A homemade outside wood boiler is an excellent way to heat water for home heating. The water can be piped through a car radiator installed in the cold-air return of a standard furnace. With the gas to the furnace shut off, the thermostat will turn on the fan to draw air warmed by the radiator through your home's duct work to heat the house. Outdoor wood boilers are also good for heating water for under-floor heating.
An outdoor wood-fired boiler is a very simple device. Two pieces of rolled steel are welded together with a larger one on the outside and a smaller one on the inside that forms an inner sleeve to contain the wood fire. The area between the inner sleeve and the outer steel holds the water to be heated. Run a piece of steel slightly smaller than a standard stove pipe through the two to vent the fire box. An iron stove door welded on one end allows wood to be added. Most pre-fabricated doors have slats under the door that can be opened and closed to change the draft in the fire box. A round steel plate the size of the outer sleeve on the back end seals and completes the boiler. When complete, the boiler will appear to be a standard barrel stove, but will be smaller on the inside, because of the smaller inner sleeve. The space between the inner and outer sleeves is the water reservoir. Water intakes and outlets will vary somewhat, depending on your use, but in general cold water should enter through the bottom and warm water should leave through the top. Installing a pressure relief valve is very important for safety.
A pump will almost always be required to circulate the water through your heating system. The size of the pump will depend on the amount of water and the distance the water has to move. A good general starting point is to consider a one-horsepower pump for most installations. If you are moving a large amount of water or moving the water a long way, consider a stronger motor.
Regulating the Boiler
A number of options are available for regulating the temperature of the water. Both require a thermometer to measure water temperature in the boiler. The simplest way is to manually regulate the draft to control the heat produced in the firebox. Watch the temperature of the water and adjust the draft to limit burn if the temperature gets too high. If the water isn't hot enough, increase the draft. Another option is to install a barometric draft regulator. Adjust the regulator until the water temperature is correct. The barometric regulator will adjust the damper to maintain the water temperature.
Very Cold Climates
Installing the boiler outside is a good idea if it will always be lit. However, in very cold climates, leaving the stove outside and exposed to the elements can result in the water freezing and damage to the boiler if the fire goes out. If you live in a very cold area, consider putting the boiler in a heated garage and setting the thermostat just above freezing to protect the boiler in the unlikely event that the fire goes out completely.
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