Abstract: An improved heating system for a building structure is described. The improved system utilizes heat generated by an enclosed natural fuel burning device such as an auxiliary furnace as a supplemental source for a standard (primary) furnace heating system. The auxiliary furnace is used to heat water or another heat exchange medium in a heat exchange member situated within the auxiliary furnace. When the water reaches a predetermined temperature, a first pump is activated and causes the water to be pumped throughout the radiators within the building and subsequently returned to the auxiliary furnace. When the temperature of the water falls below a certain level, a thermostatic switch deactivates the pump. When the temperature in the building declines below the thermostat setting for sensing the ambient building temperature, the primary furnace and a second pump are activated which in turn heat the water and circulate it throughout the structure.