Ok I have a steel chimney liner that was installed along with my add on wood furnace. The liner of course goes up through an existing masonry chimney. The owner of the store I purchased it all from told me a good way to clean this steel liner without a brush is to have a nice bed of coals in the wood burner then throw in a few cardboard boxes then open up the bottom ash door and this will clean the flue. This premonition just scares the hell out of me. Anyone do this ? I fear something hot getting on my roof and maybe catching on fire.