Glenn Herbert

Nice work! I second the advice that you can upload pictures directly to Permies. Doing that will ensure that they are always visible; I have seen too many intriguing posts which have lost all their visuals because some free photo sharing site went out of business, or someone let their website lapse.

I have a 330 gallon tank around 80 feet from the house entry, 1/2" copper underground all the way. The regulators and underground piping were installed by the gas company 30-some years ago, and I don't know what the one on the tank is set at; the one by the house is obviously set to standard 11" w.c. It powers the stove and a Polaris water heater which heats the whole house in winter (at least it did until I built my RMH to heat the main floor ).

The ignition temperature of wood is 450ish F, but it is a known issue that wood continuously exposed to high heat for a long time (months/years) will slowly break down and eventually be able to char at lower temperatures, like 200 F or so. This is probably not an issue for a cushion, as the bench surface should not be getting near those temperatures.

Here is a photo of the first fire in my RMH. I have removed the access panel for inspection and cleaning twice in about 9 years. The oven/cooktop insert didn't work so well for those purposes, but it does radiate a noticeable amount of heat as well.
https://permies.com/t/60784/a/45569/IMG_1338-w600.jpg

I built a metal access panel into the front of my RMH bell, and it gives a decent amount of instant heat while waiting for the stored heat to work its way through the massive bell walls. It also allows clear access to inspect and clean the riser and interior of the bell.

John, I read. A lot. Anything that comes into my hands. Especially anything to do with history, geology, archaeology, paleontology...

John, it may sound alien to someone from an ancient, mostly dry continent, but many areas such as Appalachia in the eastern US are full of steep rugged terrain, and when an extreme rain event hits, it can percolate to bedrock, loosen the bond of many feet/meters of soil, and let the entire hillside (sometimes with trees and buildings intact) slip downhill to crash into the valley below and jumble everything into a heap, or bury houses in the valley.

Daniel - As mentioned, you can find heating requirement calculators online. Once you have a number, batchrocket.eu has information on system sizing for the requirements, for batch box designs. J-tube designs need to be scaled as mentioned above, with an 8" J-tube being similar to a 6" batch box. When you do have a system size determined, all the chamber proportions are determined by that, and described in numerous sources including batchrocket.eu. In general, oversizing a system a bit only means that you need to run it less often; if you undersize the system a bit, you need to run it more times a day in extreme weather which may be inconvenient. Greatly undersizing can be a real problem.

Self-feeding experiments have been done by many people, but they are all experiments. I would not advise trying to do such things in a system you want to just work out of the box. Also, self-feeding implies continuous or long-term burning, which is not required by the concept of a fast, hot fire that heats a mass for continuous gentle heating.  I recommend you go with mainstream designs, and with a bell or stratification chamber rather than piped mass, to make your build as reliable as possible. To be clear, if you do not have unusual conditions, recent common designs can be expected to work reliably if followed. As far as specific layout, proportions and surfaces of your installation, those can be tailored to your situation as long as you follow the basic technical rules.

Sophie - this forum is designed with all posts sequential, not responses grouped under individual posts. The idea is to always be able to see the latest post and not miss a new answer to a previous question. Threads should be limited to one basic topic so different conversations don't get tangled together.

So the Montgolfier brothers used wood to heat their balloons, and had issues in an early flight with sparks threatening to burn the balloon.

A J-tube rocket of lightweight insulating material could provide more heat for the fuel used, and with full combustion may give less danger of sparks. A complication would be inflating the balloon to start with, as the rocket would not be able to blow hot air into the flattened balloon; not sure how modern balloonists do this, with just a fan or with the propane flame directed sideways...