Carbon monoxide is a colourless, odourless gas produced by incomplete combustion in furnaces, water heaters, fireplaces, stoves and vehicles. It cannot be detected by smell. Working CO alarms on every level of the home, plus annual professional inspection of combustion appliances, are the only reliable protection.
Where it comes from
Carbon monoxide is produced whenever a carbon-based fuel burns without enough oxygen to burn completely. In a properly operating, properly vented appliance, the small amount produced goes up the flue and outside. Problems arise when combustion goes wrong or when the exhaust does not leave:
- A cracked heat exchanger in a furnace, which lets combustion gases into the air stream the blower distributes through the house
- A blocked or disconnected flue — bird nests, debris, a vent pipe that has separated at a joint, snow or ice over a sidewall termination
- Backdrafting, where the house is depressurised enough by exhaust fans that flue gases are pulled back down the chimney instead of rising
- Insufficient combustion air in a tight mechanical room or a tightly sealed house
- Gas water heaters, fireplaces, gas ranges and space heaters that are poorly maintained or improperly vented
- A vehicle or generator running in an attached garage — even briefly, even with the door open
Never run a generator, grill, camp stove or unvented combustion heater indoors or in a garage. This causes serious poisonings every winter, usually during power outages when people are cold and improvising.
The altitude factor
There is less oxygen per cubic foot of air at 6,000 feet than at sea level. That has two consequences worth knowing. Combustion equipment must be set up for altitude — gas appliances are typically derated and require correct orifice sizing and air adjustment, and equipment installed without that adjustment burns less cleanly and produces more CO. And because your own blood is already carrying less oxygen at this elevation, carbon monoxide — which binds to haemoglobin far more readily than oxygen does — has less margin to eat into before it affects you.
This is a specific, practical reason to insist that gas appliances in Colorado are installed and commissioned by someone who sets them up for the altitude rather than straight out of the box.
Recognising the symptoms
Early exposure looks like a mild illness: headache, dizziness, nausea, fatigue, shortness of breath, confusion. The patterns that distinguish it:
- No fever. Flu-like symptoms without a temperature are a flag.
- Everyone in the house feels it, often including pets — frequently the first sign, since animals are smaller and show effects sooner.
- Symptoms improve when you leave and return when you come back.
- It started when the heating season did.
At higher concentrations: vomiting, loss of coordination, loss of consciousness. People often become confused before they recognise the danger, which is why acting on an alarm rather than investigating is the correct instinct.
Alarms: placement, testing, expiry
Where. On every level of the home, including the basement, and outside each separate sleeping area. Inside bedrooms is better if anyone sleeps with the door closed. Follow the manufacturer's height guidance — unlike smoke, CO mixes fairly evenly with air, so most units can be placed at any height.
Where not. Within about fifteen feet of a furnace, water heater or gas range, where normal brief startup emissions cause nuisance alarms. Not in bathrooms or very humid spaces. Not in direct sunlight or next to a supply register. Not in dead air space in a corner.
Testing. Press the test button monthly. Understand what that button does: it verifies the horn and the circuit, not the sensor. That is why the expiry date matters so much.
Expiry. The electrochemical sensor degrades whether or not it ever sees CO. Most units are rated for seven to ten years from manufacture, and many now have an end-of-life chirp. Look at the back of yours and find the manufacture date. An alarm older than its rated life should be replaced outright, not re-batteried — it will still pass its own test and may no longer detect anything.
If an alarm sounds
- Get everyone outside immediately, including pets. Do not search for the source, open windows first, or try to determine whether it is a false alarm.
- Call 911 or the gas utility from outside.
- Do a head count.
- Seek medical attention for anyone with symptoms, and tell them it is suspected CO exposure — there is a specific test.
- Do not go back in until emergency services or the utility says the house is clear.
- Have the appliances inspected before anything is put back in service. An alarm that sounded had a reason.
Prevention
Annual professional inspection of every combustion appliance is the substantive measure — it includes the checks a homeowner cannot perform: heat exchanger inspection, combustion analysis, draft verification and flue integrity. Our piece on cracked heat exchangers covers the furnace side in detail. Alongside that: keep the flue termination clear of snow and debris, never block combustion air openings in a mechanical room, keep the filter clean so the furnace is not fighting for airflow, and replace alarms on schedule.
Smith handles heating maintenance including combustion safety checks, CO and smoke detector installation, and gas piping. Our detector schedule guide covers the testing and replacement cadence.
Frequently asked questions
Can you smell carbon monoxide?
No. Carbon monoxide is colourless, odourless and tasteless, which is why it is so dangerous. The rotten-egg smell people associate with gas appliances is an odorant added to natural gas so leaks can be detected — it has nothing to do with carbon monoxide. A working CO alarm is the only reliable detection.
Where should carbon monoxide detectors be placed?
Install one on every level of the home including the basement, and outside each separate sleeping area, with units inside bedrooms where people sleep with doors closed. Avoid placing them within about fifteen feet of a furnace, water heater or gas range, in bathrooms, in direct sunlight, or beside supply registers.
How long do carbon monoxide detectors last?
Most are rated for seven to ten years from the date of manufacture, printed on the back of the unit. The electrochemical sensor degrades over time whether or not it ever detects carbon monoxide. Pressing the test button only verifies the horn and circuitry, so an expired alarm can pass its own test while no longer detecting gas.
What are the early symptoms of carbon monoxide poisoning?
Headache, dizziness, nausea, fatigue, shortness of breath and confusion — essentially flu-like but without a fever. Telling patterns include everyone in the household feeling unwell at once, pets affected first, symptoms easing when you leave the house and returning when you come back, and onset coinciding with heating season.