Carbon Monoxide and Your Furnace: What to Check Before Heating Season

I find them sitting on top of the furnace. Battery pulled out, set down next to a coffee can of drywall screws, because at some point the thing chirped at two in the morning and somebody dealt with it the fast way.

That chirp was probably a low battery. Most of the time it is. But a carbon monoxide detector is the only device in your house whose whole job is to tell you about something you have no other way to notice, and a disabled one is worse than none at all, because you think you're covered.

So before the heat runs in earnest: where carbon monoxide actually comes from, where the detectors belong, the expiration date almost nobody knows about, how to tell CO symptoms from a bad cold, and what to do if an alarm sounds. Some of this is on your list. Some of it is on mine.

What Carbon Monoxide Is, and Why the Furnace Gets Blamed

Carbon monoxide is what you get when fuel burns incompletely. Anything in your house that burns something makes a little of it: the furnace, the water heater, a boiler, the gas range, a fireplace, a gas dryer, the car in the garage. Normally that's fine — the combustion is clean and the exhaust goes up a flue and outside.

Two things break that arrangement.

The combustion goes bad, and the appliance produces far more CO than it should. Dirty or misaligned burners, wrong gas pressure, a restricted air supply — the flame stops burning clean.

The venting fails, and what the appliance produces stays in the house instead of leaving it. This is the more common one, and it has nothing to do with how well the furnace itself runs.

The furnace gets most of the attention because it's the biggest gas appliance in the house and it runs the most hours between October and April. It deserves the attention. It just isn't the only candidate.

The Cracked Heat Exchanger

This is the one people have heard of, and it earns the reputation. The heat exchanger is the metal chamber that keeps two things apart: the flame and its exhaust on one side, the air your blower pushes through the ductwork on the other. Those two streams are never supposed to meet. The exchanger heats up and cools down on every cycle, thousands of times a winter, for twenty-odd years, and metal that flexes that many times eventually fatigues.

When it cracks, the wall between those two sides has a hole in it. Combustion gases can get pulled into the air stream and distributed to every register in the house.

What makes this different from every other furnace problem is the silence. A failing blower motor screams at you. A bad igniter leaves you with no heat, which you notice immediately. A cracked heat exchanger does none of that. It lights, runs, makes heat, shuts off, and does it again an hour later, behaving exactly as it always has. There's nothing to smell, because carbon monoxide has no odor, and nothing to hear.

That's the whole argument for a light and a camera, which is part of what we check during a fall furnace tune-up. A crack is a shutdown, not a repair recommendation. If we find one the furnace gets red-tagged that day, and I'll show you the footage so you're not taking my word for it.

The Ones Nobody Thinks About

Look only at the furnace and you'll miss a fair share of the real problems.

A blocked or disconnected flue. Birds nest in vent terminations. Masonry chimneys shed tile and brick into their own liner. Older B-vent pipe works loose at a joint, and now the exhaust empties into the basement instead of climbing to the roof. I've walked into houses where a section of vent pipe had simply come apart and nobody had looked at that corner in years.

A backdrafting water heater. This one is the sleeper. An old-style atmospheric water heater vents by natural draft — hot exhaust rises, and that's the entire mechanism. It's a weak force. Now run a big range hood, a couple of bath fans, and a gas dryer in a house that's gotten new windows, new siding, and blown-in insulation since that water heater went in. Those fans have to pull replacement air from somewhere, and the easiest opening is down the flue. The draft reverses and exhaust spills into the room. Nothing is broken. The house just changed around it.

The attached garage. Warming a car up with the overhead door open still fills the garage, and CO moves through the shared wall and door into the house. Same story for a snowblower, and worse for a generator, which never belongs in a garage no matter how far the door is rolled up. Unvented kerosene and propane heaters in a workshop or three-season room belong on this list too.

Where the Detectors Go

The rule is simpler than people expect: one on every level including the basement, and one within hearing distance of each sleeping area. Then read the manual, because manufacturers differ on specifics and they've tested their own units.

