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question about an odler RMH post

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going to start playing with building one of these today. Overall concept, done. Ive been reading hours worth of RMH posts here on permies, and the contents of this has my attention:

""" Is 6 inches is a good minimum diameter for your exhaust stack?

6 inches is a good minimum internal CSA for a RMH. The air intake should actually be 1/4 to 1/3 of that and the exhaust flue can be as low as half the system CSA in my tests.

Is 2 inches clearance between the top of the riser and the barrel covering it a good dimension?

Remember, constant CSA. The 6 inch system has a CSA of 28sqin. So:

28/(3.14x6) = about 1.5 inch gap. In practice, just a bit more seems to work well. Paul's was 1.75 inch and that has worked well for me too. 2inch (or just over) would work for an 8 inch system with a 50sqin CSA.""""

So what does CSA mean? trying to nail down the dimensions/ratios that I will need to use. Thanks everyone.
Rich Pasto
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Cross sectional area.
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