Cristobal Cristo

I like the idea. I would suggest to consider installing the tank in such a way that it would be directly exposed to fire. I cook quite frequently on my outside dry stacked masonry stove and have noticed that pots placed on the top cast iron plate heat much slower than when exppsed to direct fire. That's why some cooking plates for masonry stoves have removable rings.

Also I would suggest to not use BBR firebox for this task. It will be heating only when burning. Massive amount of energy in the form of coals radiation will not work much towards heating the tank. Some simple L-tube would probably work the best. When the fire dies out, the coals will keep heating the water, radiating a lot of heat towards the tank's bottom.

John,

For such a room in Australia, probably a Batch Box Rocket of fi 125mm size would be enough.
Is it the only room in the building? Is it masonry, insulated? We could do heat loss calculation.
If fi 125 was chosen then you could do 60x80 cm inner bell size which with single skin would result in 82x102 cm footprint (assuming 11 cm wide bricks in Australia). It would weigh over a ton.
Steel columns would probably work if they were topped with some reinforced small slab on which you could build the heater.

Lina,

I do not know in what part of Portugal you are located, but if I did raised beds in my hot/arid summer Mediterranean climate, my plants would be roasted. If I did it on concrete/masonry floor, they would get roasted two times faster. When it's mild 30 C in late spring, the concrete gets so hot in full sun, that touching it for longer than few seconds could burn the hand.
Of course it could be probably possible with heavy watering four times a day as two times a day is sometimes insufficient for ground level vegetables.
So if you are in southern/inland part it would be better to remove it, for northern/coastal, it could be potentially kept.

I second Anne.
Pet food is one of the products on the market that is completely not needed. It's also usually very detrimental to animal's health like all junk food products. I'm talking about cat and dog food, and not some specialized products for exotics.
Feeding cat quality diet is simple and cheap:
-any meat
-hearts
-livers
-kidneys
-chicken
My cats love chicken bones, raw or cooked. They were eating them since they were kittens as soon as they could crush them.
Occasionally I give them some canned tuna, sardines (in water), fish bones and heads.
I give them milk, some sour cream and cheese. They like raw eggs and pieces of butter.
I was feeding them ground squirrels and rattlesnakes.
It's important to offer the variety, especially bones and organs. If they also hunt, variety is of a less concern.
If I don't have enough for them, they are more compelled to hunt and they are my only weapon against ultra-destructive gophers.

Ned,

What would you gain by having a panel covered with thin cob? Cob is heavy, cheap if collected on-site and works as thermal mass if applied thickly.
If the panel had a brittle minimum of 5 cm of cob on each side and let's say it would be 1x1 m then already it would weigh around 200 kg (440 lbs). Some machinery would be needed to lift and install within already preconstructed frame.

Rico Loma wrote:What is your wood of choice in NoCal?

I have eucalyptus wood chips that I produce myself, and mixed eucalyptus/oak/cottonwood that utility company drops for me once per year. I'm in Central California.

I have purchased a cheap IR thermometer with capability up to 1500 C so I will be able to measure steel temperature in my forge.
I just measured the temperatures - the firebox after two batches and soaked with heat from the coals for an hour reached 340-400 C.  We just baked hortopita from our own greens. It took only 7 minutes.
The oven on the outside has the air temperature, it can not be sensed that it's heated at all.

Two weeks ago I have fabricated a smoker insert. We have smoked a catfish and pieces of pork. Smoking process took 3 hours and around 20 l of wood chips that I was throwing over the coals once every 20 minutes.  Both smoked perfectly.

If you are turning it into a shop, you could consider to turn entire floor into a level and solid slab. Heavy machines will like it.

In theory you could pour concrete over the vappor barrier, but you do not know if the dirt under the barrier is compacted, if it gets moist and since the barrier is old, it could get damaged during slab construction and then seep moisture to the slab.
If the dirt is not compacted, entire heater can tilt or crack. It happened to Thomas here on permies.

The best would be:
- remove the barrier
-remove dirt under future slab down 4"
-compact it
-add 4" of base rock (or gravel/coarse sand) and compact it
-reinstall the barrier (if needed to cut, please make good overlaps of 12" and use proper vapor tape)
-pour slab over the barrier

I would do continuous slab under entire heater/bench. You will feel good building on it and the heater will feel good resting on it.
Please make sure that you add reinforcement, as slabs (especially longer ones) will crack if not reinforced. I would put continuous rebar #4 (0.5") on entire perimeter of the 4" slab and the same rebars in the middle to create 8x8" grid. For thicker slabs there should be two layers of rebars on top and bottom. It would require some bending, but it's worth. If you provide the shape with dimensions I can draw for you rebars with bends.
Alternatively some heavy duty remesh could be used - not the thin type from big box stores.

On the slab you can build an insulation layer using one of the techniques:
-pouring perlite/cement mix
-laying regural bricks as rowlock stretchers to create cavities to be filled with perlite or vermiculite or even broken glass
-laying insulating bricks
-laying insulating boards

You are not alone Sam. I never ever managed to grow a single carrot. I have planted thousands of seeds of various  types and at various times of the year and in various locations on my land.