I have one of those old old green monster gravity furnaces w/ asbestos ductwork. The homeowner is going to remove all the asbestos and leave the boots intact and a 12" section at the furnace for each run. My question is, can I replace all the ductwork w/ wireflex or would it be too hot and possibly melt it? Some people say I can some say it won't last long at all. Should I use alum flex instead?