Batch vs. J-Channel - Pros and Cons

I've been looking for a good comprehensive discussion/article/thread on the pros and cons of batch vs. J-channel on the Permies site and elsewhere, but have not found one.  If anyone knows of a source for this information please share, or maybe we can create the information in this thread.

Curious about:
- efficency (fuel consumption)
- build cost
- build complexity
- long term maintenance (cost and complexity)
- minimum realistic/efficent system size

Also interested in conversion of J-channel designs to Batch rockets.

Hi William;
Here is my conversion from an 8" J-tube to a 7" batch box in my shop
And here is my 8" J-Tube being converted to a 6" batch box
https://permies.com/t/138779/Breaking-News-Shop-Dragon-admitted
https://permies.com/t/148226/Dragon-Transformation-tube-Batch-Box

Fuel consumption is very close between the two styles.
An 8" J-tube requires feeding every 45-60 minutes.
A batch box requires wood every 2-3 hours.

Batch boxes cost more to build by at least a few hundred dollars depending on, creating your own metal parts or purchasing them from me.
Both styles are simple to build, J-Tubes require no metal parts in the core.
If you are a perfectionist then your RMH will work flawlessly for years.
If you are like the rest of us then you will be making changes /improvements or even repairs, from time to time.
Don't sweat about it. Once you build your RMH you will understand just how simple and easy it really is.
Rebuilds and changes are just a few hours of work no big deal.
I have done emergency rebuilds during the middle of winter. They go fast when working with clay/ sand mortar.
If you feel the need to build using overpriced refractory cement then all bets are off!
Rebuilding with refractory involved is a nightmare!
I highly recommend 3 parts clean sand to one part clay as mortar

Size?  J-tubes are commonly  6" or 8"
Batch boxes can be as small as 4" or as oversized as you care to create.     6", 7", or 8" are the common build sizes.

Hi Thomas,

Great information.  Thank you.

I'm thinking I may start by building a J, but building it to minimize issues of converting to a Batch Rocket when/if I do convert.

Hi Thomas,

I noticed you used 7" (shop) and 6" (studio) when you converted your 8" J-Tubes.

Do the 7 inch and 6 inch relate to Peter van den Berg's design dimensions https://batchrocket.eu/en/building?

The Cross Sectional Areas for 6, 7, and 8 inch are 28.28, 38.48, and 50.27 square inches respectively.

Are the reduced size of the batch boxs you built based on your BTU (kWh) requirements,  a physical design (size) constraint, or some other factor?

Thanks

Hi William;
Yes, both the 6" and the 7" batch boxes are built to Peter's dimensions.
The 8" J-tube in our studio utilized an 8" piped mass.
A 6" batch box is the largest size suggested to use with a piped mass.
That stove is problematic starting in the fall.
It has a tremendous mass that must become warm before a strong draft starts.
Use of my bypass is mandatory until warm.

The 8" J-Tube in the shop only had a single barrel.
After converting it to a stock size 7" batch.
I added a second barrel to increase my overall CSA and to provide instant radiant heat for a large practically uninsulated shop.
The brick bell has two tall towers of brick inside, as well as a few chunks of railroad track leaning in a corner as extra mass.
Once that bell warms up to 200-300F external temp. the entire shop becomes warm.
Open the shop doors to bring in equipment and it only takes a few minutes for the entire shop to feel warm again after shutting them.
Prior to having a brick bell RMH, I had a double-barrel conventional wood stove.
It could be glowing orange and you could not get closer than 5 feet from it.
15' away tools would stick to your fingers it was so cold.
Open the shop doors and it never warmed up at all.

Building RMHs was the best idea I ever had!
Converting those stoves to batch boxes was the second best!