is there a different co limit for residential furnaces, i always thought 400 ppm CO was the max allowable? i remember something about 100 ppm for residential boilers, does anybody have any info I wish they would just put the limits in our code books but i have yet to find any, recently a company went to a distant family members house a red tagged there furnace saying the heat exhanger is defective due to a sample of 140ppm CO was taken, now i stripped the 16year old kenmore down there is definately no crack i have to return because i want to take combustion readings myself just wandering if the limits are different in resi ???