Peotone CO alarm that went off at 2 a.m.: the furnace was venting into a blocked flue
A Peotone family woke to a chirping carbon monoxide alarm and did the right thing — they got out and called the gas company. The next call was to us, because the problem was the chimney, not the furnace.
Why the furnace was blamed first, and why it was not the furnace
When a CO alarm sounds, everyone looks at the appliance. But a gas furnace or water heater can be running perfectly and still spill carbon monoxide into the house if the chimney it vents into cannot carry the exhaust away. In Peotone we see this most in the older homes along the original town grid, where a masonry chimney serves the furnace and the flue has not been looked at in years.
This flue had a partial blockage — broken tile and mortar that had fallen onto the smoke shelf, plus a wasp nest near the top. Exhaust that should have risen out the top was backing down into the mechanical room. The furnace did its job. The chimney failed.
What a real inspection looks at that a flashlight cannot
We run a Level 1 or Level 2 inspection depending on what we find. That means a camera up the full length of the flue, a look at the liner, the crown, the cap, and every connection where the appliance vent enters the chimney. We check that the flue is the right size for the appliance — an oversized flue serving a modern high-efficiency furnace can stay too cool to draft and will condense and corrode.
We work to the NFPA 211 standard, which spells out clearances, liner condition, and how a fuel-burning appliance is supposed to vent. A phone light pointed up from the cleanout tells you none of that.
What we did and what every home should have
We cleared the debris, documented the tile damage on camera, and relined the flue so the furnace vents into a correctly sized, sound liner — our liners are listed to UL 1777. We also confirmed the home had working CO alarms on every level, which is what saved this family. [Observed] Most CO calls we trace to a chimney are blockage or a deteriorated liner, not a cracked heat exchanger.
If your CO alarm has ever sounded, do not reset it and move on. Get out, call for help, and have the venting inspected. Our inspection pricing is published, and we answer the phone seven days a week, 7am to 9pm.
If this sounds like your house: we handle chimney inspection across Kankakee County, and our Peotone page covers what we usually find there. Call (815) 449-4264, 7am to 9pm, any day.
Quick questions
My CO alarm went off but the furnace seems fine — could it still be the chimney?
Yes. A perfectly working furnace will spill carbon monoxide if the chimney it vents into is blocked or has a failed liner. The appliance and its venting have to be checked together.
What’s the difference between a Level 1 and Level 2 chimney inspection?
A Level 1 is a visual check of accessible parts for a system in continued use with no changes. A Level 2 adds a camera scan of the full flue and is called for after an alarm, a sale, or a venting change.
How often should a furnace chimney be inspected?
Once a year, before the heating season. Appliance flues collect fallen tile, rust, and nesting debris quietly, and you rarely get a warning until an alarm sounds.
Do you reline chimneys that vent a furnace or water heater?
Yes. When a masonry flue is oversized, cracked, or corroded, we install a correctly sized liner listed to UL 1777 so the appliance vents safely and drafts the way it should.
Published November 4, 2026 · By Jacob Dove, Friendship Chimney Sweep, Bourbonnais IL