Glenn Herbert

Radon investigation would be a good starting point, but if that does not prove to be an issue, I would start with a floor slab in the cold room (no deeper than you need for headroom) and put up a concrete block wall to main floor slab level. Many concrete companies have minimum batch delivery sizes and you would not have to pay a premium for a small amount of concrete. Interior basement walls that are relatively compact and not subject to frost heaving do not need to be reinforced concrete. Running vertical rebar in a few cores spaced across each wall and filling those cores with concrete will give plenty of strength for insurance. I like dry stacking block and surface bonding on both sides - as strong as ordinary mortared joints and much faster. Then backfill around the finished cold room walls (with vapor barrier) and pour your main floor slab.

Downward fire needs something forcing it to move downward, like a vertical chimney which creates draft. I cannot imagine that a fire would start in a cold system and move downward.

12 years ago there was much made of the idea that an RMH does not need a chimney; this was the case in Ianto Evans' location on the Oregon coast with a constant prevailing breeze in one direction and the exhaust on the downwind side, but will not be true in most locations in the world.

J-tube combustion cores are indeed reputed to be tricky in 4" sizes, but batch box combustion technology does scale up or down reliably, and moreover gives considerably more heat in the same burning time than a J-tube system of the same size.

If you can't or won't run a vertical chimney through the roof, which gives the best results, I would go through the wall as high as you safely can to increase your chances of success. A long cold exterior chimney can cause cold plugs in certain circumstances and make it impossible to get draft started without going outside and preheating the chimney.

Once you confirm that the flue does not leak and is sound, I think you will be good to go. The exhaust temperature will be much lower than a standard wood fired combustion unit, so it should be safe.

I think the practicality of wood burning for heat depends on what your house and equipment are like. If you have a big leaky house and a big stove that is not very efficient, the work involved could be overwhelming. If you have a well insulated house and a very efficient stove or best of all an RMH, the wood processing could be easily manageable.

Your garage heater follows the basics of Matt's design and works. You mention the draft is weak when cold - as the chimney warms up I would expect it to increase.

Your cabin design is different as you secondary burn chamber is smaller and shorter and the flames will be going down immediately. Your garage heater firebox is similar to the sizing for a 5" batch box, while your cabin design fits a bit less than a 4 1/2" system. 6" flue is way oversized for this, and you would have slow flow and even more cooling in the exhaust. Batch boxes depend on a chimney for good draw, and you are planning a system that will have no final chimney AND no internal riser "chimney". What do you expect to be the driver for draft in the desired direction in this plan?

I do think a 4" or so batch box is well sized for a tiny cabin as you describe, but I think it will not function without a chimney, and preferably one that goes up through the roof. If you do that, I think you will have a decent reliable heater.

I saw that and related videos a little while back. He explains the principles pretty clearly and succinctly in the first comment pinned under the video. The upshot is that that really does work, and gets energy from the following wind to go faster than the wind. It would not work in still air or a headwind.


Oops, the page jump hid my previous reply when I wrote this one

It's a ways from Georgia, but there is a school bus auction site near Slippery Rock PA (an hour north of Pittsburgh) where you can get good used buses cheap.
https://www.422sales.com/

The issue with any wood stove in a small well-insulated space is that it is going to be hard to run it slow enough to not overpower the room with heat, without being a dirty inefficient fire. I think a small rocket mass heater would give you warmth throughout the night without having to tend a fire, and not get the room too hot while the fire is burning.

A 4" batch box with the top of the firebox turned into a hot plate for cooking would probably give enough heat from a compact package. Running a bench bell along the side wall under the window to the back under the hammock would give well-distributed heat.

As you say there is no clay on site or nearby, I would make the whole thing from brick (or stone if you have any for the outer shell) with possibly a small barrel for part of the enclosure.

Sounds like a plan

Yes, you can make the firebox from standard old red bricks. Firebricks would last longer, but are not essential for a short-term build. Especially for a short-term build, fireclay slip is probably the best mortar, as it can be knocked off leaving clean bricks when you want to build something new. It may or may not meet building code where you are; it is a bedding/sealing agent only and not a cement. (US "International Building Code" requires refractory cement mortar which is strong and sticks to bricks, though I don't know how often that is actually used.)

It sounds like you have a good tall interior chimney which should have good draft. I hope both houses do not share one flue, as that might cause complications with draft.

Good advice for a cooking stove. For those who want a heating stove or mass heater, I would consider the following.

Charcoal is not the same as coal, and will not have the same issues of energy density and nasty high-temperature volatiles. It is essentially the last stage of normal wood burning. I find that when I have a deep bed of charcoal at the end of a fire in my J-tube, pushing it all into the burn tunnel so the secondary air P-channel blows on it and covering the primary air supply gives a hot fire and burns completely to ash. If you are starting with charcoal and want to optimize for burning that, I would consider configuring the core to resemble that part of the system, with a concentrated fuel bed and a restricted air supply blowing on it (preferably preheated.)