Having been in the mechanical service business for more than 22 years, I’ve had my share of carboned up boilers to contend with. In most every case, the occupants of the dwelling were unaware of the potential dangers that existed, and didn’t even know that a problem had occurred.
Of all the sooted up boilers I’ve seen, a reasonable and logical cause explained each and every case. One way or another, the incomplete combustion of flammable gas is what is responsible for the production of CO.