Yes, a gas, propane or oil water heater can release carbon monoxide into a home if its exhaust backdrafts, spills, or is blocked, or if it burns poorly. Electric and heat pump water heaters do not produce CO. CO has no smell, so a certified CO alarm outside sleeping areas is essential. If an alarm sounds, get everyone outside and call 911 from there.
- Any fuel-burning water heater produces carbon monoxide; the vent is what keeps it outside.
- Backdrafting happens when exhaust fans, dryers or fireplaces pull air down a conventional water heater's flue.
- Soot, scorching or melted plastic at the draft hood are visible warning signs of spillage.
- Health Canada: install CO alarms in hallways outside sleeping areas, test them, and replace them as the maker recommends.
- Have fuel-burning appliances inspected by a professional at least once a year, and keep vents clear of snow.
01 /Can a water heater cause carbon monoxide poisoning?
Yes. Carbon monoxide (CO) is produced whenever a fuel burns, and Health Canada lists hot water tanks among the household appliances that produce CO when they burn fuel. A gas or propane water heater is safe when its burner is clean and correctly adjusted and its vent carries all the exhaust outdoors. It becomes dangerous when the exhaust comes back into the house or when poor combustion makes much more CO than normal.
| Type | Burns fuel? | CO risk inside the home |
|---|---|---|
| Conventional gas or propane tank (natural draft) | Yes | Yes, if it backdrafts, spills or the vent is blocked or disconnected |
| Power-vent gas tank | Yes | Lower backdraft risk; still a risk if the vent leaks or the termination is blocked |
| Direct-vent (sealed combustion) tank | Yes | Lower; outside air in, exhaust out, sealed from the room |
| Gas tankless | Yes | Yes, if the vent is damaged, disconnected or blocked |
| Oil-fired tank | Yes | Yes, similar to conventional gas |
| Electric tank or heat pump | No | None from the water heater itself |
CO has no colour, smell or taste. Health Canada is plain about it: the only way to detect it is with a carbon monoxide alarm. It also notes the risk indoors is greatest in fall and winter, when fuel-burning appliances run most, which in Canada is also when houses are closed up and vents can be buried in snow.
02 /How does carbon monoxide get out of a water heater and into the house?
A conventional gas water heater relies on natural draft: hot exhaust rises up the flue, through the draft hood on top of the tank, and out a vent or chimney. Anything that weakens or reverses that draft can push exhaust into the room instead.
| Mechanism | What causes it | Clues |
|---|---|---|
| Spillage at start-up | Exhaust spills briefly before the flue warms and draft establishes | Brief; worse in cold weather and cold chimneys |
| Backdrafting | Kitchen and bath fans, clothes dryers, central vacuums and fireplaces pull air out of a tight house, and the water heater flue supplies it | Pilot goes out when fans run; soot or melted plastic at the draft hood |
| Blocked or disconnected vent | Loose vent sections, collapsed liner, bird or debris blockage, snow or ice at a sidewall termination | Heat and moisture around the top of the tank; power-vent error codes |
| Orphaned water heater in a chimney | When a furnace is replaced with a sidewall-vented high-efficiency model, the water heater may be left alone in a chimney too large or cold for it | Condensation, rust or poor draft at the draft hood |
| Poor combustion | Dirty burner, blocked flame arrestor, lack of combustion air, wrong gas pressure | Yellow, lazy or sooty flame; soot on the burner or tank |
Natural Resources Canada's Water Heater Guide explains that tightly sealed, energy-efficient homes may not leak enough air to supply combustion and draft, that other air-exhausting devices may pull air from the water heater flue, and that spillage can be a serious problem. It recommends considering a power-vented or sealed-combustion water heater in newer or tightened houses and, as a minimum, a carbon monoxide sensor near all combustion appliances. For venting in depth, see water heater venting and backdraft.
03 /What are the warning signs of a water heater producing CO?
Visible signs of spillage at the water heater, and symptoms in the people in the house, are the two sets of warnings. Neither replaces an alarm.
- Soot or black marks around the draft hood, burner door or top of the tank.
- Scorching, discoloured paint or melted plastic on pipes, wiring or fittings near the draft hood.
- Moisture, rust or stale, damp air in the utility room, especially in cold weather.
- A pilot that goes out when the dryer, fans or furnace run.
- A yellow, lazy or noisy flame instead of a mostly blue, steady one.
- Power-vent error codes or a fan that runs without the burner lighting.
Health Canada describes CO symptoms by level of exposure. Low-level exposure can cause tiredness, headaches, flu-like symptoms, shortness of breath and impaired motor function such as muscle weakness. Moderate exposure can cause dizziness, chest pain, poor vision and difficulty thinking. Severe exposure can cause convulsions, loss of consciousness, coma and death. Symptoms that ease when you leave the house and return when you come home are a classic pattern.
