A follow up post for posterity, hopefully somebody will find this of use in the future.
I was able to resolve my cat over firing issue by plugging several air inlet holes.
1. There is a ~3/8" hole in the bottom of the stove, just behind the secondary air inlet. it allows air to flow unrestricted up into the ashpan area, I plugged it completely with some ceramic insulation.
2. There are 8 small holes in the lower fireback, ~1/8" dia, you can see them if you open the front doors and look into the firebox. I plugged 6 of these holes with machine screws, sorry forgot what size. These holes allow air from the secondary inlet to enter the firebox. I played around with plugging all of them and fewer, I found plugging 6 gives the best result for my setup.
After this modification the stove is much more controllable, with cat engaged if I close down primary air the cat temps will go down, open it up and cat temps rise, perfect. With a full load of wood and air full open cat will climb to 1200-1400 and hang there for hours. Griddle will run about 600F with full air, about 400 with air full closed. I usually run about 50% open on the air and full wood loads, griddle runs 450-500F cat hangs around 800 - 1000F.
One thing that I have seen happen: If you have a big fire going and you close down the air too fast the cat will climb up to 1400 - 1500 range. Choking the air off that quickly gives you a lot of smoke for the cat to chew on. This is a well known "issue" but is easily solved by bringing the air down in steps. I usually bring it from 100% air, to 50%, wait a half hour or so and then throttle down as desired. Cat will cruise at ~700 - 800F for many hours. If you do have a smokey fire going (because your wife doesn't really understand this concept....), and the cat climbs up to 1500F, opening the primary air up will cause the cat temp to drop back to normal ranges.
Back when I had the cat overfiring I could hear the air being sucked through the secondary path, near the back of the stove. It was not a roar but definitely a noticeable "sucking sound" (to quote Ross Perot). Since I plugged up the holes I have not heard this sound, not even once.
Final thoughts:
- I consider this problem to be solved, stove seems to be operating "normally" and under control for several months now.
- A complete tear down and rebuild was not required to resolve the issue, I never did find a smoking gun or any significant air leaks.
- I don't understand what VC was thinking when they designed this stove. As many of you know the newer 2n1 models do not have a thermostatic control on secondary air flow, only the primary has a thermostat. An uncontrolled secondary air inlet seems like a bad design to me, it might work under certain conditions but when those conditions change it can run away. Trying to manage the balance of primary and secondary air over all possible operating conditions seems like a losing battle to me with one air source uncontrolled.
- By plugging the secondary air holes in my stove I have essentially "tuned" the stove my typical operating conditions.
- Not sure how effective it is to plug the hole in the bottom of the stove (#1 above) I left mine plugged but one of these days I would like to open it back up and see what the effect is. I suspect the fireback holes (#2 above) are the critical ones.
- The stove did not so this for the first 4 years of its life. So the question remains, what changed? I simply do not know, there is nothing I can find. The installation / chimney / controls have not changed one bit, only thing I can think of is different wood or there is still some mystery air leak I have not found. I have given up trying to figure it out....
Nice to have a stove I can trust again, and sleep through the night without the cat alarm going off.
I hope this adventure will be of use to someone.
