Questions about temperatures under barrel and combustion chamber

Hello all, I've been a member for some time, but not active. I have the Ianto Evans and Leslie Jackson book for reference.
I will be building two or maybe three rocket stoves in this upcoming year and need some clarity of certain aspects for my main house one.

Can someone please clarify what the temperature is likely to be directly under the combustion chamber and barrel and for how deep this temperature is likely to go in a stone Rocket Mass Heater base sitting on a concrete slab of between six inches and nine inches thick (my spirit level was wonky at the earlier stages of construction so the slab went down at varying thicknesses). The slab is sitting on four inches of polystyrene which is on plastic Damp Proof Course on top of sixteen inches of sand/ gravel and broken stone. There is also underfloor heating pipes in the concrete slab at a depth of four to six inches (just in case the rocket stove isn't suitable for our needs- my disabled hubby may need an easier to use system in the future). I'm a belt and braces girl.

My initial design had the rocket stove in a certain position so I designed the underfloor heating pipes to not be directly under the stove, but, the actual placement of walls has shown that I really could do with the Rocket Mass Heater in a different position where there will be some pipes underneath it. If the temperature directly under the combustion stove and the barrel isn't high enough to melt the plastic pipe I would like to go with the new position.


Thanks for any help.

 

The simple way to not have to worry about plastic pipes in the floor is to have an air circulation space below the combustion core. Set bricks on the area of the core and several inches all around, out to the edge of the finished surround, with a couple of inches between them so that air can freely circulate from side to side. Then add a continuous layer of cement board or bricks before starting your core construction. This will allow heat to dissipate from the floor, and be useful to you as well.  

I assume that the cob mass will still be effective though I won't get the additional effect of the floor becoming part of the mass?

Yes, the cob will still work as intended. The floor will serve as a secondary mass, even if it doesn't get very hot, as there will still be some conduction and radiation to it. Heating the concrete directly under the core would have little effect anyway, as it would be difficult for that heat to spread to where it warms the space. Air circulation allows the stored heat to move.

When you say air space underneath, is the airspace open to the room like vent ducts or sealed with stone and cob around the edges?

A sealed air space would be better than nothing, but what you really want is for room air to circulate through the spaces and carry heat away (into the room, of course).