How carbon monoxide from a furnace gets into the house, the symptoms, where CO detectors belong, and what a technician should check in Keller and NRH.
Carbon monoxide from a furnace is rare when the equipment and venting are sound, and it’s one of the few HVAC problems where “rare” isn’t reassuring enough. You can’t see it or smell it, and its early symptoms look like the flu. For homes in Keller and North Richland Hills heading into the first stretch of heating season, here’s what actually causes it, where detectors belong, and what a technician should be checking.
A gas furnace makes carbon monoxide as part of combustion. That’s normal — the system is designed to send it outside through the flue. Problems happen when combustion is poor or the exhaust doesn’t leave the way it should:
Headache, dizziness, nausea, fatigue and shortness of breath — easily mistaken for a cold or flu. Clues that point toward CO instead:
If anyone has symptoms and you suspect CO, get everyone outside into fresh air and call 911. Don’t wait for an HVAC appointment.
The useful placement rules:
Most plug-in and battery CO alarms have a service life of around 7–10 years; check the date on the back. Smoke detectors don’t detect carbon monoxide unless they’re a combination unit.
Get everyone outside and call 911 or the fire department, who can measure CO levels. If no one has symptoms and the reading turns out to be low, turn the furnace off and leave it off until a technician has inspected it. A low-level alarm is still a reason for an inspection — not a reason to unplug the detector.
| Check | Why It Matters |
|---|---|
| Combustion analysis (flue CO reading) | Shows how cleanly the furnace is burning |
| Heat exchanger inspection | Visual and camera check for cracks or holes |
| Venting and draft | Confirms flue is connected, pitched and drafting properly |
| Burner and flame condition | Lifting, rolling or yellow flame signals a problem |
| Gas pressure | Over-firing raises CO and stresses the heat exchanger |
| Ambient CO in the living space | Confirms nothing is reaching the house air |
A cracked heat exchanger diagnosis should come with evidence — a photo or camera footage of the crack. Ask for it. It’s a legitimate finding on older furnaces, and also one of the most commonly oversold ones.
Age itself isn’t the risk; neglect and venting are. Many North Richland Hills homes from the 1970s and 80s, and plenty of Keller homes from the 90s, are on their second or third furnace but still have the original flue. A furnace replaced without updating a corroded or undersized vent can draft poorly from day one. Annual HVAC maintenance with a combustion check is the simplest protection.
If your furnace is also cycling, short-running or not lighting, start with our furnace troubleshooting checklist.
We check combustion, venting and the heat exchanger on every furnace visit in Keller, North Richland Hills and across Northeast Tarrant County. Call 214-900-2533 or see our heating repair services.
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Kenny founded Absolute Comfort Mechanical after more than 20 years in the Dallas–Fort Worth HVAC trade, because he was tired of watching homeowners get sold equipment they didn't need. He still runs diagnostics himself and reviews the technical advice published on this site.
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