Can your furnace leak carbon monoxide? Here's the honest answer

Carbon monoxide (CO) is a colorless, odorless gas produced whenever fuel burns incompletely. A healthy gas furnace produces very little of it, and what it does produce stays sealed inside the heat exchanger and exits the house through the flue. CO becomes a risk only when that containment fails. Three failure paths exist:

• A cracked heat exchanger — the metal barrier between the flame and your home's air develops a crack, letting combustion gases mix into the airstream your indoor air quality depends on.
• A blocked flue — a nest, debris, or collapsed liner keeps exhaust from leaving, so it spills back into the house.
• Backdrafting — house depressurization (think a big kitchen exhaust fan in a tight house) pulls flue gases back down the vent.

All three are physical, inspectable conditions. None of them is a mystery, and none of them requires guesswork to confirm.

CO detectors are your first line, because symptoms are easy to miss: low-level exposure feels like a flu without the fever — headache, fatigue, dizziness, nausea — and it lifts when you leave the house. Placement rules that matter:

• One on every level of the home, and one outside each sleeping area — hallway near the bedrooms, not inside the furnace closet.
• Not directly beside the furnace or in the garage, where nuisance readings breed the habit of ignoring alarms.
• Replace the whole unit roughly every 7 years. CO sensors chemically wear out; a 12-year-old detector with a fresh battery is a decoration. Check the manufacture date printed on the back.

Neither Kansas nor Missouri has one single statewide rule for existing single-family homes — requirements come from the residential codes each city adopts, and most KC-metro cities, from Leavenworth to Raytown, follow International Residential Code provisions requiring CO alarms outside sleeping areas in new and remodeled homes. Code minimums aside, every-level-plus-bedrooms is the standard we'd follow in our own houses on either side of the state line.

Because CO is genuinely dangerous, it's also the most abused word in furnace sales. Three patterns to watch for on KC service calls:

1. The condemned furnace with no evidence. A tech declares the heat exchanger cracked, red-tags the furnace, and quotes a replacement — without showing you anything. A real crack diagnosis comes with proof: inspection-camera photos of the crack, or combustion-analyzer readings, that you get to see.
2. "Rust means CO." Surface rust on a 15-year-old furnace is normal aging. Rust alone is not evidence of a breached heat exchanger, and a quote that leaps from rust to replace deserves a second opinion.
3. Urgency without measurement. If nobody put a combustion analyzer in the flue or a low-level CO meter in the supply air, nobody measured anything. "You need this replaced today" without numbers is a sales script, not a safety finding.

The rule: a genuine CO finding is demonstrable. Ask to see it. An honest company will show you; we put that promise in writing on every inspection.

Here's what actually happens when the safety check is real and not a checkbox:

• Visual heat-exchanger inspection — with a camera or mirror where the design allows, looking for cracks, splits, and hot-spot discoloration.
• Combustion analysis — an analyzer probe in the flue measuring CO parts-per-million, oxygen, and flue temperature. Numbers, on a report you keep.
• Draft verification — confirming the flue actually pulls exhaust out, including a spillage test at the draft hood on older 80% AFUE (Annual Fuel Utilization Efficiency) furnaces.
• Ambient CO check — a low-level meter reading in the supply air and around the furnace while it runs.
• Gas-pressure and burner check — incomplete combustion often starts with misadjusted gas pressure or dirty burners, which are fixable long before they're dangerous.

This whole sequence is part of an honest annual tune-up — it's the single highest-value item on the checklist, and it takes measurement tools, not opinions.

Three actions, in order of urgency:

• If a CO alarm is sounding or people feel sick: get everyone outside, then call 911 or your utility's emergency line from outside. Air out first, diagnose later.
• This week: check the manufacture date on every CO detector; replace any older than about 7 years, and add units to any level or sleeping area missing one.
• Every fall: get a combustion-safety inspection before heating season — cracked exchangers and blocked flues are found in September for a reason.

The easiest way to make the annual inspection automatic is a maintenance plan — the combustion-safety sequence above is built into every heating visit, and you keep the analyzer report. Details on what the visit covers live on our heating tune-up page.