04 /Where should carbon monoxide alarms go?
Health Canada advises that the most important place for a CO alarm is in hallways outside sleeping areas. Buy alarms that carry a Canadian certification mark, such as CSA Group, Intertek or UL, which Health Canada says must appear on both the alarm and its packaging. Test them regularly, write the installation date on each one, and replace batteries and alarms as the manufacturer recommends.
| Item | What to do |
|---|---|
| Placement | In hallways outside every sleeping area; add one on every storey and near fuel-burning appliances where local rules or the manufacturer call for it |
| Certification | Canadian certification mark on the alarm and the packaging |
| Testing | Press the test button regularly; Ontario's guidance says monthly |
| Batteries | Replace as the manufacturer recommends; Ontario's guidance says yearly |
| Replacement | Replace the whole alarm by its expiry date; most have an end-of-life signal |
| Combination units | Smoke and CO combination alarms are fine if certified for both |
Rules differ by province. Ontario's Fire Code requires CO alarms in homes with a fuel-burning appliance, a fireplace or an attached garage, installed adjacent to each sleeping area, and from January 1, 2026 on every storey of the home as well. Natural Resources Canada notes that CO alarms near combustion appliances are required by building code in many Canadian jurisdictions. Check your own province's fire and building codes, and when in doubt, install more alarms, not fewer.
05 /How do I prevent carbon monoxide problems with my water heater?
- Book a yearly inspection
Health Canada recommends having fuel-burning appliances professionally inspected at least once a year or as the manufacturer instructs. For a gas water heater, that means burner, flame, venting, draft and combustion air.
- Keep vents clear
Health Canada advises inspecting exhaust vents during and after snowstorms, and Ontario's guidance says to keep outside vents for fuel-burning appliances clear of snow and debris. Sidewall terminations of power-vent and direct-vent units are the ones most often buried. See power vent in winter, and for generator and camp-stove risks, water heaters in a power outage.
- Keep the area open
Do not store boxes, laundry or chemicals against the water heater or block its air supply. Do not add insulation blankets to a gas water heater; Natural Resources Canada warns they can block air to the burner and vent.
- Watch for depressurisation
Big range hoods, dryers and fireplaces in tight houses can pull exhaust back down a natural-draft flue. A technician can test the worst case with all fans running.
- Revisit venting when other appliances change
Replacing a furnace, sealing up a house or adding a large exhaust fan can change how the water heater drafts. Ask the installer to check it.
- Consider the vent type at replacement
Natural Resources Canada notes power venting eliminates the possibility of backdrafting, and sealed combustion units are more resistant to spillage. See power vent vs direct vent.
Electric and heat pump water heaters remove the water heater as a CO source entirely, though other fuel-burning appliances may still need alarms. The choosing a water heater checklist weighs that against operating cost, and the Energy Cost Comparer prices the options with your own rates.
06 /What does a technician check for carbon monoxide?
A licensed gas technician checks draft and combustion, not just whether the burner lights. On a typical safety visit they inspect the vent connector, slope and joints; look for spillage at the draft hood with the burner running; measure CO in the flue gases and in the room with an analyser; run the house's exhaust appliances together to test the worst-case draft; check the burner, pilot, flame arrestor and gas pressure; and confirm there is enough combustion air.
If a problem is found, fixes range from reconnecting and resealing the vent, cleaning the burner, adding combustion air or a chimney liner, to replacing the water heater with a power-vent or direct-vent model. The Water Heater Troubleshooter lists CO alarms among its stop-and-call triggers, and you can request quotes for an inspection or replacement.
FAQQuestions people ask
Can an electric water heater produce carbon monoxide?
No. Electric and heat pump water heaters do not burn fuel, so they do not produce CO. Your home may still need CO alarms for other fuel-burning appliances, a fireplace or an attached garage.
Where should a CO alarm be placed near a water heater?
Health Canada says the most important location is hallways outside sleeping areas. Many owners also place one in the utility room. Follow the manufacturer's placement instructions and your province's code.
Can a gas water heater leak carbon monoxide when it is off?
When the burner and pilot are off, nothing is burning, so no CO is produced. A standing pilot burns continuously and produces a small amount that the vent normally carries away.
Will I smell carbon monoxide from my water heater?
No. CO has no smell. You may smell other combustion by-products from a badly burning appliance, but only a CO alarm reliably detects CO.
Why does my CO alarm go off near the water heater only sometimes?
Intermittent alarms often point to backdrafting when fans, a dryer or a fireplace run, or to spillage in cold weather. Treat every alarm as real: get out, call from outside, and have the venting tested.
How often should a gas water heater be inspected?
Health Canada recommends a professional inspection of fuel-burning appliances at least once a year or as the manufacturer instructs.