A kachelofen is defined by a number of standards. Only if all requirements are fulfilled the term kachelofen is correct. One definition comes from the German Industry Standard (DIN EN 15250), another one from the ASTM International (ASTM E 1602 – 1). Based on these technical standards there are national and regional laws which are even more strict (Federal State Law of Salzburg).
Some of the requirements are:
- A mass of at least 800 kg
- The external surface of the masonry heater, except surrounding the fuel loading door(s) and flue connection, does not exceed 110°C (in average on a 300mm by 300mm area)
- minimum efficiency 80%
- maximum dust 35mg/MJ
- wall thickness first draw 75 to 135mm
- wall thickness last draw 50 to 100mm
- high storage capacity
- short burning time compared to heating time
(and many more).
The basic concepts of the kachelofen are that a) energy can be stored as heat and b) energy nonetheless always travels from areas with higher energy levels to areas with lower energy levels by 1) conduction, 2) convection and 3) radiation.
The natural circle starts with sun light (radiant heat) that is converted by trees into wood. By burning the logs the stored sun energy is set free again and stored in the special fire clay of which the core of the kachelofen is built. The burning process sets free exactly the same amount of CO2 trees will use to grow the same amount of wood over the next years, so it is 100% CO2-neutral. After a very hot uncontrolled fire for typically less than an hour the kachelofen needs no combustion air any more and therefore can be closed. Now the system is sealed and air tight to a certain degree which means that no energy can leave through the chimney any more. The stored energy is trapped in the core of the kachelofen and there is only “one way out”: over the surface as healthy, cosy and natural radiant heat. Because of the high temperatures the system burns itself clean with each fire (like the pyrolysis-effect) and less than 1% of the fuel burnt (basically only the mineral parts of the wood) remains as ash in the combustion chamber.
A real kachelofen, following all regulations mentioned above, is always built brick by brick on site as it is nearly impossible to transport a fully ceramic structure of such a weight over long distances in one piece!