This article is a great source when it comes to what we should know and understand about the gas furnace and the combustion process. Generally acceptable results, where to test, thresholds ect. Keep in mind that the average home has multiple sources of potential CO (carbon monoxide), and the furnace is not always to blame. CO does not stratify and mixes/moves with airflow. Follow this guide for the allowable minimum and maximum CO (carbon monoxide) levels in the gas furnace flue.