If you have a ready supply of wood then wood-fired furnaces are a safe and economical way to drastically reduce your home energy costs. However, this is a far cry from just building a fire in your fireplace. There are a few important things to know before you start spending money. Let's start with the basics.
Most of today's outdoor wood furnaces are hydronic. This means they burn wood in a firebox surrounded by a water jacket. Heated water is then pumped where the heat is needed – your home, pool or spa or other outside buildings.
Because the furnace is outside, proper insulation is crucial for both the furnace enclosure and the water lines. The enclosure should be well insulated (an R-value greater than R-30). Using insulated pipe, such as Flexsul-Seal or Insul-Seal, to transfer the heat to the residence will ensure proper heat retention for your water lines. Once the heated water reaches your home it is used in one of two different ways:
The firebox should be sealed and provide doors for feeding and cleaning. It should include a blower to inject air in order to revive the fire quickly. This keeps the water sufficiently heated on cold winter days when you use a lot of heat.
Some manufacturers offer combination furnaces. They burn both wood and fossil fuels. This sounds nice but the fuel injectors often get clogged with soot while burning wood. When it is time to use fuels the injectors either don't work or are very inefficient.
Many of today's outdoor wood furnaces are only in the 40% efficient range. They also tend to release high amounts of particulate and gases into the atmosphere. The Environmental Protection Agency is critical of these low efficiency furnaces.
By contrast, the Michigan Department of Environmental Quality advises, "Look for a well designed unit that utilizes secondary combustion because they burn hotter and more efficiently and can be a good home heating option. A few models, such as those by Garn, Tarm USA, and Greenwood Technologies have added technology that more fully incinerate the particulates and gas emissions." In high-efficiency systems, the most noticeable difference is the use of firebrick or refractory material to create an environment where secondary combustion can occur. These units use about half the amount of wood as your typical outdoor wood burning furnace.
For a list of outdoor wood boiler manufacturers please visit here.
