If a carbon monoxide alarm is sounding right now: get everyone out of the building and call emergency services. Do not look for the source. Do not open windows first. Do not check whether it might be the battery. Move to fresh air outdoors, account for everyone who was inside, and call from outside.
That is the whole instruction, and everything below it is context rather than qualification. Nothing in the rest of this page changes what to do while an alarm is sounding.
What the safety authorities say
The National Fire Protection Association puts it this way: “If the CO alarm sounds, immediately move to a fresh air location outdoors or by an open window or door. Make sure everyone inside the home is accounted for. Call for help from a fresh air location and stay there until emergency personnel” arrive. The U.S. Consumer Product Safety Commission’s instruction is the same shape: “If you suspect CO poisoning, get outside to fresh air immediately, and then call 911.”
Why the instruction is “leave”, not “check”
Carbon monoxide is described by the CDC as “an odorless, colorless gas that kills without warning” and by CPSC as “a colorless, odorless, poisonous gas.” You cannot smell it, see it, or taste it.
That single fact determines the response. Every form of checking happens inside the building, in the air the alarm is objecting to, while your ability to make good decisions is one of the things the gas affects. The instruction is to leave because investigation is the thing you are least able to do safely and least able to judge that you are doing badly.
This is also why this page does not give you a list of physical symptoms to check yourself against. Symptom lists invite exactly the wrong behaviour: standing in the building, taking an inventory of how you feel, and deciding whether it seems bad enough to act on. The alarm is the instrument. It is more sensitive than you are, and it is not affected by the gas.
Where a sounding alarm sits among the things a CO alarm does
CO alarms make several different sounds, and they mean different things. This is the part that is genuinely useful to know in advance, and it is where most published advice is thin.
A sounding alarm — a loud, repeating alarm pattern. Leave and call emergency services. Nothing else on this list changes that.
A trouble signal — an intermittent chirp, typically once every 30 to 60 seconds. This is not a CO alarm. It is the unit reporting a problem with itself: usually a low battery, sometimes end of life. NFPA’s guidance is specific: “If the audible trouble signal sounds, check for low batteries. If the battery is low, replace it. If it still sounds, call the fire department.” A chirp that continues after a fresh battery is a unit telling you it has reached the end of its service life, or that something else is wrong with it.
An alarm that sounded and has stopped. Do not treat this as resolved. CO disperses; an alarm can clear because the gas has dispersed while the source is still there and still producing it. If an alarm has sounded and stopped, and you do not know why it sounded, it needs investigating by someone qualified before the appliances go back into normal use.
A digital display showing a low reading with no alarm. Alarms are built to sound at defined concentration-and-time thresholds rather than at any detectable trace. A display showing a low number is telling you CO is present below the alarm threshold. That is not an emergency in the way a sounding alarm is, but it is a finding, and a house that reads persistent low-level CO has a source somewhere that should be found.
Knowing which one you are hearing is worth working out now rather than at 2am. Alarm and chirp patterns are printed in the manual and usually on the unit itself.
“Can it be a false alarm?”
Sometimes the alarm sounds and no CO is found. NFPA’s own incident data makes this concrete: in 2023, US fire departments responded to 82,245 CO incidents, and separately recorded 79,051 CO alarm malfunctions and 79,286 CO alarm activations where no CO was found present.
Two things follow from those numbers, and people usually take only the first.
The first is that yes, alarms do activate without CO being found. The second, and more important, is what “no CO found present” means: it is what the responders measured when they arrived, after the building had been evacuated and often ventilated. A transient CO event that has dispersed by the time a truck arrives produces exactly that record. So does a genuine malfunction. From inside the house, at the moment it sounds, those are indistinguishable.
This is why the possibility of a false alarm does not change the response. You are being asked to leave a building for a few minutes against a risk you cannot detect by any other means. The asymmetry is not close.
