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  • by Boris Kukolj
    This is part 2 of my visit with Peter Van den Berg.
    After lunch, Peter took me to Michel Wijnja who builds wood burning appliances that incorporates his firebox design.
    Peter Van den Berg and Michel Wijnja
    Michel is a trained carpenter. He now builds custom stoves, fireplaces, mass heaters and boilers, generally with a metal frame and a firebox made of refractory concrete.
    His productions are remarkably well finished; it’s as if they came out of a factory. To achieve this, Michel takes advantage of modern technology : he designs everything in AutoCAD and sends the cut & bend lists to his local sheet metal shop which is equipped with a CNC laser cutter and a CNC press brake. The cutting quality is such that the parts are ready to be welded together and spray painted without any sanding or grinding. Michel builds his own molds and casts the parts for the firebox and sometimes the outer shell. Each unit is pre-assembled in his shop and fired before being dismantled and re-assembled at the final location.
    Michel manufactured a wood fired boiler to heat his own house which is also self-built : 1200 sqf (600 main, 450 upper, 150 shop).
    Standard walls in the Netherlands are made of (from inside to outside) 4″ Poriso insulating bricks + 3.25″ rock wool insulation + 1.25″ air space + 4″ solid clay bricks.
    There is very little heat coming out of the unit as the stainless steel sides contain 2″ of ceramic wool. Thermal storage is in the 19 tons of concrete of the radiant floors (3″ thickness). The system is tankless; between the daily firings water temperature in the floors will go from 50°C to 17°C and room temperature from 23°C to 17°C.
    Total weight of the boiler is 450 kg : 1/8″ mild and SS steel, 6 bags of castable refractory, some ceramic glass and some ceramic wool. The pump and controls and located in the space under the firebox.
    Note : Superwool seems to be the favorite brand of ceramic wool in Europe. Its fibers are water soluble which means they don’t get permanently lodged in the lungs if inhaled.
    Peter’s firebox was lengthened by 8″.
    Temperature was 100°C when we started the test.
    Walls are 1.25″ thick and insulated with 2″ of ceramic blanket.
    Loading the firebox.
    Burning about 10 kg of scrap wood per day to heat the house and shop.
    Heating costs : nil
    4:34:16 (13 min.) Testo data : 93.2% efficiency, 692 ppm CO, 10.0% O2, 191% excess air, 120.9°C stack temp.
    The box with the holes is the heat exchanger. Flue gases exit the riser in the back and down draft through the tubes piercing the stainless steel box,
    then, they exit through an opening behind the ash tray (not visible, sorry) and enter the chimney.
    Top of the heat exchanger chamber : bypass exit
    Testo numbers : this is as good as it gets : low oxygen, low excess air, low CO, adequate stack temperature, high efficiency
    Half of the octagonal riser (1.25″ wall thickness).
    The refractory concrete parts will be baked at low temperature to allow the excess moisture to evaporate. They will be assembled with calcium silicate based refractory mortar, tied together with boxing wire and wrapped in 2″ of ceramic blanket.
    Detail of firebox mold.
    This is Michel’s latest project
    For more information,
    Michel’s forum : Rocket 177 – Technologieforum (many videos of Peter’s firebox being fired)
    His blog : Houtkachels en meer (detailed pictures of Michel’s various creations)