Right now, in order
- Everyone outside — people and pets, into fresh air. Don’t stop to open windows, don’t investigate, don’t look for the source.
- Call 911 from outside. Not from inside the house.
- Tell them if anyone feels unwell. Headache, dizziness, nausea, confusion — that changes it from an equipment check to a medical call.
- Stay out until responders confirm it’s safe.
The fire department does not charge for this, and they would far rather check a false alarm than attend the alternative. Carbon monoxide kills hundreds of people in the US each year, and it does it while they sleep.
911, the gas utility, or an HVAC tech? Route by symptoms and timing
All three get called for CO alarms and only one of them belongs in the first five minutes. The deciding factors are whether anyone feels unwell and whether the alarm is sounding right now.
Anyone has a headache, dizziness, nausea, confusion, or blurred vision → 911, always, from outside. This is a medical call, not an equipment call. Fire crews carry meters and oxygen; utilities carry meters only.
Alarm is in full alarm pattern (four quick beeps, repeating) and everyone is out and feeling fine → 911 is still the safe default. If you would rather not tie up an engine and your utility has a 24/7 emergency line, they will also come free and meter the house. Either is correct. What is not correct is waiting until morning.
Alarm has stopped, nobody feels unwell, and you are already outside → gas utility emergency line. They will meter, find the appliance producing CO, and shut it off and red-tag it. Free, and they do it at 2am.
Single chirp every 30 to 60 seconds, no other symptoms → nobody. That is a low battery or an expired sensor. Replace the battery; if the unit is over 5–10 years old, replace the unit.
House is cleared and you now need the fault fixed → HVAC technician, chimney sweep, or plumber, depending on the appliance. This is the only one of the three you pay for, and the only one who actually repairs anything.
The wrong order is calling your HVAC company first. They will book you for tomorrow, charge a diagnostic fee, and cannot treat anyone. The two organisations that respond in minutes for free are the two you skipped.
After the emergency: finding the source
Once the house is cleared, someone has to identify what produced the CO — usually an HVAC technician, though the section below covers the cases where it isn’t. The usual suspects:
- Cracked heat exchanger in the furnace — the most common, and it means furnace replacement or a significant repair
- Blocked flue or chimney — bird nests, debris, or snow are ordinary causes
- Water heater venting that’s disconnected or corroded
- Gas range used as a heater — never do this
- Generator or grill run in a garage, porch, or near an open window
Don’t skip this step because the alarm stopped. The alarm stopping means the air cleared, not that the source is gone.
Nobody who came out will fix it, and the repair trade depends on the appliance
This catches people out. The fire department meters, ventilates, and leaves. The utility meters, shuts off the offending appliance, hangs a red tag on it, and leaves. Neither repairs anything, and neither is allowed to. The third call is on you, and which trade you want depends on what was producing the CO:
- Furnace, air handler, or heat pump backup → HVAC technician.
- Chimney, flue liner, fireplace, or wood stove → a CSIA-certified chimney sweep. Most HVAC companies will decline masonry and liner work outright, which is where people get bounced back and forth.
- Gas water heater venting, draft hood, or a disconnected vent connector → in a lot of jurisdictions this is a plumbing licence rather than HVAC. Say “gas water heater venting” when you book, not “CO alarm went off”, and you will get the right van.
- Gas range or oven → appliance repair, not HVAC.
- Gas piping or the meter set → the utility owns everything up to and including the meter; a licensed gas fitter or plumber owns your side of it.
If the utility red-tagged something, keep the tag and photograph it. It is the document the repair trade needs, and it is also what stops a second contractor from telling you a different story.
Before you agree to a new furnace tonight
The classic CO-alarm upsell is a cracked heat exchanger, quoted at night, with the furnace declared unsafe and a replacement offered on the spot. Sometimes it is entirely true — a cracked exchanger is a genuine reason to condemn a furnace. But it is also the most expensive diagnosis a technician can hand you, and it is very hard for you to verify from the hallway.
Three things to ask for before you sign anything:
- Photographs or camera footage of the actual crack, not a description of one. Every tech doing this work has a phone.
- The combustion analyzer reading in parts per million, and where in the airstream it was taken. “It was high” is not a measurement.
- The red tag, if the appliance is genuinely unsafe. A utility or a technician condemning an appliance documents it. No paperwork is a signal.
Then sanity-check the number. Fixr puts a full furnace replacement at $2,800 to $7,500, with a national average of $4,800. A quote far above that on the night of an alarm, with a discount that expires if you wait, is a sales technique. Unless the unit has been red-tagged and you have no heat in a cold snap, a second opinion in the morning costs you one night and can cost the contractor the job — which is exactly why the pressure exists.
If you rent, none of this is your bill. Report it in writing, keep the fire department’s or utility’s paperwork, and let the landlord procure the repair.
Detector placement, since you’re thinking about it
One on every level of the home, and one outside each sleeping area. CO mixes fairly evenly with air, so height matters less than people think — placement near bedrooms matters most, because that’s where the fatal exposures happen. Replace the units themselves every 5–10 years depending on model; the sensors expire even when the battery is fine.