Downstairs system has 10ppm of carbon monoxide. Upstairs system has 16 ppm of carbon monoxide. Due to health and safety concerns I have disable the heat function on these furnaces until the customer has resolved the carbon monoxide issue. The ductwork is old grey flex with duct board mixing boxes and it is all starting to break down. The complete downstairs system is from 1999 and the upstairs furnace is the same age with the cooling side being from 2010. I would recommend replacing these systems due to age and I would recommend replacing the ductwork for a higher efficiency.