thomas rubino

Hey Josh.
I forgot about the "Baffle", leave that out, and please don't put any restrictions in the chimney system of an RMH.
Next, regarding AI, Liz has found it extremely useful for her business. However, she is asking for keywords or descriptions for artistic work.
In this case, you are asking it to make mechanical suggestions on a subject with significant conflicting information, due to ongoing innovation.
Ask at Permies, Donkeys pro boards, Walker stoves, Peter Berg himself, even Dragontech will help answer questions.
Call me old-fashioned, but I don't believe AI is as qualified as the folks who developed these stoves.

Yes Josh;
A 33" burn tunnel is much to long.
That makes the roof of your burn tunnel 17" long.
Glenn has 8=9", I prefer 10", but have found 12" to work, but no more than that.

Your buddy is asking that slowed down 8" J to push through 6" pipes, again, not to specifications.
A recipe for an ash-plugged system.
I used a 2.5" gap on my 8" for seven years.
I also had a 48" riser. I recorded a 1100°F reading on an 8" glowing red circle on my barrel top.
The riser can be within an inch of the barrel on one side.
What kind of transition area is the barrel dumping into?

You asked why a long burn tunnel is bad.
To create the temperatures at which J-Tubes are recorded.
You must allow the gases free easy movement, although it looks like your J-Tube is burning properly, I assure you it is not.
The core portion of any RMH must be built to specifications if you expect it to work like we say it does.

In your example, the gases are slowed by their horizontal travel. When they reach the riser, they cannot get the 1800°F temperature that it is touted to reach.
This allows Ash to reach the transition area and enter the pipe system.
Increasing the riser height to compensate does not solve the problem.
After the riser, he is asking for 8" of hot gas to travel through a 6" pipe.
It fills with Ash, slowing it down to the point it will not light properly anymore.
I know this to be a fact, as my 15" burn tunnel roof, plugged with ash by mid-winter...
Prior to that, I thought I had a Bad Ass Rip Roaring J-Tube...  I did not.

I believe it needs a rebuild to "factory" specifications, if he wants factory high-performance results.

Hi Cerbu;
Those are wood gases burning, but the source of the colors is a mystery to me.

Hi Matthias;
I agree with Ben, you need a matching chimney, perhaps even a 150mm for your 5.1" Shorty.
I have both a Shorty and several 1st-generation Batchboxes.
Shorty is an incredible stove, but she does not have the power of a first-gen.
As casual as Shorty burns, she "might" flow with a smaller flue, but there is a much better chance she will not like it.
Peter has recommended the use of a bypass with Shorty, something he did  not recommend with his other designs.

You could try if you wish, but I would have a larger pipe on hand; I think you will need it.
Perhaps Peter will give us his opinion.

Josh;
You mentioned that the burn tunnel was made longer than it was supposed to be.
How long is it?   Just the roof, from the trailing edge of the feed tube to the leading edge of the riser.
Stock specs call for this to be 10" up to 12." I built one at 15"... BUT, only one time.
I had to do a mid-winter teardown to shorten that build. It seemed to burn OK, but was filling my pipes with ash.

Those two barrels share plenty of heat while burning, but they also shed plenty of heat when not burning.

3" is now considered to be a good gap for riser to barrel clearance.

My 8" J-Tube in an uninsulated greenhouse in northern Montana was run nonstop from apx 7 AM to 10 PM all winter long.
I call it extreme burning. With an exposed barrel and 45' of buried 8" piping, we could easily top 80F while it was in the low teens outside.
My chimney exhaust gas temperatures were well over 340°F; they should be 180°F- 220°F.
By 7 AM the next morning, it would be in the low forties or upper thirties and dropping steadily.

My new all-brick double bell first-generation BatchBox is in the exact location. And easily brings the room to the lower sixties with three fires a day.
The next morning, it will be in the upper fifties and creeping downward at a tenth of a degree every half hour or so.

Hi Eugene;

An 8" J-Tube has more than just a few sticks burning. With a 7.5" throat, wide open all the time, they roar.
After many modifications, they have been "maxed out" in terms of output.

A 6" Batchbox derives its power, not from the size of the load of wood, but from the size and shape of the box and the port size in the back, and receives a supercharge of cool oxygen through the secondary air tube at the throat of the port, creating a cyclone or double rams horn of fire reaching greater burn temperatures than a J-Tube can create.

Another winter project completed!
I held off last year on mudding down the tiles over Shorty's core. I wanted to confirm there was no cracking or leaking from the firebrick top.
We also had a repair to make in the core that might have required partial disassembly.
Luckily, the repair was made without a teardown.
I did discover that the ink on my Tree of Life tile couldn't withstand the high heat.
I asked Liz for a replacement tile this Christmas, and this time I placed it on two pieces of Superwool Gasket, for a full 1/4" of protection.
Monday, I mixed up some clay-heavy mortar (2 scoops of clay & 2 scoops of sand)
I taped and papered as much as I could to mitigate the mess, but I still had a big clean-up anyway...

I have more cleanup to do on the mortar between the tiles, but this job is 98% complete.

Shorty be Styling!

Fluffy leaves no doubt about what to do with 2025!
Happy New Year 2026

Hi Austin;
I suspect you are correct that your sand is not sharp enough.
I tried using different sands over the years, and I had cracking or popping issues every time.
For a straight mortar, I use one scoop of dry clay and three scoops of medium-graded sand, pure white and completely uniform.

I have had top bricks that got bumped, and they pop loose. I scrape off the dry clay to rehydrate, mix some new up, and reattach.

Nothing wrong with using refractory if it comes free; try to clean the refractory off, or use a new  brick.