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Flue placement and style - barn/shop

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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?
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Location: Victor, Montana; Zone 5b
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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
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Location: latitude 47 N.W. montana zone 6A
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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.
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Location: Southern alps, on the French side of the french /italian border 5000ft elevation
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John, this is what i'm heating the workshop and flat above, with.


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