My Tarm made it through 10 full heating seasons before the the tunnel cracked and broke. I ordered a replacement tunnel, which comes as a kit including a replacement nozzle and connecting parts. The removal of the old, in pieces, was easy, as was installation of the new nozzle and tunnel. The nozzle, which just looks like a rectangular piece of refractory with a slot in it, actually is much more complicated. It has rows of small holes in it with air passages that connect with the secondary air supply, all of which then introduces secondary air directly into the slot as the hot gases from the primary combustion chamber pass through the slot. The result, of course, is the intense gasification secondary burn which swirls in the tunnel. There are other refractory components which were still in satisfactory condition and not replaced. One important lesson learned is not to let the nozzle deteriorate too much before replacement, as an overly worn nozzle can result in wear to other refractory parts. Fortunately, that did not happen with my Tarm.