an RMH build for a trad. yurt

I'd like to document a project here, and hopefully get some ideas and feedback from more experienced folk as it progresses.

The yurt is 21' diameter, traditional style with the two central posts about 5' apart. It has about 2" of wool felt insulation. Our winter is long and most people would consider it extremely cold. I have looked at the other yurt RMH build on the forum (the one with the in-floor heat)

The feed tube must be inside, I'm not going out in the mornings to start a fire. I like the traditional yurt layout with the central hearth, a circular walking space around it and furnishings along the walls. The platform will be plywood floor over strawbale set on rocks with rubble trench / perimeter drain. (yes, i will need to replace the bales after a few years) I will probably leave a central opening in the platform to let the heater rest on mineral insulation directly on the ground. I might be able to 'sink' the RMH in the floor so that the top of the burn tube is about at floor level. I am not sure how much of the mass storage will be floor-level, or if I will make a raised bench...but I don't want it to take up too much space.

Two concerns... I want to have enough radiant heat in the system to warm the small space up reasonably quickly for those times when it's -15 or -30F and I've been away for a few days.

Second, I would really like to confine the RMH to the central hearth area. I don't know if this will give me enough exhaust length, or if there will be too many bends in the exhaust. Right now I am imagining a circle or spiral exhaust ending in a vertical stack up through the roof ring. A little radiant off the vertical stack might not be a bad thing, but i would Really like to store enough heat to keep it from freezing hard inside every night and to moderate the temperature swings.

It would have been nice to do in-floor eveywhere, but getting 10 yards of lava rock hauled in to insulate is prohibitive.

Any thoughts appreciated, cheers...

My first thought is that if your yurt is going to be stationary, you ought to look into substantially increasing the insulation. There is thick cotton or wool batt available. If you don't encapsulate your straw bales, you will likely be making a rodent hotel. That may or may not be a problem for you, depending on your perspective.

You could configure the RMH as a floor furnace and run the heat exchange duct in an insulated space under the flooring. It would be a massless RMH. You could also eliminate the heat exchange duct altogether or run the flue gas through two or more bells in the floor before sending it up through the roof.

What will you be cooking with?

Well, it's still essentially a tent and I suspect it will function best as a tent. The two inch wool felt is cozy. I worry that adding sponginess and thickness would affect the way the structure sheds water, the way it breathes, and the way it dries itself out. Yes to sealing the bales up.

It's small volume, minimal surface, round, not much mass.. takes very little to heat it, despite not being efficient in terms of insulation.

I have two winter's living now in a 144 square foot stick frame cabin that is poorly insulated with big windows, lots of surface area to volume and a drafty door. The stove is 1940's cast iron. It takes very little wood to stay warm all winter. But there is a real problem managing temperature. Half a stick is the difference between comfortable and sauna. The small fire goes out after you crawl in bed and by morning the peanut butter and coffee press are frozen solid.

What I'm hoping for the yurt is to get enough mass in a heater to dampen out the big temp swings.

I will be cooking on a two burner propane with oven for now. In my cabin cooking on the woodstove doesn't work well, as a fire hot enough to cook on is hot enough to cook you out of the cabin. I would expect the same in the yurt, unless I could decrease general radiant output except for one hot spot for cooking.

Thanks Andrew...what do you mean by floor furnace and massless RMH? I was imagining heating a floor mass.

Eliminating the exchange duct altogether sounds like it could just be a pocket rocket. All radiant. That actually might not be a bad choice for the structure, but I really do want to dampen the temperature swings.

Running flue gas through two or more bells in the floor and then up sounds like what I have had in mind initially...but I don't know what you mean by 'bell'...this is something other than a linear flow through a duct?

Bottom line, if my G.F. can wake up in the morning without frost on the sleeping bags I win....

ahhh...bells! thanks andrew. that's a whole new can of worms to think about...

