Myth #8 - CO is only caused by incomplete fuel combustion.  (Wrong!)

For the sake of argument, let's examine an atmospheric vent gas furnace. 

Atmospheric vent means the furnace depends on warm air to rise and carry the products of combustion through the flue and out of the house.  There's no powered draft inducer to push or pull air through the heat exchanger.  In the vernacular they call this a "positive draft in the flue".

If you misadjust the flame on the gas burners by closing down the primary air shutters, you'll see an increase in carbon monoxide in the flue.  If you crank the gas pressure to the burners way up, you'll over fire the furnace and see a little more CO in the flue gasses.  At any stage along the way you can say that the CO was caused by "incomplete" combustion.  But, because the CO levels are relatively low (still under 400ppm) and they are being carried out of the structure by the flue, there is NO immediate reason for alarm.  Putting it another way, incomplete combustion does not necessarily lead to CO poisoning or CO health problems.

Now, with a perfectly adjusted furnace (with 0ppm CO output) there is almost complete combustion.  But, if a 300 pound duck sits on your flue cap (on the roof) and crushes it, guess what!  You're well adjusted furnace will soon start producing very high levels of carbon monoxide because the flue is blocked.  Given a little time, the CO will migrate into the home and that's when real problems can occur.

Is it the furnace's fault?  Nope!  Is there "incomplete" combustion?  Yep!  Did the "incomplete combustion" cause the problem? 

"Incomplete combustion" is a symptom! 
It means go find out what's really wrong!