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  • Visit with Peter Van den Berg

    By Boris Kukolj

    During a trip to the Netherlands in November, I visited Peter Van den Berg at his home in Den Haag. We spent the day together and he kindly showed me what he has been working on.

    Peter is a cabinet maker by trade and worked as a composite material specialist for Fokker Aircraft and Delft University of Technology. He is now retired and is devoting his free time to the design of clean wood burning appliances, mostly incorporating rocket stove features. His designs are open source and can be found on the following forum : Home | Rocket Stoves.. Experimenters corner.. Answers questioned!

    His firebox plans are incorporated in a variety of wood burning devices, from boilers to masonry heaters including Dragon Heaters in Texas (Wood Burning Stoves, Heaters and Appliances – Dragon Heaters) and Walker Stoves (WalkerStoves.com – Home) in the Pacific North West of the United States, both contestants at the Wood Stove Decathlon.


    Peter’s present workshop is in a large uninsulated horse stable. This is his latest “batch box rocket” prototype. The heat exchanger formed by three oil drums stacked on top of each other has a very large and almost instantaneous heat output but no thermal storage. With 5 to 7 kg loads, stack temperatures won’t exceed 150°C.

    Peter’s Testo had to be fully reconditioned after getting clogged by flue gasses contaminated with the mineral oil left in the drums.


    The riser is made of vacuum formed refractory ceramic fibers and weights 1.5 kg. Inside diameter is 15 cm (6″). Temperatures increase very quickly in this environment, thus limiting the escape of unburned flammable gasses.


    The process of developing this firebox took about 4 months. Very small changes were made at a time and tested with the Testo. Defining the shape and the size of the port proved to be particularly long.

    In this system pyrolysis happens in the firebox and combustion of flammable wood gases mostly takes place in the riser.

     The P-channel (the rectangular steel tube) delivers secondary air above the port (the narrow vertical slot between the firebox and the riser). Negative pressure in this area combined with the acceleration of the gas flow due to the restriction in the port ensure the good mixing of flammable gases and oxygen.
    What Peter is trying to achieve is :

    1/ gain efficiency by lowering excess air instead of lowering stack temperature

    2/ induce turbulence through shape instead of through complex air injection.


    First step of firing procedure (cold start) : a small amount of kindling is burnt in the back of the firebox to heat up the port and the riser.

    Note : the ashes on the bottom is what was left from the previous fire.


    2nd step : burning half a load. Main air supply is on the bottom of the door. The bottom of the firebox channels the air to the back.

    12:04:23 : no Testo data yet as Peter starts recording when oxygen gets below 20%


    3rd step : full load of cordwood (about 6 kg). Most of the combustion happens in the back of the firebox and in the riser.

    12:30:27 (21 min.) Testo data : 94.3% efficiency, 115 ppm CO, 10.8% O2, 206% excess air, 93.2°C stack temp.


    Peter recorded temperatures as high as 1200°C on the bottom of the riser.

     12:55:16 (46 min.) Testo data : 92.0% efficiency, 54 ppm CO, 9.3% O2, 179% excess air, 141.9°C stack temp.

    Peter and his wife Martina during lunch break. They are looking forward to be moving to their new passive solar house which is under construction in the outskirts of Den Haag. A high mass masonry heater will be its main source of heat. The flat roofs will carry enough solar panels for the house to produce as much electricity as it uses. This time Peter’s workshop will be inside the building envelope.

    Visible in the back : vibrating table and laptop running Testo software.

    Note the open door as it was getting fairly warm.

    13:01:59 (52 min.) Testo data :
    90.5 % efficiency,
    619 ppm CO,
    12.1% O2, 236% excess air,
    130.8°C stack temp.

    Below Peter is explaining how to size the firebox and the air inlets. He feels that a 10-12 kg load is probably the maximum for a system based on an 8″ riser. Two back to back fires would make it possible to charge a large heat exchanger.

    ————————————————————————————————————————————————————————————————————————«Now the dimensions of the batch box rocket core.

    The assumption is, there should be a common base number to which all the other dimensions are related. That base number is derived from the diameter (fictional or not) of the riser diameter.

    Base dimension is 72.34% of riser diameter.
    Width of firebox is 2 times base.
    Height of firebox is 3 times base.
    Depth of firebox is 4 to 5.5 times base.
    Height of port is 2.2 times base.
    Width of port is 0.5 times base.
    Height of riser is 8 to 10 times base, measured from the firebox floor.The firebox floor do consist of a narrow flat suface the width as the port. Left and right there are 45 degree slopes in order to concentrate the glowing charcoal in the middle. Those 45 degree chamfer is part of the dimensions of the firebox. In addition, there’s also a similar shaped piece at the rear bottom of the riser.The total air inlet is 25% of riser cross section area. Riser could be round or octagon.
    P-channel is 5% riser csa.
    Main inlet plus window wash is 20%. Main inlet could be larger when starting cold and is situated level with the floor of the firebox.
    P-channel should be as wide as the port or slightly more, for the calculation of the 5% you should take the width of the port, not the actual width of the duct. This duct is hanging over the top of the port the same distance as the depth of the duct.
    The back of the p-channel which is resting against the firebox rear wall is been cut away over the height of the overhang».


    (photo, casting & metal work by Michel Wijnja)