Yesterday evening I inspected a house that had a horizontal gas, forced air furnace in the attic that appeared to be manufactured in 1987 or 1988 (Singer, serial # 788749P01), which had a carbon monoxide reading that was mostly around 4ppm, topped out briefly at 8ppm, and went down to 3ppm as it ran more. Ambient CO outside the house at eye level varied from 2-4ppm, ending at 2ppm when the furnace was at 3ppm. The house was only about 100 yards east of an elevated freeway, and the wind was westerly, and variable.
Note: I’ve had ambient readings as high as 13ppm east of freeways during rush hour.
The fact that the CO reading went down suggests the furnace is not dangerous. I’m wondering what the explanation is. The furnace had a missing filter, and undoubtedly the supply air side of the heat exchanger got a lot of dust, mites, etc., and the furnace p-robably had not been started in months. Could the slightly elevated CO be caused by this burning dust and debris in the heat exchanger?