Coaxial, which is mounted from two pipes, one of which is inserted into another. It is important to use elements of different diameters. The gas passes through the internal pipe from burning fuel, and it moves due to the operation of a special fan located in the furnace. Fresh and clean air, which maintains burning, enters the cavity between the pipes to the furnace equipment. Most often, such a design is installed in the bathhouse, since it provides excellent heat safety.
