Quick answer
Yes. Any gas furnace can produce carbon monoxide if combustion goes wrong — most often from a cracked heat exchanger, blocked venting, or backdrafting. CO is colourless and odourless, so working CO alarms on every floor are non-negotiable in Ontario (and required by law near sleeping areas). If an alarm sounds, get everyone outside and call 911; don't ventilate-and-ignore it.
Yes, a gas furnace can leak carbon monoxide, and it does so through three doors: a cracked heat exchanger, venting that is blocked or leaking, or combustion that has gone bad at the burner. A furnace that was installed correctly and is inspected every year is a low risk. One that has not been opened since the previous owner is an unknown — and CO has no smell, colour or taste to warn you.
How a furnace can leak CO
- Cracked heat exchanger — the steel barrier between combustion gases and your household air fatigues with age; cracks let exhaust mix into the airstream. Most common on furnaces 15+ years old, and sooner on oversized units that spent years short-cycling.
- Blocked or disconnected venting — snow/ice over intake-exhaust pipes, bird nests, or corroded older metal flues force exhaust back into the house.
- Backdrafting — powerful kitchen/bath exhaust fans or fireplaces in tight homes can pull combustion gases down the flue of older natural-draft appliances.
- Incomplete combustion — dirty burners or wrong gas pressure produce far more CO than clean combustion; this is measurable at a tune-up.
Ontario's CO alarm law
Ontario's Fire Code, amended in 2014 under the Hawkins-Gignac Act, requires a working CO alarm adjacent to every sleeping area in any home with a fuel-burning appliance, fireplace or attached garage. That is the legal floor — put one on every storey and one near the mechanical room, mount it at the height the manufacturer specifies, and check the replacement date on the back, because sensors expire (most between five and 10 years from manufacture). Plug-in units with battery backup are fine; combination smoke/CO units count.
Warning signs and what to do
- CO alarm sounding: leave immediately with everyone including pets, and call 911 from outside — the full sequence is in what to do when a CO alarm goes off. Do not re-enter to open windows.
- Flu-like symptoms (headache, nausea, dizziness) that improve when you leave the house — classic low-level CO exposure; treat exactly as an alarm.
- Soot streaks around the furnace, heavy moisture on windows when the burner runs, or burner flames that lift, waver or burn yellow-orange instead of steady blue — shut the furnace off at its service switch and book service.
- Alarm chirping on a regular interval (not alarming): a low battery or an expired unit. Replace it the same day, not eventually.
Which GTA furnaces carry the most risk
- 1990s mid-efficiency furnaces venting into a masonry chimney shared with the water heater — common in Burlington's post-war streets and Hamilton's lower city. A deteriorating clay liner plus a strong range hood pulling the flue backwards is the classic pairing.
- High-efficiency units with side-wall PVC terminations set low to the ground: a lake-effect squall can bury the pipes in an afternoon. The pressure switch should lock the furnace out — unless it has been bypassed or is corroded.
- Any furnace past 15 years, where heat-exchanger fatigue cracks appear; a crack that sits closed on cold steel can open once the metal is hot.
- Furnaces moved, converted or reinstalled without a permit — wrong vent slope or vent material is exactly what a municipal inspection would have caught.
How a technician actually proves a furnace is safe
A visual glance at a cold furnace proves little. A proper inspection puts a combustion analyzer probe in the flue with the burners firing and reads CO, oxygen and stack temperature, then watches whether the numbers shift when the blower starts — a shift means the exchanger is leaking air across the flame. The technician also runs a draft and spillage test on natural-draft equipment, inspects the exchanger with a camera or mirror, checks vent joints and terminations, and sweeps the house with an ambient CO meter. If an unsafe condition is confirmed, the appliance is tagged out and its gas shut off — that tag is a legal instruction, not advice. Is a cracked heat exchanger an emergency covers what happens next.
CO questions homeowners ask us
- My alarm sounded once and stopped — do I still call? Yes, from outside. Intermittent readings usually mean an intermittent spill, such as a flue that only backdrafts when the dryer and range hood run together.
- Can an electric furnace or heat pump produce CO? No — no combustion, no carbon monoxide. Gas water heaters, ranges, fireplaces and cars in attached garages still can.
- Does a high-efficiency furnace remove the risk? It reduces it: sealed combustion draws outdoor air, and the pressure switch locks out on a blocked vent. It does not protect against a cracked heat exchanger.
- Will a tune-up catch a crack? One with combustion analysis and an exchanger inspection will; a filter change and a glance will not. Ask what instruments the visit includes.
- How many alarms do I need? One adjacent to sleeping areas is the law; one per storey plus one near the furnace is the version that protects a household.
Every item on the risk list is checked during an honest annual furnace tune-up. ZK Mechanical's TSSA-licensed technicians run combustion analysis and heat exchanger inspections across the GTA and Hamilton, install and test carbon monoxide detectors, and put suspected CO problems ahead of routine work on the 24/7 emergency line: (647) 801-1252.