Plumbing in the Home/Water heater and CO


This may seem like a silly question, but can a water heater release carbon monoxide if no hot water is being used? Or can it release it anytime that it is on. (I know they don't produce carbon dioxide unless something is wrong, but I just need to know how to test it with my co detector.)

Carbon monoxide is a product of incomplete combustion of fuel. No fuel burning appliance is capable of achieving perfect combustion so if it's burning, it's usually producing carbon monoxide.  Actually, if the combustion were perfect, the only product would be carbon dioxide and water. So your statement about producing carbon dioxide if something is wrong is actually incorrect.

A gas water heater will continue to run after you stop running hot water.  As you draw off hot water, cold water enters to keep the tank full. This lowers the overall temperature of the water in the tank and the thermostat on the water heater will fire the burner to bring it back up to the set temperature. The burner will continue to run until this set temperature is reached, whether you are drawing water off or not.

The best sign of poor combustion, and therefore a problem, is sooting. Excessive soot, either in the burner area or around the vent hood at the top of the water heater is a sure sign of problems that need to be checked out and corrected.
Good Luck,

Plumbing in the Home

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Dana Bostick


Pretty much any residential plumbing questions. For "item specific" details such as a specific model of fixture, I will need to research and there may not be any useful information available. Note: I live and work in Southern California. We do not, as a rule, use hot water or steam heating systems, oil fired boilers or private water wells so my knowledge in those areas is pretty limited. There are others here on AllExerts that can probably answer those questions better.


Retired, Licensed General Contractor with Plumbing license. Active Home Inspector, Litigation Consultant and Infrared Thermographer, Online Marketing specialist.

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