I have a pre-epa airtight insert (Alpiner-not sure if that's a brand or model name) and am considering installing an add-on catalyst. The insert no longer has the metal surround and I've run a ss flex liner directly to it's 8" outlet and insulated between the 15" chimney tiles and ss liner w/vermiculite. The flue pipe is easily accessed and it's got a great draw now. I have a pretty good idea of the benefits of a catalyst (longer burn times, more heat from stove, cleaner emissons). My concerns are listed below: 1) Does an add-on really deliver on those benefits? 2) Is it a pain to remove the section of pipe containing the catalyst damper each time the liner is cleaned? Right now, I just sweep everything from top down into the stove. How do you contain the sweeping mess - bag taped to end of flue pipe? 3) Is the combustor replaceable or does it require replacing the entire unit? Comments from those who have or have used these add-on catalysts is appreciated.