For scale on the other side: a CDC study covering 2005 to 2018 found unintentional non-fire-related CO poisoning responsible for approximately 101,847 emergency department visits, 14,365 hospitalisations and at least 430 deaths annually in the United States.
The number to call, and the detail almost everyone gets wrong
There is a widely repeated piece of advice that you should call the fire department’s non-emergency number when a CO alarm goes off. It is a misreading of real guidance, and it is worth correcting carefully because the underlying advice is sound.
What NFPA actually recommends is this: “Call your local fire department’s non-emergency number to find out what number to call if the CO alarm sounds.”
The non-emergency call is the one you make in advance, on an ordinary afternoon, to establish what your local department wants you to do. The call you make during an alarm is the emergency one. Somewhere in the retelling those two collapsed into each other, and the result is advice that tells people to use a non-urgent channel during the one event where time matters.
If you have not made that advance call, use emergency services. That is the correct default and the one both CPSC and NFPA describe.
Boulder’s guidance for the related case — a smell of gas — follows the same shape. The City of Boulder instructs residents to leave the area immediately, call 911, and then contact Xcel Energy, which supplies both natural gas and electricity in Boulder.
Where carbon monoxide in a house comes from
CO is produced by incomplete combustion. The CDC names household sources including gas- and oil-burning furnaces, water heaters, gas ranges, stoves, fireplaces, lanterns, portable generators and charcoal grills. NFPA adds vehicles or generators running in an attached garage.
For heating equipment specifically, the mechanical conditions that produce it are well understood:
- A blocked, disconnected or backdrafting flue, so combustion products do not leave the building.
- A cracked or failed heat exchanger, which lets combustion gases into the air being circulated through the house.
- Improper combustion at the burner, including a furnace that has been incorrectly derated. Manufacturers are explicit about this. Goodman’s installation manual warns against lowering manifold pressure below the rating plate value, because the combination of lower air density and lower manifold pressure “may cause incomplete combustion, flashback, and possible yellow tipping.” That warning appears in the high-altitude section — relevant here, since Boulder sits a little over 5,400 feet.
- Snow or debris blocking a vent termination. NFPA specifically advises that during and after a snowstorm, vents for the dryer, furnace, stove and fireplace are cleared of snow build-up. On a Front Range house with a sidewall-vented high-efficiency furnace, that is a real winter condition rather than a theoretical one.
The safety devices built into a modern furnace are designed to catch several of these — which is the subject of the safety switches your furnace uses, and why defeating one is dangerous. It is also the strongest practical reason not to let anyone silence one.
What this page will not tell you how to do
There are no homeowner procedures here for combustion equipment, and that is deliberate rather than an omission.
Inspecting a heat exchanger, testing draft, checking combustion, adjusting gas pressure or clearing a flue are not homeowner tasks — not because they are difficult, but because the failure mode is the gas you cannot detect. A test performed incorrectly on a heat exchanger returns “looks fine,” which is indistinguishable from a correct result and is acted on the same way. That is the specific reason the boundary sits where it does.
What is yours to do: leave when the alarm sounds, call emergency services, keep alarms installed and tested, and get combustion appliances serviced by someone qualified. The CDC’s prevention advice is to have appliances serviced annually.
Alarms: placement and upkeep
NFPA’s guidance:
- Install CO alarms in a central location outside each sleeping area and on every level of the home, and anywhere else required by local codes.
- Interconnect them where possible, so that when one sounds, they all sound.
- Follow the manufacturer’s instructions for placement and mounting height.
- Choose an alarm carrying the label of a recognised testing laboratory.
- Test alarms at least once a month, and replace them according to the manufacturer’s instructions.
CPSC’s placement guidance is consistent: battery-operated CO alarms or alarms with battery backup on every level of the home and outside sleeping areas, with batteries checked regularly.
On replacement intervals, note what the guidance does and does not say. Both authorities defer to the manufacturer rather than naming a fixed number of years. CO alarms do have a limited service life, and the interval belongs to the specific unit — so check the documentation or the unit itself rather than relying on a figure quoted online.
