Flue placement and style - barn/shop

Hello. I've been browsing these forums a bit, off and on, for the past couple years. I built a barn/shop and I'm looking at eating it with a rocket mass heater. I have toyed with a couple ideas, and I'm getting closer to needing to make a decision. What I'm thinking would be my best bet overall is, because I haven't poured my concrete yet, to run my flue pipe in the dirt, with a little bit of mass directly on it, and some insulation boards to the side and bottom to help the heat go up, and then have my slab go over top of that. In my thinking, this would essentially turn my concrete slab into the thermal battery mass. I recall reading that regular concrete does not do well as the main mass directly on the flu, which was why I was thinking to have some Cobb in between, as well as allowing for some clean outs.

I had tried searching and didn't see anything. Has anyone currently on here try this or know someone who has tried something similar, and what were the results?

Hi John, thanks for asking questions about rocket mass heaters. What you are proposing is possible but may not function as well as hoped. Some more information may be helpful as well--what is the size of the shop what are the low temps going to be like when you are trying to heat the shop? Is the shop insulated?

So I first would recommend insulating under the entire slab as this will hugely improve the rmh ability to heat the space. Even if you insulate under and to the sides of the pipe, the heat will distribute through the concrete and be sapped by the available mass beneath it.

Second, if you do insulate to the sides and under the pipe (flue) then the ability for the stove to shed its heat to the mass will be greatly reduced and defeat the point of the stove. With the insulation keeping heat from permeating on 3/4 of the sides, and then the cement floor heating up above it, it won't build up and store your heat for later release--instead it will just continue on out of the building out of your chimney.

The better option if space is the issue, is to build an rmh with a belled bench which tend to take up less space then the traditional piped mass, then have it exit the building. Belled benches also tend to have more radiant immediately released heat than a piped mass which will hold heat longer but take longer to release that heat into the space.

As for the cement--temps above 212 will start to break down cement. It can be used in parts of the mass, but shouldn't be used in the stove itself where combustion is much too high.

Here is a link to a thread discussing heating floors with the RMH-- https://permies.com/t/5831/underfloor-heating-system-rocket-mass

Hi John;  Welcome to Permies!
Daniel was correct.  This idea will not work as well as you wanted.  
The only heat you would gain would be radiant heat from the barrel.  Not much better than a box stove.
Using a brick bell above your slab would do a much better job of holding heat.
The bell can be a bench if you like or just a large brick / metal box.
In my shop I use a 7" batch box with double barrels and a large brick bell.

John, this is what i'm heating the workshop and flat above, with.

https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

Five years old now. And i like it. Freezing outside, 21C° in now. After the fire has been out for 12 hours.

Thanks for the replies. I should add that I do have 1.5" foam all around the base of my barn to help isolate the soil in the barn from the surrounding soil. I also have a vapor barrier around the outside perimeter to help create an insulation blanket.

From the video in first linked thread, the description of what theu did seemed pretty close to what I was thinking. I do agree to not put any iso around the pipe, but from that other thread, it sounds and looks like they just cobbed around the pipe on the dirt and then built a wood floor over it, so their mass is in contact with the lower soil.
I will have a vapor barrier between my mass and the lower soil as well...

One of the reasons I want to have in floor ad opposed to a bell or bench is to pipe some of the heat over to the bathroom to make sure my toilet and pipes don't freeze. While it may not be a perfect solution,

I am, at least temporarily changing from total concrete to 411 limestone with a thin cement top coat for the floor. Much cheaper for now. Maybe in the future I'll get concrete. But I still think having the pipe in contact with at least some of that. I will have ti get better pipe tho, as what I was gonna use would smash when i compact the stone...

Edit to add: Am I just off my rocker on this? Anybody else want to chime in? Or have I added more clarity?

Most definitely the stove will work if built correctly, and it will most likely keep the shop from freezing depending on the size of the space. It will not function to its highest performance though. Having the surrounding soil sealed off with a membrane and some rigid foam helps quite a bit, but you will definitely still lose a lot of heat to the ground. Basically you will be feeding a thermal battery that will never be quenched.

Daniel,

Thanks again. I just relaized I forgot to say I'm im Northern Ohio and the barn proper is 26x48 with an additional semi-seperated 10x28 bay that shares a full wall with my garage.

I also have 4 south facing 3x11ft tall clear windows (plexiglass wall panels that I'm hillbilly double paning with other plexi and caulk inside. They help raise the interior barn temp pretty significantly on a sunny day on their own. Although, this is the first time I've had any floor besides dirt in there. I've got 2/3 of my limestone/portland floor installed right now.

I see what you're saying about the mass below sucking up a bunch of heat... Wouldnt the same thing happen to an RMH with bench or bell sitting on a concrete or stone slab, though? It wouldn't be spread out as much, but I would think the same physics would come into play...

Hi John;
With a bell ,either a brick or a metal bell. The heat rises quickly, only as it cools does it drop towards the floor where there is a chimney waiting to suck it out.
Not much heat left to loose into the slab.
With a piped mass, again most of the heat radiates up. Some does transfer down which is why we encourage insulating below the pipes.