This invention relates to wood or coal burning devices such as stoves or furnaces. 
It has long been recognized that wood and coal contains a very substantial potential of thermal energy which can be released by burning.  The early wood or coal burning furnaces were, however, very inefficient in utilizing the thermal energy released during combustion.  In most of these stoves or furnaces, ambient air was heated by being brought into contact with the outside walls of the furnace.  A problem associated with these furnaces is that most of the thermal energy generated during combustion is lost through the flue or chimney. 
One way to increase the efficiency of heat exchange is to suspend a fire or combustion chamber within a housing so as to create an air chamber through which the air to be heated can circulate.  The air in the chamber is heated by the thermal energy passing through the walls of the fire chamber.  For example, in U.S. Pat.  No.  4,140,101 to Glover, entitled WOOD BURNING STOVE WITH FORCED AIR HEATING and U.S. Pat.  No.  4,206,743 to Niemela, entitled WOOD BURNING APPARATUS, air enters the air chamber through an inlet located near the bottom of the air chamber, rises by convection current through the chamber, and then exits out an air outlet located near the top of the chamber.  Ambient air is thus heated by passing through the air chamber adjacent to the fire chamber as well as by coming into direct contact with the outside walls of the furnace housing. 
In U.S. Pat.  No.  4,128,094 to Lewis, entitled HEATER, air is circulated in a circular pattern around the fire chamber in order to increase the surface area in which the circulating air is exposed to the hot fire chamber walls. 
The above stoves or furnaces are much more efficient than stoves which do not incorporate an air circulation chamber Nonetheless, the bulk of the heat generated by combustion is still lost through the flue. 
Another disadvantage to many wood burning furnaces is that the log grate is suspended within the fire chamber itself.  The ashes falling to the bottom of the fire chamber act as an insulator to decrease the transfer of thermal energy across the chamber walls below the grate.  This causes an increase in heat loss through the chimney, thereby decreasing the efficiency of the system. 
Therefore, a need exists for a wood or coal burning device which includes structures whereby the maximum amount of heat may be extracted from the device by the ambient air circulating through it.  Further, a need exists for a wood or coal burning furnace which can easily and safely be maintained.