I utilize a handheld UEI CO detector when I do my service calls. it is sensitive enough to go down to one part per million. I have held the detector at the opening of the external flue pipe of a 92 percent furnace and have only read a high reading of 13ppm.
Hypothetically if the furnace had a crack in it would the supply air read this high of a level of CO?
In cases of hair line fractures of the heat exchanger would any CO be detected?
In a situation where CO is leaking into the occupied space do the levels build up thus increasing the ppm count?
So in this case will the furnace need to operate a length of time to obtain a realistic walk around reading?
What is proper technique in using my detector in conjunction with furnace operation to get a thorough assessment of outputs and to protect my clients.