Jess Reyes wrote:Ah thank you Glenn!
One more question about the ducting inside the mass- I've seen some recommendation that the first meter or so be stovepipe flue-grade (e.g. stainless steel), rather than HVAC ducting-grade... is that necessary?
Caveat: I'm also working up to my first RMH build. I've been looking into this same question.
The common concern with using HVAC ducting-grade in the early part of the run is reactions of the zinc to high temperatures. Assuming your HVAC ducting is galvanized, if it gets hot enough then the zinc may oxidize, melt, or vaporize. It seems like melting or vaporizing are pretty unlikely in most cases but oxidizing might happen. Another issue is that the HVAC ducting is not quite as robust as stove pipe so it may not last as long. If you're building up a solid bench around it then this may not be a major issue.
There are a bunch of threads on permies about this very question as well as some discussion over on
http://donkey32.proboards.com/. For my build (also 8"), I've decided to go with about 6' of black stove pipe coming out of the barrel, then HVAC ducting, then black stove pipe for the vertical run up to the high-temperature chimney (which is already in place, required by code, and probably a good idea for all of the ceiling/roof penetrations anyway).
Hope this helps.