I have read several posts that say only metal ductwork should be used on wood furnace installations. I understand that the temps can get very high if the fan is not running due to a power failure or fan failure. Does this mean no flexible ductwork anywhere on the system? (ie. main trunk is metal but take offs are flex) At some point it would have to be safe wouldn't it due to air temp drop or metal duct acting as a heat sink? I can see the tie in from furnace to main trunk not being flex and maybe even a minimum distance of metal trunk but I gotta believe there are alot of add-ons that hit existing systems that would not have all metal duct or proper clearances from combustibles. So whats the code say in your state?