I came across some overstock, large diameter spiral tubing I can get for uber cheap. Does going big (20 inch diameter) on the exhaust, through the thermal mass have any downside?
I know it will flow slower, and could be made shorter due to the larger surface area to transfer heat through. That should not effect the draw through the stove, if anything, I would think it would enhance the draw on the system. The up tube at the end of the exhaust would still be 8". The uptube could even be inserted down into the larger tubing to form a heat bell in the larger tubing, so that the gas has to lose it's highest temp before going up the last leg of the exhaust.
I had seen where a guy had used half barrels on an outdoor system, and said it worked fine. On the other hand, I have seen many comments about keeping CSA constant through the whole system. Here are the up side and down side as I understand them.
Up side; Less surface restriction to flow (reduction of smoke back, better draw through system), more surface area for heat transfer per ft, no need to zig zag pipe back and forth to heat a large bench.
Down side; Typically more costly. You could not cut your distance in half just by doubling your pipe size, while you have more pipe surface area per foot, you lose some thermal mass area per ft, for the available pipe surface area. You need a larger thermal mass to accommodate the larger pipe.
I am sure I have missed a few things. What issues do you see?
I did a quick drawing to illustrate the mass per foot compared to the surface area per foot of two pipe sizes.
Clearly, the smaller the pipe, the more mass per linear foot that is near the pipe available for relatively fast heat transfer. This is both a benefit and a detriment depending on what you are trying to accomplish. The smaller pipe will take longer to heat the mass, due to more mass per surface area of pipe. So if you are looking for heat transfer in the shortest amount of time, bigger is better. The downside is the mass will have to be bigger to take advantage of the bigger pipe. It would be interesting for someone who has thermal dynamic transfer modeling capabilities to do a modeling test to see how different pipe size transfer the heat into the mass. With that kind of data, we could engineer for the sweet spot in our thermal mass. Don't get me wrong, I love the seat of the pants engineering, but I am going to have three systems running, and even with these side by side systems the differences are not going to be definitive because of the many variables of location, room size, insulation, convection paths, etc.
Shane, that's me with the half barrels. It works very, very well. If you haven't done it, head on over to Donkey's board and read all you can about "bells." You'll find a lot of solid info on larger chambers for mass. Here's a starting point:
You've got a pretty good handle on some of it, the trade offs regarding mass vs. size. In some spaces it's an advantage, in others not so much. Depends on what you are going for. I've helped on a few half barrel system builds in dwellings at this point, and they work great for folks who need to spread the mass load over a larger area due to floor construction or layout. They are also great for areas where you want quick response and don't need the long term storage. Keep in mind half a 55 gallon barrel is only around 11" high. With a 2" cob layer underneath, you would have 5" of cob over the top to reach seating height of 18". That 5" is only that thin in the center, it rapidly gets thicker towards the sides as you build out. You can get a LOT of mass built up on them if you choose to. Not only that, but a regular width bench is hard to heat with a single run of 8" flue, you'll typically see the runs double back in a bench that is 24" + wide when using flue pipe, so while they still hold more mass than a larger chamber, the difference isn't quite as great as you are thinking.
I think your large flue would work really, really well for some applications, give it a try. You can agonize mentally over this stuff forever, the best way to learn is just start throwing systems together and see how they work. Mud is free. Good luck on your builds!
Oh, edited to add that if you go with that 20" pipe, I'd cut it in half if I were you. Easier to achieve seating height and the bottom half of the pipe will most likely not transfer much heat to the mass. Just a thought.