Carbon monoxide - in mixed water and heating boilers?

first any burning of gas will produce hydrocarbons thus carbon monoxide. appliance age and performance will dictate the c.o. amount its producing. if a flue was blocked, carbon monoxide can backdraft into a home thru the draft diverter. there are levels of carbon monoxide which each gas appliance is allowed to produce by the american gas ociation in a vent. if these levels are exceeded to an unsafe amount, yes carbon monoxide can be fatal. bottom line is, no carbon monoxide is good. gas appliances should be checked by a licensed proffessional if in doubt and carbon monoxide detectors are always recommended in a home as a safeguard. best of luck