Yep, you're right, I knew I missed something. Like Greg mentioned though, most of the 8" flue boilers will narrow to the 6" flues without a problem. If you have an adjustable draft (like most are) they will usually encompass the difference, or tapering. To be honest, I don't remember my flue size on the double wall stainless system I have now....either 6 or 8"...gotta check that, it's been awhile (10yrs. or so). The KAA2 has a 6".
I also mentioned previously that the KAA-2 doesn't have the fuel oil backup option, and that the KA4 was extinct, so for those of us who already possess a fuel oil system, and where the KAA-2 will handle the heating chores, the fuel oil backup issues are a moot point, providing space isn't an issue.
The difference in gross btu output between the Harman VF3000, and the Keystoker KAA2 is about 5 - 6k btu, but the difference in price is almost $2k. Most requirements (or choices) for heating don't fall in such small parameters as 5 or 6k regarding btu output, so it's either smaller or larger in most cases. I think that might've been part of the thinking as to why Keystoker discontinued the KA4, which by the way was more or less a match to the Harman, sans the water coil option (Harman), and quite possibly the fuel oil backup. Their thinking was the same as I mentioned here regarding the KAA2. Since the KAA2 was rated at 90k btu gross output, and the KA4 was 96, why produce both ? The difference between them was very minimal as far as output, and covered most of the same area there, more than not. The biggest differences was boiler water capacity (24, 40 & 55 gals.) and hopper size. As I mentioned before, hopper size on the KA4 was much closer to the KA6 though....225lbs. for the KA4, 275lbs. for the KA6, as opposed to 100lbs. for the KAA2
Harman only has the one stoker boiler (VF3000), and I'm sure they targetted the average household when they designed it that size. In my case, as with many, a good thourough researching of this site/forum provides valuable information with which to make an educated decision regarding the most applicable type, brand, or size of coal unit to buy. And, I might add, which falls within most of our meager budgets...
Many guys on here (such as yourself) bring up various points, small or large, that we might miss......which in effect could change our decision on a particular model or make.......we all appreciate that, believe me.
Overall, in my situation, the Keystoker was the "hand's down" winner overall. But that was for me. I covered my dislikes with the Harman, and none of them really involved the workings of the machine at all, except maybe for the DHW coil being an add on option, whereas the KAA-2 (as well as the other Keystoker boilers) were included. My issues with Harman were with the dealer/customer policies that the company has in place, as well as the insistance on the "professional installation" and it's affect on the warranty. Which by the way is only 5 yrs. in relation to Keystokers 10.
Those issues, along with the price difference, are some big detractors.
One of the big things I considered the other day, was that due to the big demand now for coal burning boilers (and furnaces), and since Keystoker is so back logged, many will cave in and buy the Harman instead of going without, only later to realize that Harman's policies weren't to their liking. Their machine is a great unit as far as reliability, but nowadays, they all are, for the most part.
I almost went the Harman route, but I hesitated for lack of further research on them, their warranty, etc. I'm glad I held off and waited.....
Sorry so long.....