8" J-tube with half barrel bench

I'll probably be building an 8" J-tube with half barrels soon.

My question now is, how many barrels can I use without running the risk of too much condensation and poor draft?
I have gathered that an 8" J is roughly like a 6" BBR, so that would mean a bell surface (CSA) of 5,3m^2.

With an entire barrel exposed on top of the riser (~2m2) that only leaves 3 more half barrels (0.85m2 each, plus two half lids).

In my first design idea I would need 4 half barrels. So maybe I have to partly cover the barrel over the riser?

Any input is welcome.

Hey Ben, a bit of RMH history
Years ago, I asked Matt Walker how many half barrels an 8 " J can push, and his answer was 5 barrels.
Matt's 1/2-barrel idea was well before Batchboxes became mainstream. At that time, nobody knew the parameters between them.
He built his half-barrel J-Tube in a bar back east, stunning the fledgling RMH world!
Meanwhile, Lasse Holmes & friends were playing with fire on the west coast and invented the Batchbox design, which Peter then refined into an ultra-efficient powerhouse, and inserted it into a stratification chamber (Bell), which is known today as First Generation Batchboxes.
Matt was also the innovator who disassembled his 8" J-Tube with a piped mass and installed one of the new 6" Batchboxes into the 8" piped system.
This was where we learned that a 6" batch and an 8" J-Tube were comparable in performance output.

Thank you Thomas for that update in RMHistory.

5 barrels would be more than enough

Here's a first sketch:


The core and a small part of the bench are going to be in the kitchen.
Then the bench goes through the wall and a bigger bench is going to be located in the living room.
At the end of the bench it connects to the existing chimney (the little square on the right). Maybe the chimney will get a metal insert, maybe I'll use it as is. There's no option for a new chimney at another point. The roof was just renewed and we don't want to penetrate it anywhere else.

The biggest challenge I see at the moment is not having any bypass option. I'll be installing an opening where we connect to the old chimney to have an option to prime the chimney.
I could also install an extra exhaust pipe on the top of the bench, allowing the warmest air to exit in the beginning. That could act as kind of a bypass. Although the exhaust would still have to travel all the way to the end of the bench. Just not sink to the bottom.
Any opinions?

Looks like a good plan. Just remember that when you join half barrels at 90 degrees, you will have to extend the top of one barrel over the joining one to cover the whole space. This means either shortening the run covered by the barrel, or adding a partial "triangle" of barrel to fill the gap. So for your plan, the best thing would be cutting filler parts from a third whole barrel rather than alter the shape of the layout.

Thanks Glenn. Yes, I was counting with another barrel for scrap cuts.

Does anyone have some insight on the (lack of) the bypass? Is a "top of bench" bypass better than nothing?
If I imagine the flow of the gases, I would say theoretically yes.

So I'm still wondering about the usefulness of this "top of the bench" bypass.



I would only gain about 6" height difference between the regular opening into the existing chimney and the "top of bench" bypass opening.

I will go for a T (or other opening) anyways. For inspection and priming. But I'm wondering if the bypass will be useful.

Hey Benjamin,     I personally don't think a bypass at the far end of the bench with only a 6" height difference will do a heck of a lot. Perhaps just a cleanout to access the base of the existing chimney where a small fire could be initiated if the draft is needing a boost would suffice?

Hey Gerry,
thanks for the answer.

I don’t see any other way. But then there are lots of benches with the chimney at the far end.

When I go check the site on Sunday I’ll keep your idea in mind. I like it.