Why don't propane/NG appliances produce lethal amounts of CO?

I know there must be a simple answer to this, but why don’t gas cooktops, ovens, heaters and water heaters produce CO? Appliances with bigger burners have direct exhaust venting, which might direct most combustion products out of the house, but cooktops and ovens don’t (they’re often operated without an active exhaust fan).

I know that misadjusted heaters and water heaters can be a CO hazard. Do they only produce significant levels when misadjusted? Does propane and natural gas burn without producing CO if the combustion is stoichiometric?

Yes. Burning any hydrocarbon will produce CO2 and H2O, and nothing else if the combustion is stoichiometric.

Actually, it it very difficult to get stoichiometric mixtures of propane or natural gas and air to completely combust to only CO2 and H2O. If you have stoichiometric quantities of reactants, you would end up with CO2, H2O, H2, CO, unburned methane, and carbon (soot). If you started with propane (C3H8), you would also have some ethane and methane along with the unburned propane.

The reason a stove does not pose a CO danger is there is a lot of excess air (oxygen), the amount of gas burned is small compared to the volume of the kitchen, and you generally can see the combustion, or at least, would notice if it started producing a lot of soot. With a furnace or water heater, you generally cannot see if the burner is producing soot and you will typically burn a lot more gas, so you have a lot more H2O and CO2, which is best to vent to outside of the inhabited area regardless of the CO content.

Cecil did a column on this question.

My furnace is 40,000 BTU/hour. The biggest burner on my stove is 20,000 BTUs/ hour. That would boil a 2qt pot in a few minutes. If I tried to heat my house with my gas stove then there would be danger of CO. But keeping the fire low it put out a small amount of CO.