Inspect the flue piping for any signs of cracks that could cause a leak, and also check for proper upward pitch. This is the last path for flue gases to travel before they leave your home. Most furances have an induced draft motor and although the burner area is in a negative pressure system, the flue piping is in a postive pressure system. This means any leak in the flue piping will likely cause the flue gases to be pushed out into the home.

Upward pitch of the flue pipe is of paramount importance to check for because of the simple fact that heat, and thus hot flue gases, will rise.