By double wall I hope you mean a class A insulated chimney. Just double wall in uninsulated will get too hot and could set the roof on fire. On a 2x4 you could likely "get away" with a 6" as long as the stack total is 12', but if you go to an 8" you would then be set for your next larger evaporator, a 2x6 and could use the same thru the roof stack. It all depends on what your plans are for 5 years out.
If you are likely to get a 2x6 in 3-5 years, the long term will cost more now but less in the long run. If you are unlikely to expand to where you need a 2x6 until 6 years or beyond a 6" is likely the best call.
Last edited by maple flats; 11-05-2018 at 03:39 PM.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.
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