What is wrong? › Commercial HVAC
CO alarm: leave, call 911, do not silence and carry on
The CDC counts more than 400 unintentional CO deaths a year not linked to fires, and the symptoms feel like flu or a hangover, so people stay inside.
What is probably happening
- A cracked heat exchanger or bad venting in the rooftop furnace section, pushing combustion gas into the supply air.
- Gas cooking equipment or a water heater venting badly, especially if the kitchen hood is off or the building is under negative pressure.
- A generator, forklift or vehicle running near an outside air intake.
Do this in the next five minutes
- Evacuate everyone, staff and customers, to open air.
- Call 911 from outside. Tell them a CO alarm is sounding and how many people were inside.
- Ask anyone with headache, dizziness, weakness, upset stomach, vomiting, chest pain or confusion to be checked by the responders.
- Do not go back in until the fire department has taken readings and cleared it.
- Then call a licensed commercial HVAC contractor to test the heat exchanger and venting before the heat is used again.
Do not
- Do not silence the alarm and open a door. You cannot smell CO and the reading tells you nothing about the source.
- Do not restart the heat until the heat section has been tested with a combustion analyzer.
- Do not assume a battery fault. Treat every alarm as real until the fire department says otherwise.
Who to call
911 first, then a licensed commercial HVAC contractor
The fire department measures CO and makes the building safe. The HVAC contractor finds the failed heat exchanger, flue or vent and fixes it.
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What it should cost
$100 to $250
Service call and diagnostic visit, before any repair
Source: Angi, HVAC repair cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the commercial hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Will you test the heat exchanger and the flue with a combustion analyzer, and give me the readings in writing?
- Can you check the building for negative pressure with the kitchen hood running?
- Does the quote lock out the heat section until the repair is done, and who signs it back on?
- Are the CO alarms in the right places for a building with a rooftop furnace? Who checks that?
End the call if
- A contractor who restarts the heat without testing it.
- A whole-unit replacement quote with no combustion readings behind it.