Glenn Herbert

You can try all the tricks you want, but Mother Nature has seen them all and will not be fooled. It is a law of physics that you cannot get more energy/work out of a system than you started with, and you cannot even get all of it back if you do anything with it.

I would advise you to research in and around Vancouver, where Dale got his trailer. What part of Canada are you in?

I fully agree with using a bell instead of a piped mass. I do think that with your climate and likely intermittent or light heating requirements, a J-tube would be quite practical, and easy for a novice to build. With the core and riser in the location you show, you can build a single-skin brick bell to clear the riser and have enough space to the stairs above to be safe. (I would not feel safe with a barrel under the stairs at all.)

I would consider a narrow tall bell (just big enough for good airflow around the riser) with a bench-height extension to the left. This would all need to be sized for the right internal surface area. Batchrocket.eu gives the complete information on this; an 8" J-tube has been found to be similar to a 6" batch box for bell sizing purposes. If you can exit nearly straight up, I believe a 6" chimney will serve you well. An 8" chimney would be bigger than you need, and you would have lazy exhaust and possible condensation issues. I would not be concerned about oversizing a J-tube, as you can burn just one load a day if your heat needs are slight, and not overheat the house.

The stair location looks ideal for evenly heating your house.

For structural support, I would suggest adding a couple of posts between the existing ones under the bell, to equalize the loading. I also like your idea of making the mass exposed on the bedroom side.

There is not a simple cutoff point for temperature; higher temperatures accelerate the breakdown process. Paper stored in an attic can experience the effect over decades.

Is there any gap between the bench and the wood posts? If they are in contact, there may be an issue. How close are the heat ducts to the back of the bench?

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...