I've been using an 8-inch add-on catalytic combustor for the first time this season, and it's working out pretty well. I've got it hooked up to an indoor boiler. I noticed that one night when I forgot to engage the cat before going to bed, the fire was pretty well out the next morning. That never happens when the cat is engaged. And, I get zero creosote in my insulated ss chimney. The only problem I have had is accumulation of ash dust on the cat element. It ran fine for 2 months before I had to take it out and clean it. Then it went another month. Then another week. Finally, I discovered that my conventional manual damper was out of position so that when I thought it was open it was partially closed, etc. This allowed the boiler's forced air draft blower to blow dust directly onto the cat element, plugging it up. I never have the cat engaged when the damper is open. So having fixed that problem, I think I'm good to go. However, I also noticed that most of the dust/ash that makes it past the damper winds up in the cleanout. Presumably most of it is heavy enough to fall to the bottom of the cleanout (diagram) and not be carried up into the cat honeycomb. I would like to find a way to make sure that all the dust ends up at the bottom of the cleanout, so that I never have to clean dust out of the cat again (wishful thinking). Any of you engineering types or other wood-burning gurus have any ideas how to do that? A basic diagram of what I am talking about is attached. Oh yes, I did fix the damper. Don't know why they make them the way they do (loose & wobbly), but I just drilled a couple of small holes through the cast iron damper and the steel rod that it hangs on and put a couple of cotter pins through it. Now it does what I tell it to do, which is the way I like things.