Use of this renewable energy is a great environmental advantage. Because the firebox is quite large (44" x 36" x 30", 56" x 48" x 36" or 72" x 60" x 40", depending on the model), a good burn time between firings can be achieved, even when using waste wood or other lower quality wood. The burn times can vary from about 12 hours on up to 96 hours or more, depending on the heat load and the size of the outdoor wood furnace. The combustion air intake is thermostatically controlled to maintain a constant water temperature of about 185 degrees F. This controlled combustion can extend burn times between fills to literally days when low heat demands exist. About 80% of the outdoor wood furnaces being operated provide 100% of the heat for the buildings, homes and domestic water that they service, though the installation manuals of all the listed units require that a backup heating system be in place.
The building can be heated through any means of hot water heat transfer. As strange as it may seem, the greatest percentage of these outdoor wood furnaces (boilers) are installed with existing forced air systems. Although, when you think of the number of heat pumps installed with forced air systems, which have the same type of fin tube exchanger, the high number of forced air systems used does correlate. The water-to-air heat exchangers provide a very efficient transfer of heat to the entire building through the existing ductwork. These water-to-air exchangers are installed below the air conditioning coil, if present. They are very easy to install but do require a large investment.