What is a B Vent or Natural draft gas fireplace?

These type of fireplaces do not have a sealed combustion chamber and relies on the buoyancy of hot gases to vent effectively. They must be installed either with an existing chimney that meets the local building code standards or they can be installed using a B vent which is a vent pipe that must be installed to the same parameters of a masonry chimney but can be enclosed in combustible material. They draw air (oxygen) necessary for combustion from the area surrounding the fireplace. The combustion by-products or flue gases are hot and naturally rise up through the chimney which provides a route for these hot gases to escape from the house. As the fireplace continues to vent these flue gases up the chimney, the walls of the chimney heat up, which allows the flue gases to retain more of their temperature and buoyancy thereby increasing the speed with which they exit the chimney.