Many posters here have expounded about using the returns to circulate heat from their wood stoves. If I get time tonight I will produce code language limiting or prohibiting this practice. There are real dangers present. Individuals have not factored or taken preventive measures to insure safety. The current heating ducts are never exposed to an open flame appliance or the possibility of exhaust gasses they produce heat in a sealed heat exchanged compartment. Doors are not constantly being opened in the compartment likewood-stoves . As much as we may want to believe wood stoves are not all that tight not compared to a sealed combustion chamber and sealed exchanger compartments. One of the first code aimed at this practice, is no flexible ducts can be used to extract or transmit heat from a solid fuel burning appliance. Floor joist pan bays cannot be used because all ducting has to be non combustible, the wood of the joist is combustible. To be safe a smoke-damper would have be installed within the system, also a monoxide sensor or alarm . The monoxide sensor or smoke detector operated damper, would either be activated and close down the system to eliminate further spread of smoke and monoxide. I have ASked NSF to look into this. Expect more codes to address usage of systems not listed for this purpose or designing compliant safe systems.