Can a Water Heater Leak Carbon Monoxide?
On this page (6 sections)
The Short Answer: Yes. A gas water heater makes carbon monoxide (CO) when combustion is poor, and CO can enter your home if exhaust doesn’t leave through the vent. Common causes are a blocked or disconnected vent, exhaust pulled back by fans, a failed power-vent blower, or a starved burner. CO has no smell. If a CO alarm sounds or people feel ill, leave and call 911.
How exhaust ends up indoors
Gas water heaters vent in one of three ways, and each fails differently:
| Vent type | How you recognize it | How CO gets in |
|---|---|---|
| Natural draft (atmospheric) | Metal vent pipe rising from a hood on top of the tank | Blocked chimney or cap, disconnected pipe, or exhaust fans pulling exhaust back out of the hood |
| Power vent | Blower on top, plastic or metal pipe out a wall | Blower failure, separated joints, blocked termination |
| Direct vent, condensing tank, or tankless | Two pipes or a concentric pipe to the outside | Separated or cracked pipe, blocked termination, damaged seals |
Combustion problems make it worse. A burner starved of air, or with a clogged flame-arrestor screen, produces more CO, often with yellow flames and soot.
Warning signs
- A CO alarm going off, especially when the water heater is running.
- Headaches, dizziness, or nausea that get better when you leave the house.
- Soot or scorch marks around the burner door or the draft hood.
- Heavy condensation on windows near the heater.
- A vent pipe that is disconnected, rusted through, or sagging.
- Melted plastic or discoloration at the top of the heater.
The CPSC carbon monoxide fact sheet (opens in a new tab) describes the symptoms and recommends CO alarms on every level of the home and outside sleeping areas.
What to do
- If the alarm is sounding or anyone feels ill: go outside, call 911, and don’t go back in.
- If you notice warning signs but no one is ill and there’s no alarm: turn the water heater’s gas control to OFF if you can do so quickly, and call a qualified gas technician or the gas utility. Many utilities will check for CO.
- Don’t run the heater to “test” it, and don’t tape over separated vent joints.
- Install or test CO alarms before using the heater again.
A rotten-egg smell is fuel gas, not CO. That is also an emergency; see water heater closet smells like gas.
What the technician will check
A technician measures CO in the room and in the exhaust, checks that the vent draws properly with exhaust fans and the dryer running, and inspects the full vent run and termination. They clean and inspect the burner and flame arrestor, check combustion air openings, and measure gas pressure for over-firing. On power-vent units they test the blower and pressure switch. If the furnace shares the vent, it gets checked too; see water heater and furnace not working.
Repair or replace
Most CO causes are fixable: reconnecting or replacing vent pipe, adding combustion air, cleaning the burner, or replacing a blower. Replacement is needed if the tank leaks into the combustion chamber or the heat exchanger of a tankless unit is damaged.
Frequently Asked Questions
Q: Can an electric water heater produce carbon monoxide? A: No. Electric resistance and heat-pump water heaters don’t burn fuel. If a CO alarm sounds in a home with an electric water heater, look for another source such as a furnace, range, or attached garage.
Q: Does the white vapor from my water heater vent mean CO? A: Not necessarily; on high-efficiency units it is often water vapor. See white smoke coming from water heater. If an alarm sounds, act on the alarm.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
Editorial standards and corrections · Prepare a useful service request