When our CO alarm went off one January night I shut the furnace down and called a tech out the next morning. He put a combustion analyzer on it and the flue numbers were way off. Here's the thing I learned the hard way, a visual look at the heat exchanger catches almost nothing. Most cracks hide where you can't see them. Insist on a proper scope with the blower pulled, and have them check the flue pipe too. Ours turned out to be a rusted-out secondary exchanger that nobody would have found peering through the burner opening. Second tip, call the manufacturer's warranty line yourself instead of going through the HVAC company. I registered the serial number, confirmed what was covered, and they sent out an authorized contractor. That took all the sales pressure out of it. Get every reading in writing with the actual numbers, because if CO is coming back high the unit should stay off until it's fixed, and a legit tech will red-tag it anyway. In our case the furnace was 15 years old, so paying the labor on a partial repair made less sense than putting it toward a replacement. The new unit runs quieter and the gas bill dropped enough that we actually noticed the difference.
How I finally sorted high CO readings on my gas furnace - steps that actually worked
Nov 03, 2026
Last reply: 6 hours ago
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I went through almost the same thing two winters back with a 20 year old Trane. Our detector was one of the plug-in combo units and it chirped twice in one night, then quiet. I nearly talked myself into thinking it was a fluke until I read the display said carbon monoxide. Kept…
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I went through almost the same thing two winters back with a 20 year old Trane. Our detector was one of the plug-in combo units and it chirped twice in one night, then quiet. I nearly talked myself into thinking it was a fluke until I read the display said carbon monoxide. Kept the furnace off and booked someone for the morning, and I'm glad I did because the readings were bad enugh that the tech would not let us run it at all after that.
Funny thing, ours turned out the same alarm but the furnace checked out fine. Turned out the water heater was backdrafting because someone had replaced the vent with the wrong size and it wouldn't pull once the house got sealed up tight in winter. Tech put an analyzer on the draft hood and there it was. So yeah, don't let anyone stop at the furnace if the numbers are clean, the other gas appliances can be the culprit too.
The thing that changed it for us was a low-level CO monitor with a digital readout. Our regular alarms never made a sound, but that little unit was showing small bumps every time the furnace kicked on. Called a tech becasue of those readings and the flue had a partial blockage from a birds nest. Never would have known otherwise, the regular alarms only care once levels are allready high. Might be worth having one on each floor if your house is like ours.
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