The foundation furnace in its classic solution is built of fireclay elements in the place of its setting. In addition to the door, Zduńska buckles or connecting pins and connections of the flue pipe with the chimney it does completely without metal parts. The fire is lit in the combustion chamber lying on the foundation of the furnace. The combustion chamber in terms of its shape and size must be adapted to the smoke ducts connected to it. The whole, in turn, should have such dimensions, so that the chimney is able to carry the exhaust gases from the combustion chamber through the smoke ducts and pull them into its interior. The core of the stove is surrounded by a wide chamotte wall or lined with a tiled coat. This bricklaying can have different dimensions. From the thickness of the wall depends, how long the tiled stove will maintain a high temperature on its external surface.
There is talk of a light, medium or heavy type of construction. The wall thickness of the light furnace is 8-10 Cm . With the combustion chamber completely filled with dry wood (residual humidity approx. 15 -18 percentage) a heating output of approx. 1150 W/m².
The average type of construction is said to be, when the wall thickness is approx. 10,5-12 Cm . The complete filling of the combustion chamber allows to obtain approx. 930 W / m2 heating power. The heavy type of construction has walls that are thick 12,5-14 cm and gives a heating capacity of approx. 700 W/m².
The heating time depends on the type of construction: for light foundation furnaces it is approx. 1 hours, while for heavy-built furnaces it is extended to over 2 hours.
Sufficient oxygen is essential for clean combustion. Because foundation furnaces do not have a separate air supply to the combustion chamber, this must be done through their door. Therefore, in these furnaces, double doors are usually used, which are external, tightly closing plate have a second part. It protects against burning sparks and is equipped with sufficiently large air gaps. Foundation furnaces do not have an ashtray. The ash remaining at the bottom of the furnace serves as a primer for the next fire. In a well-made foundation kiln, a very high temperature of wood firing is created. The outer door of the combustion chamber can be closed only then, when the wood no longer burns open (Yellow) Flame (after ok. 1 Hours.), and a blue glow rises above the heat. These doors are constructed in this way, that when they are closed, the air supply to the combustion chamber is interrupted. The glowing fuel then consumes the remaining oxygen in it. Since cold air no longer enters the combustion chamber, its temperature rises above 1050°C. At such a high temperature, oxygen bound in the wood is enough to keep the heat for many hours. The wood is then burned completely, in addition to a few non-flammable minerals and metals. So there is almost no ash left.
The foundation furnace gives its heat to the living room mostly in the form of radiant heat.
Fireclay elements in the combustion chamber heat up, accumulate the thermal energy of the fire and slowly direct it to the outer surface of the furnace. There is no air circulation in the room which causes the dust to lift, and there is no excessive heat build-up under the ceiling.