My wife and I designed our house for DIY radiant heat. I installed copper heat exchangers in the firebox and stovepipe of the Fisher Grandpa Bear stove I grew up with, and pumped water through it to heat our radiant floors. After a hard winter spent feeding the stove, I realized that a rocket stove would better fit our needs. (Rocket stoves are efficient, convection-driven, biomass-fueled stoves that burn hotter than conventional stoves.)
It took me two to three days to build what I call the “Dragon” rocket stove boiler. It has three main components: the burn chamber, where combustion starts; the riser tube, which is the convection engine that fires the stove; and two heat exchangers. The heated water is stored in a 330-gallon tank separate from the stove. I designed a closed system, meaning no water enters or leaves the system. To build your own “Dragon,” take a trip to your local salvage yard and size up the scrap-metal components, avoiding aluminum.
Learn more about this rocket stove boiler project.
• 1⁄4- to 3⁄8-inch-thick steel for base/grate
• 1⁄16- to 1⁄8-inch-thick steel for burn chamber inner and outer walls
• 1⁄16- to 1⁄8-inch-thick steel for riser tube base
• 1⁄8- to 1⁄4-inch-thick steel for riser tube/connecting tube and riser tube outer wall
• Two 50-foot coils of 1⁄2-inch soft copper tubing
• 3⁄4-inch rubber hose rated for hot water
• Reciprocating saw
• Grinder or plasma cutter for cutting metal
Burn chamber. The burn chamber consists of a base, a grate, an inner wall, a soft copper tubing heat exchanger, and an outer wall with a lid. The diameter of your outer wall should be approximately 1 1⁄2 inches larger than the diameter of your inner wall.
I constructed the base by welding part of an old water heater around the bottom rim of a semitrailer dual-wheel spacer or brake drum. You can use anything that’s made of at least 1⁄4-inch-thick steel and has some kind of a grid/spoke structure to allow air and ash to pass through.
Fit the inner chamber wall over the base to hold firewood. I cut the bottom and top off an old 100-pound propane cylinder.
Wrap 1⁄2-inch copper tubing around the burn chamber to create the initial heat exchanger. Leave approximately 10 inches sticking out beyond the top and bottom of the inner chamber wall. (See “Heat exchangers,” below.)
Cut a notch into the bottom of your outer chamber wall for the copper coil to exit through. Slide the outer chamber wall down over the copper tubing. Make sure the lid fits over the outer chamber.
After you’ve assembled the stove, you’ll want to mark and cut a hole in the base skirt/grate and insert some 2-inch steel pipe to serve as the air intake. Install the inlet at a 45-degree angle, somewhere near the center of the grate. Some of the debris falling out of the stove may be hot coals, so it’s important that the inlet empties downward into a fireproof container.
Riser tube. This is a vertical tube in which the hot gas and flames rise, creating a strong convection force that develops the draft. While the outer chamber wall would not be necessary on the riser tube, if you plan to vent straight out of the riser tube directly into a stovepipe, it will add some insulation to help keep the heat transferring more effectively from the riser tube into the secondary heat exchanger. It will also improve the safety of the riser tube and elbow assembly, which can glow red-hot.
Connect the riser tube to the burn chamber with a 90-degree elbow (or fashion a 90-degree elbow by cutting a pipe at a 45-degree angle). Fashion a support and outer chamber wall to house the riser tube. Wrap 1⁄2-inch copper tubing around the riser tube as its heat exchanger. Put the stovepipe directly inside the riser tube and vent outside the building.
Heat exchangers. Use 1⁄2-inch copper tubing. If you’ll be using a flaring tool to make the plumbing connections, it won’t matter whether you get copper tube size (CTS) or refrigerant-dimensioned tubing. Leave the tubing coiled. Remove any protective rubber caps from both ends and set them aside. Rotate the coil and adjust the loops so that when the coil stands vertically, both ends of the tubing are pointing up. Place one end of the tubing in a garden hose, and turn it on about half-flow until water comes out the other end. This will force all the air bubbles out of the tubing. Remove the hose, taking care to retain the water inside the tubing, and replace the caps on either end. Place the tubing in a freezer for about 12 hours. Ice will push off the caps and protrude a couple of inches from both ends. This step is necessary to keep the tubing from kinking when bending it around the riser tube. To create the exchanger coil for the riser tube, gently straighten approximately 20 feet of the tubing and tape it to the riser tube.
The tape will hold the tubing in place as you quickly wind the copper around the riser tube. The result will be an artful coil reminiscent of moonshine stills. Repeat the process similarly for the burn chamber, placing most of the coil toward the bottom third of the chamber, where most of the heat will be concentrated. Use wire instead of tape to hold the copper when winding this coil.
Use stainless steel hose clamps to connect the 3⁄4-inch rubber hose to both of the rocket stove boiler’s heat exchangers. The rubber hose should handle temperatures up to 212 degrees Fahrenheit. I use a 330-gallon steel tank to store our hot water. Water is by far the most efficient medium for thermal mass heat storage, so we have approximately eight hours of buffer time after a fire goes out before our house starts cooling off. The water is piped from the house into the stove, out of the stove into the tank, and out of the tank back into the house. Another advantage is that the tank moderates the temperature swings in the stove. This ensures that I never risk piping overly hot water into the cross-linked polyethylene (PEX) lines in our concrete floors (which are rated up to 185 degrees under pressure).
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