Thermal battery on wooden floor?

Greetings from Denmark!
First time posting - Long time lurker.

I am building a RMH cob bed/couch. 8" Duct 55 gallon system - By the book.(..or rather pdf.)
The flooring in my small cabin is set in two levels. The lower level flooring is bare ground. The upper level is concrete slab with a wooden floor on top.
I have build the "firebox" on the bare ground level and I am hoping to place the thermal battery on the raised wooden floor.
I´m concerned with fire hazards and I would greatly appreciate any input:

1. Since I don´t have the firebox on the wooden flooring,- how would you recommend insulating between the wooden floor and the thermal battery? (I´m hoping for less insulation than 4 inches, less expenses and less work)
2. I could remove the suspended wooden floor where the thermal mass will be and build it directly on the concrete slab (more mass). Is this the better option? (Again, I´m hoping for less expenses and less work.)
3. I´m planning to elevate the horizontal flue, by 1½ feet, immediately after it leaves the exhaust. Will this create problems with airflow?

Thank you in advance.

Jim, iirc you have lecca clay balls in Denmark. I would remove the floor, and make a 4 inch bed of those. Heat will travel through. But slowly. It might heat the slab. But lightly. If you put the mass directly on the slab, it will heat the ground bellow, and you will loose heat through conduction to the ground outside the hut.

By horizontal flue, you mean you won't have a vertical chimney?

This doesn't work. You'll have wind the wrong direction, one day, and you will smoke the whole shack. If it doesn't happen at night, and you die during your sleep.

Jim Tag wrote:3. I´m planning to elevate the horizontal flue, by 1½ feet, immediately after it leaves the exhaust. Will this create problems with airflow?

If you mean between the burn unit and the mass, that should be OK. I think using two 45 degree turns would be better than two 90 degree turns if that is possible.

My shack is up on frost piers, so there's about 3-4 feet between the ground and the joists. My mass is on the wooden floor, with insulation underneath it. That works pretty well, but the floor was designed to handle the mass.

And my system was pretty flaky with blowback problems until we extended the vertical final chimney above the roof.

Satamax Antone wrote:Jim, iirc you have lecca clay balls in Denmark. I would remove the floor, and make a 4 inch bed of those.

That is a great idea. Thank you!

By horizontal flue, you mean you won't have a vertical chimney?  

I do have a vertical chimney. ½" steel pipe.

Ron Helwig wrote:

If you mean between the burn unit and the mass, that should be OK. I think using two 45 degree turns would be better than two 90 degree turns if that is possible.

Point taken. I will use two 45 degree turns. Thank you.

Ron Helwig wrote:My mass is on the wooden floor, with insulation underneath it.

What materials did you use for insulation?

Jim Tag wrote:

Ron Helwig wrote:My mass is on the wooden floor, with insulation underneath it.

What materials did you use for insulation?

Typical insulation. Fiberglass batting IIRC.