The pictures below show flues that are not properly connected to the chimney of the home. Openings are present where gas can leak out into the house. A reading was taken with a carbon monoxide detector - levels were indeed elevated in the basement and the occupants of the home were in danger of carbon monoxide poisoning.
Check the flue pipe connections on your heating system, or better yet have your oil or gas contractor do it during a tune-up and cleaning. They can be easily fixed but can be fatal if not maintained and checked.
