After about two months of 24/7 burning my boiler, yesterday I had to clean the 8-inch add-on cat element. Wasn't sure what I'd find when I opened it up, but it was just covered with fly ash. I took the unit out and gave it a good going over with a shop-vac. Given that the hard-firing days and nights are still to come, it looks like once-a-month ought to do the trick, with a nice, hot bath at the end of the season. I think it will be fairly easy to get at the element without removing the cat unit, because I can go in through the bottom of the clean-out Tee with the vacuum cleaner hose. It's only about 2 feet. I was pleased to see no surface degradation, cracks or other signs of wear and tear on the element. My hope is to get two heating seasons out of it. I think I understood Craig to say that generally speaking, as long as the ceramic substrate is intact, the catalytic coating should be as well.