Concentric vents make it easier to have a two-pipe installation for high efficiency furnaces. These vents simplify installation and only require one hole through the wall or roof where the pipes terminate. Without using these vents, the installation would require cutting two holes thorough the home (2 Inch or 3 Inch depending on the furnace), one for each pipe. These vents save time and money by reducing the amount of work required. Available in 2 Inch and 3 Inch intake vent sizes.