Carbon Monoxide Alarms and Your Heating System

Carbon monoxide is colorless, odorless and produced by every fuel-burning appliance in the house when combustion goes wrong. You cannot smell it, and the symptoms it produces look like a winter virus. A working alarm and an annually inspected heating system are the two things that stand between a fault and a household, which is why a combustion check is part of every furnace repair and safety inspection we run.

What do you do when a carbon monoxide alarm sounds?

Leave the building, then call. That order matters — carbon monoxide impairs judgment before it impairs movement, so the decision to go should be made on hearing the alarm, not after investigating. A full-alarm pattern is not a low-battery chirp; a chirp every minute or so with no pattern is almost always a battery or an end-of-life signal.

1. Get everyone out, including pets, and take a head count outside.
2. Call 911 from outside, or from a neighbor's house. Tell them a carbon monoxide alarm is sounding and whether anyone has symptoms.
3. Do not go back in for phones, coats or the alarm itself.
4. Let responders ventilate and measure. They carry meters; a house that reads clear to an instrument is a different situation from one that does not.
5. Before anyone sleeps in the house again, have every fuel-burning appliance inspected — furnace, boiler, water heater, fireplace, range and any unvented heater.

Where should carbon monoxide alarms be installed?

Carbon monoxide alarms belong on every level of the house and within earshot of every sleeping area, because the alarm's job is to wake people. Beyond that, follow the instructions printed on the alarm itself and check current Minnesota and Wisconsin requirements, which differ from each other and change.

• Outside each sleeping area, and inside bedrooms where someone sleeps with the door shut.
• On every level, including a finished basement and a walkout level with a bedroom.
• Where you will hear it from bed — an alarm in a basement utility room three floors down protects nobody at 3am.
• Not directly above or beside a fuel-burning appliance, where normal start-up spikes cause nuisance alarms, and not in dead air behind a door or in a cabinet.
• Interconnected alarms, where one sounding sets them all off, are worth the difference in a tall old house with a stairwell.

Why do CO alarms expire, and how do you check yours?

A carbon monoxide alarm's sensor degrades with age, and every alarm carries a manufacturer end-of-life date — usually printed on the back, along with the date of manufacture. After that date the unit is no longer a CO detector, even though the test button still makes noise. The test button tests the horn and the circuit; it does not test the sensor's ability to detect gas.

Take each alarm off the wall this fall and read the back. If the date has passed, or there is no date because the alarm predates the practice, replace the unit outright. Replace batteries on the schedule the manufacturer states, and after a real alarm event follow the manufacturer's guidance on whether that unit should be replaced — heavy exposure can shorten a sensor's life.

Which heating faults actually produce carbon monoxide?

Carbon monoxide in a house comes from incomplete combustion or from exhaust that does not leave the building. In Twin Ports homes those two conditions have a short list of common causes, and cold weather makes most of them worse.

Common household carbon monoxide sources and what drives them

Source	What goes wrong	Season it shows up
Cracked heat exchanger	Flue gas leaks into the supply air stream	Deep cold, when the furnace runs hardest
Backdrafting water heater	Exhaust spills into the room instead of up the chimney	Cold snaps, and after a furnace upgrade
Blocked or iced vent termination	Exhaust cannot leave; combustion goes incomplete	Freezing drizzle, blowing snow, drifting
Attached garage	A vehicle or generator running with the door down	Any time, worst in winter warm-ups
Unvented space heater	Combustion products go directly into the room	Cold snaps and power outages
Blocked chimney	Nest, debris or collapsed liner obstructs the flue	First fire of the season

The backdraft case deserves particular attention in older Duluth and Superior houses. When a high-efficiency furnace is installed and the old water heater is left on a masonry chimney sized for two appliances, the water heater is now alone on an oversized, cold flue and can struggle to draft — the orphaned water heater problem that a good installer solves at the time of the furnace swap, not afterward.

What deep cold and blowing snow do to venting

Extreme cold and wind-driven snow are the local aggravating factor. A condensing furnace's PVC sidewall intake and exhaust sit low on the wall, and freezing drizzle, drifting, or a roof avalanche can close them off — the situation described in what to do when snow or ice blocks a furnace vent. Most of the time the pressure switch catches it and the furnace refuses to fire, which is the safety system working. The dangerous version is a partly blocked natural-draft chimney on an older 80% furnace or a water heater, where the appliance keeps burning and the exhaust finds another way out.

Out in the townships this matters more, not less. In Barnum and Duquette a lot of heat comes from propane appliances in older buildings with masonry chimneys, and in Brule and the Douglas County townships the same is true with the added variable of wood backup. Clearing snow away from outside vent terminations after every storm is homeowner work and it is worth doing.

What a technician checks that an alarm cannot

An alarm tells you carbon monoxide has already reached the living space. A combustion analysis tells you the furnace is heading that way before it gets there. A technician measures flue gas composition, draft and temperature rise, inspects the heat exchanger visually and with a camera, checks that vent piping is intact and pitched correctly, and confirms that the water heater drafts properly with the furnace and the bath fans running.

A short-cycling or repeatedly-relighting furnace is worth investigating for the same reason — a system that lights and drops out is not burning cleanly, and the pattern described in ignitor versus flame sensor failures is one a technician should read rather than a homeowner guess at. If your furnace is behaving differently this year, book a furnace diagnostic and combustion check rather than watching it. And never disable, jump or tape over a safety switch to keep a furnace running. Those switches exist for exactly this reason.