Can a Furnace Leak Carbon Monoxide? How It Happens

A gas furnace is built to keep exhaust and house air apart. Carbon monoxide only reaches your rooms when the heat exchanger or the vent fails, and there are four ways it happens. An electric furnace burns nothing, so it has no CO to leak.

The Minnesota Department of Health describes CO as an odorless, colorless gas formed by the incomplete combustion of fuels. It counts an average of 14 Minnesotans killed by unintentional CO poisoning each year and about 300 emergency department visits, and it notes that cases run higher in the winter months (MDH on carbon monoxide), when a Minnesota furnace barely rests.

How does a furnace keep carbon monoxide out of the house?

The burners fire into the heat exchanger, a set of metal chambers. Flames and exhaust stay inside them while the blower pushes house air across the outside, and an inducer motor pulls the exhaust out through the vent: metal pipe into the chimney on an 80% furnace, PVC out a side wall or roof on a 90%+ furnace. The heat exchanger's wall and the vent are what keep combustion gases out of your air. Safety switches, such as the pressure switch and the rollout switches, catch many failures, but they don't replace working alarms and a yearly inspection.

Four ways furnace exhaust gets into the house

A cracked heat exchanger

EPA lists cracked furnace heat exchangers among the sources of combustion gases in homes. Cracks tend to form where the metal flexes most, after years of heating up and cooling down or after a furnace has overheated on a clogged filter or a starved return, and you usually can't see one yourself. Our post on cracked heat exchanger symptoms covers how a technician confirms it.

A blocked, leaking or disconnected vent

EPA warns that blocked, leaking or damaged chimneys or flues release harmful combustion gases and "even fatal concentrations of carbon monoxide," and tells homeowners to keep flues and chimneys connected, in good condition and not blocked. In Minnesota the usual blockage is winter itself: drifted snow or a frost ring over a high-efficiency furnace's PVC pipes, which our guide to clearing a frozen furnace vent shows how to handle. On older furnaces, look for metal vent pipe that's rusted through or pulled apart at a joint.

Backdrafting

EPA's list of CO sources includes back-drafting from furnaces, gas water heaters, wood stoves and fireplaces, and it notes that fireplaces and wood stoves without their own outdoor air supply can back-draft into the living space, particularly in weatherized homes. Run the bath fans, a big range hood and the dryer at once in a tight house, and the house can pull air down a chimney that a water heater depends on. Where the furnace and water heater share an old chimney, a new high-efficiency furnace can leave the water heater alone in a flue that's now too big for it, a liner question our AFUE explainer covers and one that often shapes a Saint Paul furnace quote.

Flame rollout

Rollout switches sit beside the burners and shut the furnace down when flames leave the combustion area. Lennox lists the usual causes as a cracked or obstructed heat exchanger, a flue that can't push exhaust out, a failing inducer motor and gas pressure out of range, and it advises treating even one trip as a prompt to investigate. A boom at ignition can push flame out too; see our guide to furnace noises.

What are the symptoms of carbon monoxide exposure?

MDH describes early exposure as a mild headache and breathlessness with moderate exercise. Continued exposure brings more severe headaches, dizziness, fatigue and nausea, then confusion, irritability, impaired judgment and coordination, and loss of consciousness. EPA adds that low concentrations can cause fatigue in healthy people and chest pain in people with heart disease, and that CO can cause flu-like symptoms that clear up after you leave home. That pattern is the clue.

MDH says people with asthma or emphysema, heart disease, or anemia or sickle cell anemia are at greater risk, as are older adults and young children.

Minnesota's CO alarm law in brief

Minnesota Statutes 299F.51 requires an approved and operational carbon monoxide alarm within 10 feet of each room lawfully used for sleeping, in every single-family home and every unit in a multifamily building. Our CO alarm placement guide covers what the statute's definitions mean, where else to put alarms and how long they last.

When the CO alarm goes off

1. Don't ignore it. That's MDH's first line.
2. If anyone has symptoms, get everyone out now and call 911 from outside. EPA says to get medical attention and tell the doctor you suspect CO poisoning.
3. If everyone feels fine, EPA's steps are to air the house out and turn off fuel-burning appliances, including the furnace. MDH says to call your gas utility or a qualified technician. Make that call from outside. If you're not sure whether anyone has symptoms, treat it like step 2: everyone out, then call 911.
4. Leave the appliances off until a qualified technician has checked them and the chimney, as EPA advises. Don't relight anything yourself once the alarm goes quiet.
5. Smell rotten eggs? That's gas, a different emergency. Leave without flipping switches and call your gas utility or 911 from outside: CenterPoint Energy in Minneapolis, Xcel Energy in Saint Paul.

If the furnace is red-tagged

A technician who finds a furnace unsafe can red-tag it: shut off the gas to the appliance and tag it so nobody restarts it. Leave it off, even on a cold night, and see our red tag guide and no-heat checklist for what to ask and how to keep the house and pipes safe until the repair.

Where we find CO problems in Minnesota houses

• Older city houses. In Minneapolis and Saint Paul, the furnace and water heater often share one old chimney, and the metal connector pipes rust at the joints. Our pages on furnace repair in Minneapolis and furnace repair in Saint Paul cover that housing.
• Attached garages. MDH says not to run or idle a vehicle in an attached garage, and EPA adds even with the garage door open. Remote starts on cold mornings make that easy to forget.
• Generators during outages. MDH notes poisonings from generators used during power outages.

How often should a furnace be checked for CO problems?

MDH recommends a yearly check of fuel-burning appliances by a qualified heating contractor, and EPA says a trained professional should inspect, clean and tune up the furnace, flues and chimneys annually. Our furnace tune-up is $129, or $219 with the air conditioner, and it is the visit where the burners, venting and combustion get checked.

National cost guides such as HomeGuide put a heat exchanger replacement at $1,000–$3,000, and on an older furnace a new one often makes more sense: ours run $4,500–$9,500 installed, roughly $4,500–$6,000 for 80% AFUE and $6,500–$9,500 for 96–98%. A service call is a $79 trip charge, waived when you approve the repair, with a written, flat-rate price before any work.

What to do next

Test your CO alarms, read the date on each one, and keep the furnace vents clear all winter. If you've seen soot, a tripped rollout switch or flames that change when the blower starts, leave the furnace off and call 612-429-3465 for furnace repair. Hours are 7am–9pm, seven days, and no-heat calls get priority.

Common questions about heating

Sources

• Minnesota Department of Health: MDH on carbon monoxide health.state.mn.us
• Minnesota Rules and Statutes (Office of the Revisor): Minnesota Statutes 299F.51 revisor.mn.gov
• U.S. Environmental Protection Agency epa.gov
• Lennox manufacturer documentation lennox.com
• HomeGuide cost guides (2026 editions) homeguide.com