How my rocket stove boiler works

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.

2 thoughts on “How my rocket stove boiler works”

  1. Cool web site, I’m going to have fun looking at all your projects.

    Can you explain your refractory mix that you used and also I noticed that you changed your fresh air intake both with the tube your using and also I noticed you change the position of the air holes in your feed chamber.

    Thanx, Click

    1. Thanks, you are right about the change on the fresh air intake. When I forced the air in the bottom, it worked like a blast furnace and gasified the fuel in the feed tube. It increased heat output and fuel consumption too much. It would be a great way to fuel a steam boiler, but was just too much for a heater. By feeding the air the other way, it preheats the fuel and provides just enough air to maintain a clean, even burn.

      The refractory mix I used this year was a bit different from my original. This year I used cat litter, perlite, sand, a bit of portland cement, and paper pulp. The cement helps it hold its shape better during the curing process and further out from the inner part of the stove, gives it more durability. The paper pulp helps prevent spalling and adds air space which further increases insulating properties.

      The perlite adds insulating properties, and the cat litter is high in magnesium oxide, which stands up to heat remarkably well (found in commercial refractory). The cat litter is prepared for use by adding hot water and stirring/grinding until it has a clay like consistency. The sand is mostly used as a cheap heat resistant filler.

      Enough cement is added to hold the mix together. I add just enough water to give it the consistency of concrete.

      For ratios, I use 2 parts prepared cat litter, 2 parts perlite, 1 part sand, and 1 part paper pulp. I then add water and portland cement until it has the workability I prefer. It should be very similar to concrete ready to pour.

      As a kind of funny aside, be sure your perlite doesn’t have fertilizer in it. I couldn’t understand where the urine smell was coming from in the mix until I realized the perlite contained fertilizers.

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