Carbon Monoxide Alarm Going Off in Winter With a Gas Furnace?

Your CO alarm is going off. Do this first.

Leave the house. Everyone, including pets, into fresh air. Do that before you read the rest of this.

Then call 911 or your gas utility's emergency line from outside. Most utilities send someone within an hour and it's usually free for a CO or gas call.

A carbon monoxide alarm on a gas furnace in winter is an emergency, full stop. CO is a byproduct of burning gas, and a furnace that's venting it into your living space can be lethal in a few hours.

You don't need to figure out the cause right now. You need to get out and get help. The diagnosis happens after the air is clear and a pro has a meter on it.

Why this happens in winter specifically

Your furnace works hardest on the coldest nights. Long run cycles mean more combustion, more exhaust, and more chances for a small venting problem to become a dangerous one.

Cold also seals up houses. Windows stay shut, and modern homes are tight. CO that would have leaked away in October now builds up.

Snow and ice do their part too. A sidewall vent (common on 90%+ high-efficiency furnaces) can pack over with snow and choke off the exhaust path.

And heat exchanger cracks behave worse when it's cold. The metal expands more from a hard cold start, which can spread a crack that stayed hidden in mild weather.

What's actually causing it

Four things cause the overwhelming majority of furnace-related CO alarms, and a fifth that's easy to overlook.

The classic is a cracked heat exchanger. That's the metal chamber that keeps combustion gases away from your breathing air. When it cracks, CO rides your ductwork into every room. It's the most common serious cause on furnaces past 15 years.

Next is a blocked or disconnected flue. If the exhaust can't get out, it comes back in. Snow, a bird nest, a fallen vent cap, or a rusted-apart vent connector all do this.

Bad combustion is third. Dirty burners or wrong gas pressure make an incomplete, CO-heavy flame. You'll often see it as lazy yellow flames instead of tight blue ones.

Don't forget the water heater. If it shares a chimney with the furnace, or a big exhaust fan is depressurizing the house, either appliance can backdraft and set off the alarm.

Last, and least likely, the alarm itself is expired. CO sensors die at 7 to 10 years. But you never assume that until the house is confirmed clean with a meter.

The full sequence, step by step

Once you're outside and safe, here's the order that keeps you out of trouble.

Get everyone out first. Air is your medicine for CO, and you want it before you do anything else.

Call for help. 911 or the gas utility. Let them clear the house with their equipment.

After they say it's safe, shut the furnace off at the wall switch or breaker and leave it off. This is the single most important thing you can control.

Air the place out. Open doors and windows on opposite ends for cross-flow.

Write down what the tech will want: furnace age, model number off the data plate, flame color if you saw it, any soot or rust, and whether a water heater shares the flue.

Book a licensed HVAC tech for a combustion analysis and heat-exchanger inspection. Don't relight until they clear it.

• Everyone and every pet outside
• Call 911 or the gas utility
• Furnace off at the switch or breaker
• Ventilate with cross-flow
• Gather furnace details for the tech
• Schedule a licensed inspection

What this costs in 2026

The diagnostic visit itself runs about $150–$700. That covers the tech's time, a combustion analyzer reading, and a heat-exchanger inspection (sometimes with a camera scope).

If it's a blocked flue or a rusted vent connector, you're often in the $150 to $500 range for the fix, depending on parts and access.

A cracked heat exchanger is the expensive one. The part plus labor typically lands between $1,500 and $3,000, and it's a big job.

Here's the honest math: on a furnace older than about 15 years, most techs will tell you to replace the whole unit for $4,500 to $8,500 rather than sink $2,500 into an old cracked one. A new heat exchanger in a tired furnace is money you'll spend again soon.

The DIY side is small: a new CO alarm or two ($25 to $50 each), and clearing snow off a vent termination (free).

What you can do versus what needs a license

Your safe lane is short and that's on purpose. Combustion equipment isn't a DIY playground.

You can shut the furnace off, air out the house, check for snow on an outside vent, replace expired CO alarms, and gather info for the tech. That's it.

Everything past that (heat exchanger inspection, combustion analysis, gas pressure, burner cleaning, flue repair) needs a licensed HVAC tech. It's not just skill, it's the meter. You cannot eyeball CO.

A crack can be invisible cold and wide open hot. A pro tests under load with an analyzer, which you don't have and shouldn't fake.

Can this wait? No.

This is one of the few home problems where waiting can kill you. There's no version of "I'll deal with it after the weekend."

The most dangerous scenario is the alarm that stops on its own. The furnace cycled off, or the house aired out, and it goes quiet. People go back to bed. Then it runs again at 3 a. m.

If the alarm sounded once for a real reason, the source is still there. Keep the furnace off and keep the alarm powered.

The only thing you're allowed to "wait" for is the tech's arrival, with the furnace off and the house ventilated in the meantime.

Older furnaces and older houses

If your furnace is an 80% AFUE unit venting into a masonry chimney (common in homes built before the mid-2000s), pay extra attention. Those setups are more prone to vent connector rust-out and backdrafting.

Furnaces installed in the 1990s and early 2000s are now hitting the 20 to 30 year mark, right where heat exchangers fail.

Tight, newer homes have the opposite problem. They're so well sealed that a small CO leak or a backdrafting appliance builds up faster because there's less natural air exchange.

Either way, the fix is the same: get it inspected and don't guess based on the house's age.

Safety rules that aren't optional

CO doesn't announce itself. No smell, no color, no taste. Your alarm is the only warning you get, so treat it like the smoke it can't produce.

Never go back to sleep on an unresolved alarm. Overnight is when most fatal exposures happen.

Don't run the oven, a grill, or an unvented space heater to stay warm while the furnace is off. Those add CO, not fix it.

Keep at least one CO alarm on every floor and within about 15 feet of each bedroom. Test them monthly and replace them at 7 to 10 years.

How a tech actually diagnoses this

So you know what a good visit looks like: the tech starts with a combustion analyzer in the flue while the furnace reading CO in parts per million.

They'll inspect the heat exchanger, often with a mirror and light or an inspection camera, looking for cracks, rust, and separation at the welds.

They check the flue and vent connector end to end for blockage, disconnection, proper slope, and rust-through.

They confirm gas pressure and burner condition, and look at flame color and shape for signs of incomplete combustion.

If a water heater shares the vent, a thorough tech runs a worst-case depressurization test with exhaust fans on to catch backdrafting.

A cracked heat exchanger or a failed combustion test means the furnace gets red-tagged and stays off. That's the system working, not overkill.

The short version

Out of the house, call for help, furnace off, don't relight until a pro clears it. That's the whole plan.

The likely causes are a cracked heat exchanger, a blocked or disconnected flue, or bad combustion, all of which a licensed tech confirms with a meter.

Budget $150–$700 for the diagnostic, and be ready for a replacement conversation if the furnace is older than 15 years.

The one thing that matters most: don't talk yourself out of the alarm. It's cheap to be wrong about a false alarm and catastrophic to be wrong about a real one.