A donkey boiler is a water-heating system installed outdoors. Based on the "donkey boilers" on ships' decks for use when the ships are in port, donkey boilers are an economical way to warm water for use indoors or outdoors. The simplest systems involve a metal drum filled with water and heated by a wood fire. Steel drums come in sizes ranging from 5 gallons to 55 gallons. Choose the right size for your needs. A 5-gallon drum is large enough for an outdoor shower, but the largest drums can be used as auxiliary hot water sources for homes.
Build a concrete slab for the donkey boiler. Excavate an area 6 feet long on each side to a depth of 4 inches. Nail 2-by-4-inch pieces of lumber together to build a frame that fits inside the edges of the excavation. Tamp the bottom of the excavation smooth.
Install the supports for the barrel. Set the bottoms of two pieces of pipe in the ground inside the form about 4 feet apart, then cross the pipes until they form an "X." Wrap wire around the spot where the two pipes intersect. Push the bottoms of the pipes into the ground. Repeat the process with the other two bars, making sure the intersection of the X on the second set is at the same height as the intersection of the X on the first set. The supports form two X shapes that the barrel rests atop.
Mix the concrete according to the manufacturer's recommendations. Pour concrete into the frame and use a bow rake to work it into the corners and around the pipes. Smooth the surface with a board. Let the concrete set for 24 hours and remove the frame.
Prepare the steel drum. Unscrew the two bunghole covers. Install a drum faucet in one of the holes according to the manufacturer's instructions. Drum faucets screw into place or are tapped into the hole until the faucet fits snugly. Set the steel drum in place on top of the pipe supports with the open hole farthest off the ground.
Insert a hose into the steel drum and fill it with water. Build a wood fire on the concrete pad under the drum to heat the water. Drain heated water from the drum by attaching a hose to the drum faucet or opening the spigot to fill a bucket.