Hi
Permie people! I have questions. But first I want to say how grateful I am for this forum! I've learned SO MUCH from reading through all these great conversations about RMHs. Thanks!
Background: I'm living in a tipi in NW Montana and I want to keep from freezing overnight by sleeping on a
RMH. I have taken a basic play-with-fire
class from Ernie and Erica in Santa Cruz, CA, and helped a little in building the
RMH they designed for a house in Bonny Doon, CA. I bought Ianto Evans' book, read it a couple of times, and downloaded Ernie and Erica's Annex 6" plans. AND I found a
local friend who has built 5 RMHs of his own! And that's when I started getting confused.
My friend heats his home through -15°F Montana winters with a very atypical RMH. The burn chamber is in the basement. The insulated heat riser is very tall (runs from basement to 1st floor) but still exits into a 55gal barrel and a short horizontal manifold before going into the vertical mass that extends to the second floor. But basically, he built it using Ianto Evans' numbers for an 8" system and he says it rocks.
He built all his other (horizontal-run) stoves with 8" CSA cores and says when he's experimented with 6" or less he's seen a much dirtier, less-efficient burn. He also says that a bigger core will result in more BTUs pumped into the mass faster, for a quicker heat-up time. He considers anything smaller than 8" to be a compromise on the power of RMHs. BUT! He has used 6" flue with 8" cores and says they work great! He said that the height of the final exit flue makes more difference than consistency in CSA.
Enough rise and the warm air creates enough draw to make up for the greater drag of a smaller flue.
So this is my confusion. I always thought the golden rule of RMHs is that the CSA of your flue must always be the same or greater than the CSA of your core. How can that be true, if the 8"-to-6" systems work? One idea I had is that his cores are all brick, and thus square, and so the turbulence might result in an effectively smaller CSA of moving gasses, making a smaller flue possible?
I want to design for 6" flue because it's way easier to find, but he recommends an 8" core. With a flue that exits through the smoke flaps of the tipi, I'll have a pretty tall rise and respectable draw when the air in the flue warms up. Any opinions?