't heal, and a furnace that age is also closer to other failures that make replacement the more sensible call next to a repair.
Newer furnace, one bad part
A furnace that's only a few years old can still crack an exchanger, usually from a defect or a install issue rather than age. If the cabinet, blower and controls are otherwise in good shape, a contractor can often replace just the exchanger and leave the rest alone. That keeps a furnace that still has most of its life left from being scrapped over one failed part. The smell and the soot mean the same thing either way. What differs is how much furnace is still worth saving around that crack.
Winter nights average 20°F statewide, and that's the stretch where a furnace runs longest, heating metal walls for hours at a time instead of cycling on and off. A heat exchanger that's already cracked gets worked hardest on nights like that, and whatever's leaking into the air stream leaks longer. You might not notice anything different about how the house feels. The furnace still runs, the rooms still warm up. If you've caught a smell or a symptom that lines up with what a cracked exchanger does, getting it looked at means you're not the one calling around trying to find a contractor who can come out and test it. Send the form once and that search is off your plate. Source: NOAA average winter low temperatures, Illinois
NOAA nClimDiv 1991-2020 county normals weighted by homes
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Furnace running clean
No smell when it kicks on. No soot around the burners, no rust creeping across the cabinet. Air moving through the ducts is just air. This is what a sealed exchanger looks like from the basement floor, nothing to notice, which is the point.
Furnace showing its age
Rust along the cabinet, a furnace old enough nobody's sure of the year. These are the ones worth a closer look, especially if a smell has shown up when it runs. Surface rust alone isn't a cracked exchanger, but it's reason enough to have someone check.
Before you get your furnace repair
After you get your furnace repair
Deal With It Tonight, or Later
Smell it, ignore it
The furnace keeps running the same as always, so there's nothing forcing you to stop and check it out.
Whatever's leaking into the air keeps going through the ducts every time the burners fire, all winter long.
Soot or rust around the burners gets worse the longer a crack sits there unattended.
Headaches or a stuffy feeling near the furnace room get written off as a cold or dry winter air.
You're still the one who has to notice the smell again, remember it, and decide tonight's the night to deal with it.
Smell it, call it in
A contractor can test the exchanger and check carbon monoxide levels directly instead of you guessing from a smell.
You skip the part where you search for who repairs furnaces in Illinois and read through reviews at ten at night.
No calling three different companies and explaining the same smell and the same basement three different times.
Once someone licensed looks at it, you know whether it's a repair or a replacement instead of wondering.
The furnace gets dealt with before another cold night asks it to run for hours at a stretch.
Why does my furnace smell different when it kicks on?
Walk by the basement and notice it before you can name it. A smell like formaldehyde right when the burners fire, gone a few minutes later. Soot collecting around the burner compartment that wasn't there last time you looked. Rust spreading across the cabinet near the bottom. Maybe the pilot or burner flame looks yellow instead of blue, or flickers when it should hold steady. Upstairs, anyone who spends time in the rooms over the furnace might get headaches or feel sick to their stomach for no clear reason, especially with the furnace running for hours on a cold night. None of this is loud. The furnace keeps heating the house the same as always, which is exactly what makes it easy to notice once and then talk yourself out of.
A furnace like this one runs quiet and sealed, no smell when the burners fire and nothing collecting around them.
An older furnace like this one, run for years, is the one worth checking first if something smells off near the vents.
Why does a cracked heat exchanger only show up when it's cold out?
You've had the furnace running for weeks now with no trouble, but the smell you noticed on the first cold snap hasn't come back since. It's tempting to think it was nothing, maybe dust burning off, and move on. But a furnace with a cracked exchanger doesn't act the same way every time it fires, and that's worth understanding before you write it off.
Metal expands when it's hot
The crack in the exchanger opens and closes with the metal around it. When the burners fire and the walls heat up, the metal expands and the crack can widen enough to leak. On a mild day, the furnace might only run a few minutes at a time, not long enough to heat the metal all the way or open the crack much. On a cold night, when it runs long and hard to keep up, the metal gets hotter and stays hot longer, and the crack opens wider for more of that time. That's why the smell shows up on the coldest nights and seems to disappear in between. The furnace isn't fixing itself. It's just not working hard enough to show you.
Winter nights average 20°F statewide
At 20°F, your furnace isn't cycling, it's running long stretches to keep up. A crack in the exchanger gets hours of heat instead of minutes, and more of whatever's leaking has time to reach your ducts before the burners shut off.
NOAA nClimDiv 1991-2020 county normals weighted by homes
A smell from the furnace is worth checking, not waiting out
A cracked heat exchanger comes down to this: a smell near the furnace, soot or rust where there shouldn't be any, or a carbon monoxide alarm going off, especially on the cold nights when the furnace runs longest. None of that is normal, and none of it goes away by itself. The crack stays until someone fixes it.
You don't have to figure out whether the smell was something or nothing, or sort through contractors and compare quotes while you wait and wonder. Send this and that part of it is off your plate. You'll have someone licensed to call about getting the exchanger looked at, tested, and either repaired or replaced, instead of a list of numbers you'd have to work through yourself on a January evening.
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Signs of a cracked heat exchanger in Illinois

Furnace runs, nobody's sure

Checked by someone licensed

Furnace running clean

Furnace showing its age

Smell it, ignore it

Smell it, call it in