By Daniel Watkins, Noble Chimney Sweepers.
A carbon monoxide alarm is not a chimney inspection, but for most homes it is the only venting monitor in the building. When it chirps in a warning pattern or goes into full alarm, it is reporting one thing: combustion gas that belongs in a flue is ending up in your living space instead. That is a chimney and venting question as much as it is an appliance question, and around Lockport and the DuPage County line it surfaces most often in the first cold stretch of the season.
What a carbon monoxide alarm is actually measuring
A CO alarm measures carbon monoxide concentration in the room air over time. It is not a smoke detector and not a gas leak detector. Carbon monoxide has no color and no smell, which is the entire reason the device exists. Illinois requires carbon monoxide alarms near sleeping areas in homes with fuel-burning appliances or an attached garage. If yours sounds, treat it as real, get everyone outside, and call the fire department first. The chimney conversation comes after the house is safe.
One detail catches people out: most alarms are time-weighted. A low concentration has to persist before the horn sounds. So a unit that goes off at 6 a.m. after the furnace ran all night may have been reading a slow, steady spill for hours before anyone heard anything.
The venting faults behind most chimney-related CO calls
Carbon monoxide reaches the room because the exhaust path failed somewhere. The usual culprits are a blocked flue, an undersized or deteriorated liner, a disconnected or rusted-through vent connector at the appliance, and a chimney that has cooled so far it never establishes draft. Nests, leaf packs, and fallen tile shards are the blockages we pull most often. On an older masonry chimney serving a modern furnace or water heater, a cracked clay liner lets flue gas migrate into the void spaces of the chimney and drift back into the basement.
None of that is visible from the floor. It takes a proper look at the flue interior, which is what a chimney inspection is for. When the liner itself is the fault, chimney relining is the repair that makes the flue tight and correctly sized again.
Why early September is when this surfaces in Woodridge
Two things change at once. Furnaces and water heaters start running longer cycles as nights cool, and houses get closed up after a summer of open windows. A chimney that limped along in July, when the appliance fired briefly and the house leaked air freely, now has to carry a sustained load in a sealed building. Any blockage that accumulated over nesting season is still sitting up there. Homeowners booking chimney and dryer vent service in Woodridge ahead of October avoid the December wait, when every sweep in the southwest suburbs is scheduled out for weeks.
What we check when an alarm points at the chimney
We start at the appliance and work up: vent connector condition and slope, the connection into the chimney, the base of the flue for debris, the liner along its length, and the termination at the top for a crushed, blocked, or missing cap. We also look for a setup mistake that has become common in this area, where a water heater is left venting alone into a large chimney after a high-efficiency furnace was moved to a sidewall vent. That oversized flue never warms up, so it never drafts properly, and it spills.
Noble Chimney Sweepers is certified, bonded and insured, owner-operated, and I have been doing this work for 14 years. You get the price up front before anything starts, and our rates live on the pricing page. The full lineup is on our chimney services page, or call (708) 432-5747. We answer 7 days a week, 7 AM to 7 PM.
Frequently asked questions
Does a carbon monoxide alarm mean my chimney is blocked?
Not necessarily, but a blocked or failed flue is one of the most common causes. The alarm only tells you gas is in the room. Finding out why requires checking the appliance, the vent connector, and the chimney itself.
Where should carbon monoxide alarms be placed in the house?
Near sleeping areas on every level, following the manufacturer instructions for mounting height and clearance from appliances. Illinois law requires them within a short distance of bedrooms in homes with fuel-burning equipment.
Can a fireplace set off a carbon monoxide alarm?
Yes. A fireplace with a closed damper, a blocked flue, or poor draft can push combustion gas into the room. Gas log sets can do it too, which is one reason an annual check matters.
My furnace vents out the side wall now. Does my chimney still matter?
It does if the water heater still vents into it. A chimney sized for a furnace plus a water heater is often far too large for the water heater alone, and an oversized cold flue is a classic spillage setup.
How often should the flue be inspected if I only have gas appliances?
Once a year. Gas appliances leave less visible residue than wood, so problems build quietly. Corroded liners, rust flakes at the base of the chimney, and nesting debris all accumulate without any obvious sign indoors.
Will a chimney cap prevent this?
A cap prevents a large share of it by keeping animals, nests, and rain out of the flue. It will not fix a cracked liner or a bad connector, so a cap is one part of the answer rather than the whole answer.
Is a carbon monoxide alarm a substitute for an inspection?
No. An alarm is a last-line warning that something already failed. An inspection finds the fault before it ever produces a room full of exhaust gas.
How fast can you come out after a carbon monoxide event?
Call us at (708) 432-5747 once the fire department has cleared the house. We cover Woodridge and the surrounding southwest suburbs 7 days a week, 7 AM to 7 PM, and we quote the price before we start.
Published September 1, 2026