Short answerA gas furnace produces carbon monoxide whenever it burns, and a sound heat exchanger and flue carry it safely outdoors. The risk comes from cracked heat exchangers on furnaces over 15 years old, blocked or disconnected flues, and backdrafting. Install CO alarms on every level and outside bedrooms, and if one sounds, get everyone out of the house and call 911 before anything else.
Where the carbon monoxide comes from
Natural gas burning with enough air produces carbon dioxide and water vapor. Burning with too little air, or with the flame disturbed, produces carbon monoxide as well, and every gas furnace makes some. That is normal. The combustion happens inside the heat exchanger, a sealed metal chamber, and the flue gases exit through the inducer motor and up the vent to the outdoors. The air you breathe passes over the outside of the heat exchanger, picks up heat through the metal, and goes to the registers. As long as that metal wall is intact and the vent works, the CO never enters the living space.
The dangerous scenarios are the ones that break that separation. A crack or rusted-through spot in the heat exchanger lets flue gas mix with supply air. A blocked, disconnected or corroded flue lets gas spill into the basement instead of leaving the house. A house that is depressurized by a big kitchen exhaust fan or a tight new envelope can backdraft an atmospheric water heater or an older 80-percent furnace, pulling exhaust down the chimney and into the room.
Why heat exchangers crack
A heat exchanger is thin steel that heats up and cools down thousands of times a season. Every cycle expands and contracts the metal, and the welds and bends are where fatigue eventually wins. A furnace that short-cycles because it is oversized for a 1,100-square-foot Westland ranch, or because a dirty filter trips the limit switch every few minutes, goes through many more cycles than it was designed for. Condensation is the other killer. An 80-percent furnace that runs with a restricted return or a cold flue can condense water vapor inside the exchanger, and the resulting acidic moisture rusts the steel from the inside.
The age pattern is consistent. We rarely see cracks under 12 years. Between 15 and 20 years they become common, and on a 25-year-old furnace that has been run hard through Michigan winters with infrequent filter changes, a crack is more likely than not. The furnaces of greatest concern across our service area are the mid-1990s to mid-2000s 80-percent units still running in Ypsilanti Township, Garden City, Inkster and the older parts of Canton, many of which were installed during a replacement wave and have not been looked at since.
Symptoms in people, not just equipment
Carbon monoxide has no color, no odor and no taste. Low-level exposure feels like the flu without the fever: headache, dizziness, nausea, fatigue, a vague feeling of being unwell. The tell is the pattern. Symptoms that are worse in the morning and better after you leave for work, that affect everyone in the house including pets, that show up when the furnace starts running in October and ease when windows open in spring, are not the flu. Higher exposure brings confusion, vomiting, chest pain and loss of consciousness.
If anyone in the house has these symptoms and a CO alarm has not sounded, do not assume you are safe; the alarm may be missing, expired or in the wrong place. Get fresh air and call for help.
What to do when a CO alarm sounds
Leave the house. Everyone, including pets. Do not open windows first, do not go to the basement to check the furnace, do not reset the alarm to see if it does it again. Once you are outside, call 911. The fire department will meter the house and tell you when it is safe to re-enter. Then call us or another HVAC company to find the source before the furnace runs again.
A separate situation is the smell of gas, the rotten-egg odorant DTE adds to natural gas. That is not carbon monoxide, but the response is the same: leave, and from outside call DTE at 800-947-5000 and 911. Do not flip light switches or use a phone inside the house.
Where to put CO detectors in a Michigan house
Michigan's residential code requires carbon monoxide alarms in new construction and in homes where a permit is pulled for work on fuel-burning appliances, but a lot of the existing housing stock in Washtenaw and Wayne County has none. Our recommendation regardless of code:
- One on every level, including the basement, where the furnace and water heater live.
- One in the hallway outside each sleeping area, within about 10 feet of bedroom doors, because the exposure that kills people is the one that happens while they sleep.
- Not inside the furnace room itself or within a few feet of the furnace or water heater, where normal startup can cause nuisance alarms, and not in a dead-air corner.
- At the height the manufacturer specifies. CO mixes evenly with air, so wall-plug units, shelf units and ceiling-mounted combination units all work when installed per the instructions.
- Replace the whole unit at the end of its rated life, typically 7 to 10 years; the sensor degrades even if the battery is fine.
A combination smoke/CO alarm is fine. A single CO alarm plugged into an outlet in the kitchen and nothing else is not enough coverage for a two-story colonial in Saline with bedrooms upstairs.
What we check on a furnace visit
A proper furnace tune-up is the main defense, and the two parts that matter for CO are the ones a quick look cannot do.
Heat exchanger inspection. On most furnaces we can see a good portion of the exchanger with the burners removed and a light and mirror or a borescope. We look for cracks at the bends, rust-through at the bottom of the cells, and discoloration that points to a hot spot. We also watch the flames when the blower starts; flames that dance, lift or change color when the blower comes on are being disturbed by air pushing through a breach.
Combustion analysis. A digital combustion analyzer in the flue reads CO in parts per million, oxygen, stack temperature and efficiency. A clean, properly adjusted furnace reads very low CO in the flue. Rising CO, especially CO that climbs as the furnace runs, points to a flame problem, a cracked exchanger, or a venting issue. We also check draft at the flue and inspect the vent connector, chimney liner or PVC for corrosion, sags and disconnections, and we check that the water heater flue is drafting too.
If we find a cracked heat exchanger, we shut the furnace off and tag it. That is not a sales tactic; it is a furnace that is mixing flue gas into your supply air, and no responsible technician will leave it running. From there the decision is replacing the exchanger under warranty if it qualifies, or replacing the furnace, which on a 20-year-old unit is almost always the better path. Our repair or replace furnace guide walks through that call, and if you are looking at a new unit, furnace installation estimates are free.
Older homes, chimneys and orphaned water heaters
One specific Southeast Michigan scenario deserves its own warning. When a 90-percent furnace replaces an old 80-percent unit, the new furnace vents through PVC out the side of the house, and the old masonry chimney is left serving only the water heater. That chimney is now far too large for the small flue gas volume from a 40-gallon water heater, the gases cool and condense before they reach the top, and the chimney can fail to draft at all. The result is an "orphaned" water heater spilling exhaust into the basement. The fix is a properly sized chimney liner or a power-vented water heater, and it should be part of any furnace installation where the chimney is involved. If your furnace was swapped to high-efficiency in the last decade and nobody mentioned the chimney, have it checked.