The new burnpot with the fins DIDNOT resolve the issue. About 6 weeks ago I had a group of three including a engineer from Harman along with the local Harman dealer visit checking out my boiler. After checking everything one of the harman reps (not the engineer) thought the reason for the bubble was not enough air passing through the burnpot. I wasn't to impressed with the engineer when he mentioned that " nothing jumped out at him " as reasoning why this bubble occurs. The rep adjusted the draft (white screw) on the controller for the combustion blower so it gets full voltage for maximim RPM all the time, so much for this being used as a draft adjustment per owners manual. It had been adjusted to spec from the owners manual. I should be receiving another burnpot with a different grade stainless (more heat resistant) burning surface plus the burning surface will be a heavier guage in a couple weeks to try. I have noticed that the bubble that had started to form on the present burnpot, it hasn't got any worse since the tech adjusted the controller for maximum RPM. Could it be not enough air going through the burnpot created by adjusting the draft to spec, I'm thinking it is, the reason that some owners of the PB 105's and PF100's are not having the burnpot bubble issue.