I am currently building a octogon shaped sauna. I am wanting to heat it with a rocket stove mass heater, the feed tube will be on the outside of the sauna. The barrel on the inside. There will be a bench on 7 of the 8 walls (one being left open for the door) that will contain the exaust tube. the sauna will be approx 160sq feet. The exhaust tune will be 30 or 40 feet long.
My question is should I go with 6" tube or 8" tube. Is 40 feet to long for the exhaust tube?
I want this space to get hot... Real hot. Will a rocket stove get this to sweaty temps in not to long of time?
40' is about the recommended limit for a 6" RMH without considering bends, so I think you need an 8" system to power a bench that long. This would make way more than enough heat to make the space hot, and keep it hot for many hours. Unless you expect to use it every day or so, you probably don't want the thermal flywheel of a full-mass bench; you would probably do best with a minimum-mass bench that lets heat radiate from as much of the duct as possible, with maybe a wooden bench surface with airspaces surrounding the (possibly lightly-cobbed) duct. I might even start with a layer of cob only on the first bit of duct to moderate radiation to that bench, then add more cob if you decide it needs it for smothing out the heat profile.
Do you have experience with heated benches in saunas? You might want the advice of an experienced builder/operator to say whether it is really a good thing to have a major source of the heat coming from the bench you are sitting on. It might get dangerously hot in order to make the sauna air hot enough.