Short version of my question: Can I substitute a concrete block foundation instead of a poured concrete slab under an rmh?
Re-stated with context:
A small guesthouse building, not yet built, on skids so it can be moved in the spring.
I'd like to heat it with an rmh this winter. I'm trying to find the best way to properly support the rmh with its own foundation. I understand that a 4" concrete slab would be appropriate, but for ~5'x10' that's impractical for me to construct. So I'm hoping I could use concrete blocks instead.
I have the new rmh builder's guide and the 4 videos but haven't fully studied them yet. What I know so far is that it will be a 6" system because I already have a 6" ID heat riser and 20' of 6" metalbestos chimney. I wasn't going to start on the rmh build or even its plan yet, but it seems to make sense to do its foundation now and place the building around it.
Ideally I'd want to set those concrete blocks side by side into a 5'x10' rectangle below grade on a few inches of gravel but not not mortar them together; that way it would be more moveable next year but it sounds less strong. Or maybe just mortar the perimeter blocks?
As I said, I haven't got all the details yet. The "Annex Heater" seems the closest fit so I will probably follow those plans as closely as possible.
My question right now is the suitability of a support pad made of concrete blocks and whether they must be mortared together or filled with concrete.
Mortaring the blocks together or filling cores with concrete will have little effect if it really wants to settle unevenly. I think in your situation you would be best off making a double layer of blocks with joints staggered, or maybe better a layer of hardibacker or cement board on top of the single layer of blocks to prevent any separation of blocks from telegraphing into the cob.
If you make the flue path as much as possible sealed with metal outside of the high heat zone, you will have a safeguard against the cob mass cracking.
How much do you plan to heat the guesthouse this winter? If there will be long periods of no heat, you may have a problem with frost heave getting under parts of the system and moving it differentially; nothing short of a solid reinforced concrete pad would protect against that.