Too much mass for mass heater? 18in off ground

In Alberta, Canada (Think...North Pole)
I am half finished building a 350sq ft building - started as an underground cellar now pondering it as a guest house.
I have built and am currently living in a spectacular round cordwood house. Which is heated by a large Amish-built wood cook stove Which is nice (and it works) but for my new building, I want a Rocket mass heater. I bought the 4 DVD's and the 6" RMH plan and have a couple questions...As it was going to be underground, it is built very sturdy, Post and Beam, with the floor about 18in off the ground. I am now going to build perlite filled earthbag walls around my 2x4 walls. (I bought 600 cu yds of Perlite & the grenhouse guy thought I was nuts) - Please see attached picture/plan
The floor studs (in squares) have already been installed but could be removed. The ceiling is only 6ft high. In your opinion, should I
a) Put the RMH on the ground - The wood feed box and the barrel would be above floor level and the cob mass would be the floor in the square with the barrel. I would leave the other two floor squares above the ducting. So the cob mass would not be accessable.
b) Build up the ground (18in of dirt or dirt/cob) and build the RMH on top of this built up area (have the cob mass be a bench within the room)
c) Reinforce the floor squares and put the RMH on top of the floor as a bench/bed. I think it might be nicer to be able to sit on it rather than have it under the floor.

Is there any issue with too much mass? Will mass under the stove pipes even get warm or does heat only rise? What if I expanded the bench to be a 3' wide bed, would the extra mass detract from the purpose or is the more the merrier with mass heat storage?
Any help would be appreciated!

Megan, straight away, i give you my opinion and feelings. Based on what i want, may be not what's best.

First of all consider that a rmh isn't may be the best for a building which is not occupied all the time.

This word of caution being said. I see two options. your floor isn't gonna cope with weight, that's prety much for sure.

First option, concrete blocks and may be a slight slab/foundations whatnot on the ground, going to your 18 inches, and then a horizontal batch rocket; think open fireplace; and a proper massonry bell on top. Or even a double bell. You could make it with one barrel for usual layout, or two barrels on top of eache other like Peter van den berg workshop heater, more quick heat, non storage or not too much storage bell. Then go through a small massonry bell for grabing the last calories. Tho, remember, you can go over the top at exctracting the heat, and your system stalls.

Or, since your building is not huge, i'd try the underfloor aproach. Remove the floor joists, make a bed of perlite on the ground, thick enough, so your heat doesn't radiate there. Build a normall 6 incher with mass out of cob, insulate the sides too, so heat doesn't go wasted. A good four inches of perlite underneath on the sides you could put some of your perlite bags too. Make it nearly flush with your surounding floor, side insulation going to the surounding floor, and finish with stone, like slate, or some flat pieces of something, tiles, may be. Obviously you have the feed flush with the floor, and just the barrel in the room. At a perfect height for cooking as well One advantage is you haven't wasted any room, and a little plus, uyou can sweep breadcrumbs, dust and whatever has fallen on the floor into the feed To avoid falling hazard, a little cast iron grid could be found, to put on top of the feed, recessed so it's flush.

Hope this helps.

Max.

Thank you so much, Max.
I do think I want to go with a mass heater, even if the building may not get used much in the next couple years. I have lofty plans of living there in the future... Of quitting my job in the city and living in the woods full time. The shit keeps piling up around the fan but refuses to hit it....
Anyhow, I agree that the floor squares won't hold the heater. But I am loath to waste lovely heated benches and to not enjoy them. I am leaning towards removing all three floor squares and lowering my living space to include the cob bench. I could even fashion stairs as long as I kept the room insulated from the rest of the crawl space, it would be it's own room- just lowered to the ground. I wouldn't have the great option of sweeping the dust bunnies down the wood hole but I guess I can't have everything.
4" of Perlite under the brick...sounds good. But no earthbags under, right? (combustible...) Will the perlite alone be compact-able enough to hold the bricks without settling and wrecking the airtightness of the heat riser? Will perlite and clay form a more compacted surface?Will the perlite, which is popped up quite fluffy now, get packed by the weight of the cob and become less valuable as insulation?
Thanks,
Meagan

Meagan.

