flue and bench size for tiny house RMH

Sourcing materials for a RMH, cabin is 250-300sqft, r-20 walls, r-30 roof&floor, in hardieness zone 3.  As a primary heating source,   what would be an ideal flue size and bench mass volume.  Other than the heat riser and bench mass, the construction will be heavy steel (at least 1/4").
Because common pipe is in whole inches, what consideration should be given to interior diameter through the tubes?

Nathan Prince wrote:Sourcing materials for a RMH, cabin is 250-300sqft, r-20 walls, r-30 roof&floor, in hardieness zone 3.  As a primary heating source,   what would be an ideal flue size and bench mass volume.  Other than the heat riser and bench mass, the construction will be heavy steel (at least 1/4").
Because common pipe is in whole inches, what consideration should be given to interior diameter through the tubes?

No no no and no!   https://permies.com/t/52544/metal-burn-tunnel-heat-riser

Nathan Prince wrote:Sourcing materials for a RMH, cabin is 250-300sqft, r-20 walls, r-30 roof&floor, in hardieness zone 3.  As a primary heating source,   what would be an ideal flue size and bench mass volume.  Other than the heat riser and bench mass, the construction will be heavy steel (at least 1/4").
Because common pipe is in whole inches, what consideration should be given to interior diameter through the tubes?

Nathan I have a reasonable amount of experience with these wonderful creations and can tell you that....

Ideally, interior csa (cross-section surface area) should be consistent throughout the system, with your transition area/ash pit (from barrel to thermal mass) being at least twice csa, more if you can manage it, and tapering down to original csa to travel through mass. If you're doing a batch box, like the one shared in this thread (a little easier to manage in terms of fuel size and length of burn), interior csa is consistent from the heat riser onwards, with a larger batch box, primary air intake at 20% riser csa, and secondary air intake at 5% csa. http://batchrocket.eu/en/9-english/7-building#dimension

This link should also give you some ideas about sizing, but my suggestion would be to build no smaller than a 6 inch system, which can push through a horizontal run of about 30 feet. Any 90 degree turns reduce air speed and subsequent horizontal run length about 3-5 feet. There is something quite special about 8 inch systems, if you can manage one this size...up to 40 feet of horizontal run.

Glad to see that Satamax responded to your metal proposition. The link i've shared also addresses suitable materials. I made a batch rocket with an insulated converted wood stove, and although it worked well it was never intended for long-term use.

Have you seen the thread about Matt Walker's tiny house cook stove and heater?  It seems like a near-perfect solution to heating and cooking in a small space - I'm sourcing materials ready to build one in my tiny house.

Hi Nathan; As everybody has told you metal is a poor idea. IT WILL NOT LAST LONG !!!  Burra's suggestion about the Matt walker tiny house stove is a excellent idea. A cooking stove and a riser less mass heater all in one.   Matt also makes and sells a 6" ceramic fiber core J tube kit that would work awesome for you. He also sells just the building plans for any of his stoves.( http://walkerstoves.com) With a 300 square foot cabin you definitely do not need a batch box or an 8" J tube . A 6" J tube is plenty and it would run you out of your small highly insulated cabin if you ran it for more than a few hours at a time.  If your concern is about making cob / finding clay and sand in your area there are options that call for quite a bit less than an all cob bench.

I like Donkey's approach when questioned about RMHrs. Somewhere in one of the videos in the cyclone batch thread, of which Nathan's post was originally a part, he talks about trying to talk people out of doing a rmh to get at their specific requirements and expectations...discusses how a traditional j tube typically requires alot of attention during a burn cycle, whereas most people are accustomed to a wood stove scenario in which you load it up, let it burn, and go off to something else. I have experienced the same in my builds, which is where my preference for a batch emerges...don't have to process the wood down quite as much, and with lots of doings afoot, it's nice not to have to pay as much attention to the fire.

I also know from experience that smaller than 6 inch systems are tricky to get working well - any rmh build requires figuring and tinkering, but larger ones perhaps less so - at least in my experience. Plus increased btu's and thermal mass means less overall time spent paying attention to the heater, which frees up more time for other awesome permaculture/natural building endeavors. But having said that, Nathan's original query again was posted in the cyclone batch thread, which featured a 4 inch system working really well in a small space. Peter does say it may well be close to the edge of what can be accomplished without running into dirty burns and smoke back.

The thing Satamax shared is cool too, with masonry as the barrel - reminiscent of Kirk's cyclone batch with a masonry bell over the riser. However, just want to point out that ideal heat riser is circular, then octagonal, then square, with rectangular being the least preferred option. As discussed by Peterberg in the link i posted. I think the one in the Hermon Heater has riser dimensions of 4.5 x 9 inches, if i heard that part correctly.

Mother Tree - that is such a beautiful piece of work shared from Matt Walker. Really elegant - hope you have great success with that, and looking forward to seeing the results!

Ok, this is a trailered tiny house, regular RMH construction wont float for weight and durability.  Steel was the obvious choice for durability, but it sounds like high carbon is a no no for safety and service life.  Perhaps a set of refractory tubes similar to a heat riser for the feed and burn tunnel nested into a pipe casing (for rabbiting) (the burn tunnel opening could be blinded off for servicing)
The bench mass will be a split level sub floor ondol, unceremonious and removable in an engineered trough so gross weight doesnt prevent trailering, but the heater could still be used as secondary heat while uninstalled.
Im not commited to any style of construction, I just have a-typical parameters.  As beautiful as they are, I dont want to disassemble cobb art every time I move.  Thanks for the insight, the link to alloy operating temperatures was very helpful.  

Nathan Prince wrote:Ok, this is a trailered tiny house, regular RMH construction wont float for weight and durability.  

Did you see the "minnie mouse" in the original thread? First part of this video...