An add-on wood furnace is a unit that is placed next to or on top of an existing furnace. The add-on wood furnace uses the same ductwork as the existing furnace and blows heated air into the house.
The add-on wood furnace has its own firebox, where you burn wood to heat the air that is blown into your home. Most units have a blower fan to help circulate the air and distribute heat evenly. The size of the firebox will determine how long you can run the furnace before needing to reload it with wood.
Add-on furnaces are less expensive than installing a whole new heating system, and they are also easier to install. If you already have a fireplace in your home, adding an insert or stove is another option for using wood to heat your home.