You absolutely want a bench. May be you could use option number two, mixed with number one. Or three

Forget about lowering your floor.

Heat travels through solids by conduction, in all directions. The only place where convection occurs is in the pipe itself. And you can also lower the flue tubes lower than the feed. Tho, problem occurs with condensation.

So here we go with "mixed option"

How skilled are you with concrete blocks? Or Aerated Concrete Autoclaved blocks (xela/hebel/ytong) You seem all right to buy lotsa perlite. Why not a few grey energy blocks

Lay some on the ground, flat for the base of the whole surface of the thing, they are insulating. And you have already come up 8 inches of your 18. They have 4 inch ones for the walls. Put some of theses on the perimeter of the base, then, at this point, either you chuck the flue tubes in for low heated bench, which doesn't overheat your butt. Or start building your cob mass in the recess/base you've made in your floor with thoses blocks. It will heat up by conduction. Then build your conventional rmh on top of this. Why do i insist about insulating the crawl space, you could use the heat radiated by the mass to warm the whole floor by convection. And it wouldn't even need to be insulated underneath, as hot air goes up. Still under the masss to avoid conduction with the ground. But i bet all the crawlies of the world would gather in a heated crawl space!

You like this one?

All good and then you confused me. After the tenth re-reading and some further figuring, I think I'm getting clearer.
How does this sound...
If I started out insulating the ground 4-8inches (with what - I'll get back to) for the entire 12x 6 area and built 18" high perlite earthbag walls around it to seal it from the rest of the crawl space. So I had a living space that was a foot or so lower than the wood floored area. Then I built the mass heater on top of that, into a bench/bed.
Cob on the heater would meld into cob on the low earthbag wall. See picture. I like it and think it would work.

Just one issue - what insulation can I put over the ground & under the heater that will neither melt nor disintegrate if I put a billion ton mass heater on it...? Aerated Concrete Autoclaved blocks look good but they seem to be only sold in Australia, Greece and Florida, China and Vietnam. I'll go back to wondering if clay/perlite might work. Earthen floors are supposed to be loadbearing. I will research adding perlite to an earthen floor for insulation....
Any added ideas?
Meagan

That plan sounds all right to me. Remember that the mass might not go over 150F°

Perlite cob could be all right. Don't know, i don't have any clay over here. So i use cement, concrete, ciment fondu etc!

Check the insulation factor.

Imho, you would need at least a R of 6, but that's european numbers. The R numbers in the states seem to be different! Well, the R factor of 8 inches of glass wool of the best quality. That seems to be in the range of R22 R26 in the us.

Have you checked the half barrel system? That's a good alternative to flue tubes, when it comes to cheapness.


http://donkey32.proboards.com/thread/560

Pish easy to make mass that way

Euh, edith: you could stuff a pair of half barrels under your mass, under floor level for your flue. And have just mass above, or even may be re do another layer of half barrels, for a second bell above the first one.

Thanks for all your help!
No clay? How odd. I have far, far, far too much, You could make pottery out of my field.
Half barrels look nice but I tend to do things the hardest way possible and they too easy.
Thanks again!

There is more to the issue than doing things the hard way. The whole point of rocket heaters is efficiency. Here is a nice short summary of why bells are more efficient than flues. The 1/2 barrel approach is a bell.

http://blog.dragonheaters.com/wood-heat-storage-flues-vs-bells/

FYI, if you are having a struggle with support for the large area of thermal mass, the chimney flue build also on the blog site has a footprint of only 30"x36". See this thread.

https://permies.com/t/27091/rocket-stoves/Rocket-Heater-Masonry-Stove-Built

If you are concerned about the weight of the mass all you have to do is put extra supports under the mass. Concrete blocks or treated wood posts.