thomas rubino

Hey Sam, are you using Clay/sand mortar or refractory?
In my builds, I only use clay mortar.
Refractory is expensive, and you have to chip/grind it off if you want to reuse bricks.
Clay mortar is inexpensive; it pops right off, and you can rehydrate and reuse it!

50-52" are about the tallest risers that I have seen used.
I believe I have read that if it's too tall, it starts losing efficiency.
As far as the top gap between the barrel and riser, I always went with 2.5" to allow for fly ash buildup on the top of the riser.
On the 6" Batchbox stoves, we use 12" as a minimum clearance.
With a J-Tube, a larger top gap would give you a lower barrel temp but a higher mass temp.
You might give 7" a try...

To me, the most important place is the exit chimney.
At home, that is all I look at; really, it's the only one that really matters.
But other locations around the bell or riser are pretty neat to see, too.

Hi Ben;
One for sure in the exit chimney and another at the top of the strat chamber, at the farthest point from the core

I don't have a proven technical reason, other than guessing an incomplete burn.
One year, I did a summer rebuild on my 8" J-Tube, with a piped system.
I extended the burn tunnel to have a 15" roof. (If 10" was good 15" must be better, right???)
For the first 3 months, it seemed to work great!
During the 4th month, I noticed that it seemed to be not as rockety, by the fifth month, it was stalling and smoking back.
This 5th month happened to be February in northern Montana.
The J-Tube was in my wife's uninsulated artist studio...
I was lucky enough to catch a sunny 35F day to open the stove up.
I found my transition area full of ash AND my 8" pipe was reduced to apx 1.5" at the top of the pipe...
That ash made it all the way through the horizontal pipe to the vertical turn to go up the chimney.

GOOD GRIEF!   I was frantic.
This was where I learned just how fast and easy a J-Tube rebuild/repair can be.
I started at 8 a.m., and I was relighting at 1 pm!  
Core rebuilt to an 11" burn tunnel roof, transition area, and pipes vacuumed and blown out.
Starting to cool off in the room but quickly warmed back up as soon as the dragon started roaring!

Hey Sam, looking at your photo, the feed tube appears to be rather shallow.
Normal depth for a J-Tube, feed tube is 16"  
Wood is commonly cut to 16"; this keeps your wood entirely contained, with no chance of a piece tipping over onto your floor.

Just be sure not to make the burn tunnel roof more than 10-12". Too long will work, but it will fill the system with ash before spring.

Excellent post, thank you for sharing!
Sometimes you need to make do with what is available.
Two seasons is pretty good for a metal J-tube.
This season, with firebricks for your core, I think you will be very happy!