If you have an existing oil or gas furnace with the accompanying ductwork, you may wish to consider an Add On Wood Furnace. Unlike a wood stove, this wood furnace has its own blower and is designed to have ductwork attached to it. The problems encountered with wood furnace add ons are due to the duel blower situation. Two blowers sharing the same ductwork presents an airflow problem. If the return trunk line is tied into both furnaces, one blower will try to pull backwards thru the other.
A seperate and distinct return is recommended for the add on. The supply trunks can be shared, but should be shared in a dynamic way to direct the air in the proper direction, and not plenum to plenum which might also cause a backward flow thru one unit.
Another method would be to use zone damoers which would open or close the connector ducts depending on whcih unit is operating. These dampers would prevent backfeeding but would restrict operation to one unit at a time, both could not run simultaneously.
You CANNOT vent the wood furnace into the same chimney flue as the other furnace. You cannot vent the wood furnace into a flue used by any other appliance, it needs its own chimney flue. The chimney flue cannot be brick lined and cannot be gas vent or type B. It must be a lined chimney or an all-fuel type chimney. The size must be at least the size of the exhaust on the furnace, and no bigger in area than three times the area of the furnace exhaust.
Many unique building codes apply to wood burning furnaces. The duct clearances will be larger than standard clearances and there will be combustible air requirements. Best to check with your local building department before purchasing. Also, most manufacturers will not stand behind the warranty unless the installation is by a qualified technician, so do-it-yourselfers should be advised. Most homeowner insurance companies might feel the same way.