im in the process of installing an add on wood furnace. i was at menards last night just to see what they got and what i can use. my question is has anybody ever used the aluminum flex duct to run from the add on furnace to the plenum of the existing furnace? also the add on furnace was bought used and there are 8 out of 12 firebricks that are cracked, does this hurt the operation of them or is it just cosmetic? thanks