rocket mass underneath a concrete pad

I have a 32x24 barn which I want to put some kind of RMH system in it and I would also like a concrete pad just big enough to work on my vehicles. As far as the 'mass' of the stove, I like the idea of using a 1/2 barrel. Since I have a dirt floor, it would just be a matter of digging down far enough to bury the 1/2 barrel on top of a layer of perlite for insulation, and then rocks and cob on top of that. I kindof had the idea of making the top of the mass more or less level with the floor. My question is: would it be feasible to have a concrete pad on top of the mass? (I should probably gen up a rough sketch to explain my idea).

Hi Dan;
We encourage experimentation here so if you build this way, and it works  then document it so it can be shared.
I do see some possible issues that I will share with you.
If I understand  correctly, you want to make an rmh  and plumb it into a half barrel system.  All good.
Next you are thinking you want to bury that 1/2 barrel 3-4 ' deep. Sitting it on top of perlite for insulation.  
This is  a problem area.  To bury that mass in the ground, it must be heavily insulated  (think 4" foam board) on the bottom and up the sides to ground level.
Any less and your heat will absorb into the earth (Nothing but the sun can heat the earth) .  
If you insulate your mass from the earth and then pour your slab over top I think you will not be very happy with the result either.
To start that slab would need to be every bit as insulated as the mass was. If you do that, I suspect that you might see a temp climb on the slab but it will be minimal and not the toasty warm pad you are hoping for.
Leaving your insulated buried mass open to the room (not under a slab) would release more heat into your shop. But mainly your heat will be coming from the barrel over your riser.  
The buried mass will radiate but only from the top not the sides.

If it were me I would  build an above ground rmh with a brick bell, you will be finished quicker and your room will feel warmer.

If you keep your half barrels as close to the surface as possible, it may work. A 6" layer of reinforced concrete will transmit heat pretty effectively. To be honest, I would turn the half barrels (well bedded in stabilized perlite) cut side up and build forms so you can pour your work slab with maximum bottom surface exposed to the hot gases. Corrugated metal would be a good form material so you wouldn't have to try to remove wood from under the slab.


One caution: a serious masonry mass is going to take hours to heat up in the morning and stay warm hours after you are gone at night, so for an intermittent occupancy space, you may want a lower mass heater that can be more responsive. If you want to keep the space warm round the clock, go for the big slab.

Interesting idea Glen!    I never considered putting the barrels open side up.
Would certainly transmit heat better in that aplication.

thomas rubino wrote:
To start that slab would need to be every bit as insulated as the mass was.

I assume you mean that the portion of the slab not over the stove, or where it's in contact with the ground.