When 1 cubic foot of natural gas is burned with just enough air for complete combustion, it produces 11 cubic feet of combustion products (as shown in the illustration on this slide). This total includes 1 cubic foot of carbon dioxide and 2 cubic feet of water vapor formed in the burning of the gas. It also includes about 8 cubic feet of nitrogen, which did not take part in the burning of the gas. In virtually all burners, some excess air is used. This volume of excess air is added to the combustion products flowing from the flue outlet. A provision is made on most equipment to vent (remove) these flue gases from the home.
Excerpt from Gas and Oil Heating – Preparing for the NATE Exam