I still advocate for thicker insulation, but you can always add that later. Look up floor furnace on the internet to get an idea of what I am referring to. The ones I dealt with in Texas were natural gas burners with massive cast iron manifolds installed in a metal lined box in the floor, open on top and covered with a grate. They worked on simple convection. Cold air from the floor finds its way into the box and is heated by the furnace and the heated air rises up into the room. (Don't let rodents get in the box. Burnt rat and mouse urine smells awful and permeates everything.)

What I recommended earlier would work to heat the air in the yurt efficiently, but would not address the temperature fluctuation issues, especially without additional insulation. This would not be a problem if you could automatically feed the stove during the night, as with some camp pellet stoves.

It sounds like you really want a mass heater but don't want to haul in heavy material. Check out some of RMH configurations at this site and at Donkey (donkey32.proboards.com). I think you could keep everything pretty well contained in a 5 ft diameter area in the center.

An RMH without the M is not a pocket rocket. Adding additional risers with barrels, or adding bells would dump more heat into the air before sending the flue gas up the chimney. Bells can be made with heavy material to retain and slowly radiate heat.

Do you have clay on site? What heavy material do you have available?

I have big piles of smooth field stone nearby that would be great mass. I have clay and sand. I don't mind hauling some heavy stuff. I don't want it to eat up all of my space. I hoped the mass could be mostly stone if i could figure out a thin mortar that would work..

I had initially thought of some sort of earth floor hypocaust. The ground up here freezes hard for a long time, and it would be a pretty much infinite heat sink under a yurt. I had thought I would insulate the hypocaust floor /foundation with a thick layer of scoria / lava rock held in place by a perimeter band of lava rock 'earthbags'. Then I figured out how many yards of lava rock that would be and how much it would cost to get a truck load of it out here...that's what I don't want to haul. i suppose I could use foam board and concrete slab, but i don't really want to go there for a tent that's probably going to move someday. If I ever build a house it will have a massive foundation and super insulation, but right now it seems overkill for a fairly small tent.

I do want to confine it to a five foot circle in the center and the linear ducting in ianto's book seemed better suited to along a wall. I found the donkey site while learning about bells. I am going through it. Perhaps mass in a circular bench bell or platform.

Actually the yurt is pretty cozy with straight up radiant heat, that's really what it's designed for, so I don't think I'd bother with trying a convective air heating system.

I'm not even sure that an RMH is appropriate, but if I could get one to do the job it would be great.

My earth moving tools are a wheelbarrow and a pickaxe, and I've got a lot of other projects to get done too, so I don't want to get too caught up in a big foundation project.

thanks for your thoughts!

If you haven't read this one at Donkey's yet, I would recommend you start with it.

http://donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=113

It's long, but full of info. You could build a round single bell about 4' OD and probably fire it only once a day and have good results.
I would suggest experimenting before making anything too permanent.

Not exactly a bell, but this gives an idea of what I'm thinking.

http://www.youtube.com/watch?v=-jzKKIHhTU0&feature=autoplay&list=PL3D195EA85C11F929&lf=results_main&playnext=2

Nother one to read

https://permies.com/t/10653/stoves/Yet-another-portable-RMH

hmm...and I also like this little one, it's getting pretty close to right..6" stove in tiny hut
If i had a circular bell / bench with the heat exchange barrel right in the center of it what do you think the transition from the barrel space to the bell space would look like? ie would it have to enter at one spot, or could i just build in multiple entry ports from the barrel into the bell? It would be nice to get uniform heating around the bell...

Then one exit port, slightly lower than the intakes, going to an elbow with clean out, and a vertical exhaust?

It would be nice to 'sink' the heat exchange barrel right into the bell, but I suppose then I would not get enough heat difference between the heat riser gas and the downflow barrel gas to drive the system well...

This thread has a cutaway drawing of the bell design that I was thinking about. No traditional steel barrel, instead the masonry becomes a large barrel (bell) and is charged with heat and then allowed to radiate to the room over many hours. The hottest gases go straight up to the top and as they pass heat to the masonry, fall and then exit out the stack. As the heat storage nears capacity the burn will get fussy, so you will have to learn how much you can burn to charge the system and how long it will radiate after the fire is out. I'm planning to do something similar to heat my basement and moderate the upstairs temps.

http://donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=40