I get a lot of questions pertaining to how my rocket stove furnace actually works. Hidden beneath a layer of concrete and 4″ of cast refractory are the inner workings. Unfortunately, I didn’t have time to take pictures as I built it, so I’ve taken the time to make a simple diagram of its internals. Take a look:
We’ll begin with the right hand side at the feed tube. It is made of three lengths of 2′ long 8″ black stovepipe. The joints are held together and sealed with red RTV silicone. It sits about a foot off the bottom of the firebox, making a total of 7′ of wood capacity. Combustion air is fed through the top around the fuel. It is preheated from waste heat collected from where the feed tube sucks heat from the firebox.
The firebox is cast around a 8″ cylinder and is 4″ thick on all sides. At the ash cleanout, I made a door out of refractory with a rebar handle. It, too, is 4″ thick. The heavy insulation retains all the combustion heat possible to ensure a complete burn of all wood, tars, and wood gases.
The heat riser is 2′ long horizontally, takes a 90 degree turn and continues up for another 2′. The purpose of the heat riser is to combine the wood gases and tars with the remaining combustion air to eliminate all smoke. The end result is a mixture of hot gases consisting mainly of carbon dioxide and steam. The flame stops before the end of the heat riser if the fire is maintained properly.
The hot gases then rise another 3′ through a chimney with 2″ thick insulation. Within this insulated chimney is 50′ of 1/2″ copper pipe at the bottom and 60′ of 3/8″ copper pipe at the top. The pipe is coiled and placed snakelike in the chimney to pull as much heat from the exhaust gases as possible. The water is pumped in from the cold side of the system at the top and exits at the bottom.
The exhaust takes another 90 degree turn and exits about the same temperature as a dryer vent would.
Many people have voiced concern over creosote production. In a normal wood stove and pipe situation their concern would be legitimate. Because of the heavy insulation, the creosote has been burned in the firebox and heat riser before the exhaust ever comes in contact with the copper coils.
If you have further questions, feel free to leave a comment below.