In planning my rocket mass heater installation, I’ve got some challenges. The firebox is outside, the radiating tank is inside, half the thermal mass is inside, half is outside (sun porch). And according to what I’ve been reading, every 90 degree elbow is equivalent to 10 feet of straight duct. Ouch.
I’m planning a 7″ system because I’m planning to use 8″x6″ rectangular steel tubing for my riser. That cross sectional area translates closest to 7″ round pipe. Because I have to have some of the thermal mass inside and some in the sun porch, I will have to calculate my turns carefully to ensure I don’t add so much equivalent length I slow the flow excessively.