What a correct diagnosis looks like
If a CO event is investigated at your house, a correct diagnosis is instrument-led.
It should include actual combustion measurements taken with a combustion analyser rather than an opinion about how the flame looks; verification that the venting path is intact and drawing, including the termination outside; and, where a heat exchanger is suspected, a stated method and a stated result rather than a verdict.
A finding that justifies significant expense should be demonstrable. A cracked heat exchanger in particular is a physical condition that can be evidenced. If it cannot be shown to you in some form, the claim has not been supported — and that is worth knowing, because a heat exchanger is one of the conditions most often asserted during a sales conversation.
Equally: if an alarm sounded and the answer you are given is that nothing was found, ask what was measured and where. “Nothing found” after an evacuation and ventilation is an expected result, not an all-clear on the source.
Frequently asked questions
Should I be worried if my carbon monoxide alarm goes off? Treat it as real every time. Get everyone out of the building and call emergency services. Carbon monoxide cannot be detected by human senses, so the alarm is the only warning available.
Should I call the fire department if my carbon monoxide detector goes off? Call emergency services when an alarm is sounding. NFPA recommends calling your local fire department’s non-emergency number in advance to find out what number they want you to use — that advance call is not a substitute for calling during an actual alarm.
Can a carbon monoxide alarm go off falsely? Yes, and it should still be treated as real. In 2023 US fire departments recorded 79,051 CO alarm malfunctions and 79,286 activations where no CO was found on arrival — but “not found on arrival” also describes a genuine event that dispersed after evacuation. You cannot tell which you have from inside the building.
How long do you have to be exposed to carbon monoxide to get sick? It depends on concentration and duration, which is precisely why the response is to leave rather than to assess. There is no threshold you can usefully judge from inside the house, and attempting to judge it is the delay that causes harm.
My alarm chirped once and stopped. Is that carbon monoxide? An intermittent chirp is normally a trouble signal rather than a CO alarm — usually a low battery or end of life. NFPA advises replacing the battery, and calling the fire department if it still sounds afterwards.
The alarm went off and then stopped on its own. Is it fine now? No. CO disperses, so an alarm can stop while the source remains. Have it investigated before the appliances return to normal use.
Where should CO alarms be installed? Outside each sleeping area and on every level of the home, per NFPA and CPSC, following the manufacturer’s placement instructions.
After the emergency
Once emergency services have cleared the property and it is safe to be inside, the source still needs finding before the equipment goes back into service. Boulder HVAC Pros works across Boulder and the surrounding communities. Call (720) 764-0399, or see emergency HVAC repair in Boulder.
If you are weighing whether a heating problem needs attention tonight or can wait, see which no-heat situations are genuine emergencies. The combustion checks that belong in an annual service are covered in what a real pre-winter tune-up includes, and the wider context in winter comfort, safety and heating costs in Boulder.
Sources
- National Fire Protection Association, Carbon Monoxide safety (response instruction, trouble signal, placement, testing and replacement, incident statistics) — https://www.nfpa.org/education-and-research/home-fire-safety/carbon-monoxide
- U.S. Consumer Product Safety Commission, Carbon Monoxide Information Center — https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center
- Centers for Disease Control and Prevention, About Carbon Monoxide — https://www.cdc.gov/carbon-monoxide/about/index.html
- City of Boulder, list of Boulder utilities (gas smell instruction) — https://bouldercolorado.gov/list-boulder-utilities
- Goodman, Installation Instructions, GM9S8 / GC9S8 gas furnaces (manifold pressure, incomplete combustion, high-altitude provisions) — https://documents.alpinehomeair.com/product/goodmangm9s8gc9s8installationmanual.pdf
- NOAA National Centers for Environmental Information, GHCN-Daily station inventory (Boulder station elevation) — https://www.ncei.noaa.gov/pub/data/ghcn/daily/ghcnd-stations.txt
