Glenn Herbert

Yes, raising half barrels on a layer of rocks and sealing with cob would work fine. I have not used thin layers of cob on a bench top, so cannot personally advise on the best thickness - many others here can do that.

Cob reinforced with some straw will keep cracks from opening, and a chimney which draws decently will ensure negative pressure inside once the system is warmed up, so any leaks would draw into the bench.

If you make a mass cob bench, you don't need to sift any stones out for the bulk of it, only for the surface layers. In fact, a popular technique is called "cob lasagna": a layer of cob, a layer of rocks as big as will fit comfortably, a layer of cob, repeat until full. Most rocks will be more massive for the volume than clay, which is helpful.

I do think it would be easier for you to make a bell, especially a half-barrel bell, than ducts winding through the mass. It would give much less friction, and thus much less chance of difficult or slow starting, forever after.

Does your barrel have a removable lid? If so, cut out enough of the barrel floor to fit over the riser, seal it to the manifold, and just remove the lid to inspect/clean the riser (in conjunction with the cleanout door at the base of the manifold).

Some people have removed the top of an inverted barrel leaving an inch of horizontal metal, and installed a circle of heavier steel with a gasket fitting into the recess, sometimes just held by gravity.

If you have not built your bench yet, I would advise you to make it a bell (a simple hollow brick box) rather than ducted channels. You will have much less friction, and cleaning will be simpler.

I agree that a simple bell will be considerably easier to build than one with a bench; if you can fit a simple rectangular bell with the right ISA under the stairs, do it. My main thought with the "bench" extension was to get enough ISA under the sloping stairs.

A small chair or two that store under the stairs and can be pulled out when you want to sit close to the fire would work nicely. We have a couple of old wooden little child-size school chairs, very sturdy and perfect for sitting and feeding the fire.

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 ).