Carbon Monoxide and Your Furnace: A Fox Valley Homeowner’s Checklist

Start With Detectors, Not the Furnace

Carbon monoxide is colorless and odorless, and the early symptoms (headache, nausea, dizziness, fatigue) look like the flu that is going around in February. A detector is the only reliable warning you will get, and it is also the cheapest item on this list. Illinois law requires a CO alarm within 15 feet of every sleeping area in homes with fuel-burning appliances or an attached garage. We recommend going a step further.

• One on every level of the house, including the basement where the furnace and water heater live.
• One near each sleeping area, in the hallway outside the bedrooms, so it wakes you up.
• Test monthly, replace batteries twice a year, and replace the whole unit at the age printed on the back, typically 5 to 10 years. Sensors wear out even when the test button still beeps.
• Know the plan. If an alarm sounds, everyone goes outside, you call 911 from outside, and nobody goes back in until the fire department clears the house. Then call us to find the source.

If you smell gas rather than hear an alarm, the rule is different and just as firm: leave the house right away, call 911 from outside, and call your gas utility’s emergency line from outside before calling us.

What a Cracked Heat Exchanger Is, and Why It Matters

The heat exchanger is the metal chamber inside the furnace where the burner flames live. Combustion gases, including carbon monoxide, travel through the inside of it and out the vent. Your household air travels across the outside of it, picks up heat, and goes to the ducts. The two streams are never supposed to meet.

Years of heating and cooling cycles flex that metal thousands of times a season. Eventually it can crack, usually at a seam or a bend, and combustion gases mix with the air the blower is pushing into your bedrooms. A furnace that has overheated repeatedly from a clogged filter gets there sooner. Cracks are often hairline and invisible from the front; the cell has to be inspected with a camera and mirrors, or a combustion test that shows the blower disturbing the flame. A cracked heat exchanger is almost always the point where the conversation moves to furnace replacement, because the part is expensive, the labor is most of a day, and a furnace old enough to crack one is usually old everywhere else.

Warning Signs You Can See From the Furnace Room

None of these prove there is a CO problem, but each one is a reason to shut the furnace down at the switch and call rather than keep running it.

Venting and Condensate on 90%+ Furnaces

Most furnaces installed in the Fox Valley over the past 15 years are condensing furnaces rated 90% AFUE or higher. They pull enough heat out of the exhaust that it leaves as a cool, wet gas through PVC pipe rather than a hot one through a metal chimney. That is great for efficiency and it introduces two things to watch.

The first is the vent terminations. In our area they usually run through the rim joist or a sidewall, low to the ground, and February is when snow drifts, ice and frost can block the intake or the exhaust. A blocked vent trips the pressure switch and the furnace shuts down, which is the safety system working. The problem case is a partially blocked or separated vent that lets exhaust recirculate into the intake or leak into the basement. Keep both pipes clear and make sure the joints are glued and supported.

The second is condensate. A 90%+ furnace produces gallons of acidic water a day. It drains through a trap and a small line to a floor drain or a condensate pump. When the trap clogs or the line freezes, water backs up into the inducer housing, the pressure switch trips, and the furnace locks out. Water under the furnace or a gurgling inducer are the tells. Older 80% furnaces skip the condensate but rely on a metal flue and chimney liner that can rust or crumble; those get a visual inspection every year.

What a Tune-Up’s Combustion Analysis Actually Catches

The single most useful CO safety step after installing detectors is a fall furnace tune-up that includes combustion analysis, not just a visual once-over. We put a combustion analyzer probe in the flue and measure what is coming out of the furnace while it runs. That gives us numbers the eye cannot: carbon monoxide in the flue gas in parts per million, oxygen and carbon dioxide levels that show whether the burners are getting the right amount of air, and flue temperature that shows the heat exchanger is doing its job. We also check the room for ambient CO with the furnace running.

Those readings catch what a flashlight misses: burners making CO because gas pressure has drifted, a heat exchanger leaking enough to shift the numbers when the blower starts, a vent that is not drafting, and a furnace that has been slowly getting worse for two winters. On the same visit we inspect the heat exchanger, clean the burners and flame sensor, check the rollout and limit switches, test the pressure switch, clear the condensate trap and confirm the vent terminations. If something is off, you get a written finding and a flat-rate furnace repair price to approve before any work, and if the heat exchanger is the problem we say so plainly and quote a replacement.

The semi-annual maintenance plan is $240 a year and includes that fall furnace tune-up plus a spring AC or heat pump tune-up, each up to a $130 value, one free business-hours service call, priority scheduling and 5% off services. That is up to $390 in services for $240, and the combustion check comes with it every fall. Skipped the fall visit? A February tune-up is still worthwhile.