What I'd add from the field:

• Don't mount it directly over the furnace or water heater. Appliances can put out a brief, harmless puff at startup, and a detector sitting on top of that will nuisance-alarm until somebody pulls the battery. Give it real distance from the equipment.
• Height matters less than you'd think. Carbon monoxide mixes with room air instead of layering at the ceiling like smoke. Follow the manufacturer's guidance and put it where you'll hear it.
• Skip the dead spots. Not in a bathroom, not beside the stove, not in a corner behind a door, not where a supply register blows across it.
• Combination smoke and CO units are fine. One less thing on the wall.
• Test them monthly. The test button checks the horn and the circuit, not the sensor's remaining life. That's a different problem, and it's next.

One more thing worth knowing: a standard detector alarms on levels that hurt you quickly, not on low concentrations you might breathe all winter. That's what it was designed to do. If your house has a history of draft problems, a low-level monitor that reads out an actual number is a reasonable upgrade — that's the kind of thing that catches a backdrafting water heater before it becomes an emergency.

The Expiration Date on the Back

Carbon monoxide detectors expire, and I've never once had a homeowner know that before I said it.

The sensing element has a service life, generally seven to ten years. When it's up, some models chirp an end-of-life signal that sounds a lot like a low battery — so people put a fresh battery in, the chirping stops for a while, and everyone assumes it's handled. Others just become unreliable without saying anything.

There's a manufacture date or a replace-by date on the back of the housing. Take yours down tonight and look. If there's no date on it anywhere, that alone tells you it's old enough to replace. A new one runs somewhere around twenty-five to sixty dollars, which makes it one of the few genuinely cheap things in this trade.

Telling It From the Flu

Carbon monoxide symptoms are dull and general, which is what makes them dangerous. Headache. Dizziness. Nausea. A heavy, foggy tiredness. Trouble concentrating on something ordinary. In heavier exposures, vomiting, confusion, loss of consciousness.

Read that list in November and it's the flu. Or a cold. Or a rough week.

Three things separate them, and they're worth committing to memory:

• Usually no fever. Flu-like symptoms without a temperature deserve a second thought.
• More than one person at once. Everybody feeling lousy the same week, pets included — a dog that's suddenly lethargic is a real signal, since animals are smaller and show it sooner.
• It gets better when you leave. This is the one that matters most. If the headache clears an hour into the workday and comes back after dinner, that pattern is telling you something specific about the house.

If that describes your last few weeks, get outside and call for medical help before you call a heating company. Blood work can confirm CO exposure, and it's not a diagnosis to make from an article.

What We Can Check That You Can't

Your half of this is short: working detectors on every level, within their date, tested monthly. Keep storage away from the furnace and water heater. After a heavy lake-effect snow, walk the house and make sure the sidewall vent pipes aren't buried in a drift — thirty seconds, and it's one of the more common ways a modern furnace ends up shutting down.

Our side needs instruments and the cover off. We inspect the heat exchanger with a light and a camera. We put a combustion analyzer on it to measure what the burners are producing instead of eyeballing the flame. We run a draft test and check for spillage at the draft hood with the exhaust fans running, which is exactly how you catch a backdrafting water heater. We verify manifold gas pressure against the data plate, because overfiring stresses the exchanger. And we look at furnace, water heater, and venting together, because in a lot of these houses they share a chimney and they interact.

I was in a basement in Georgetown Township not long ago on a furnace repair call — no heat, failed igniter, twenty-minute fix. While I was down there I ran a draft test on the water heater beside it, more out of habit than suspicion, and it spilled. Badly. The homeowner had put in a new range hood over the summer. The furnace they called about was fine. The appliance nobody was thinking about was the problem, and there was no detector on that level of the house.

That's most of what nearly forty years in West Michigan basements has taught me about this: the thing that gets you is rarely the thing you were worried about. Which is the honest reason I'd rather do a furnace maintenance visit in September than a diagnostic in January. Not because a tune-up is magic, but because it's the only time anyone looks at the parts of the house that don't announce themselves.

And if what's really on your mind is that scorched smell on the first cold night, that one has its own answer. It's usually just dust.