The add on wood furnace is a cheap heating instruments connected to the furnace plenum and existing ductwork. It can either be used to supplement the current heating system or function as the primary heat source. Usually, these burn either wood or coal. This aspect permits to switch between fuels when one becomes cheaper than the other.
The installation of an add on wood furnace is a bit tedious job. After installation, one gets two furnaces, thermostats, blowers, plenum and chimneys.
The installation has to be done by a licensed HVAC professional. An expert who has been installing wood furnaces for the last 30 years, doesn't mean their first attempt at installing an add on wood furnace will be a complete success.
Another problem found is, a convenient way to store and transport a winter's worth of firewood without making hundreds of trips up and down the basement stairs.
Looking for the least costly way to heat the home will always involve some ifs and buts. There may be some drawbacks involved with, buying cheapest appliance to burn the cheapest fuel.