RMH vs Batch box & chimney questions.

I have been researching to build a RMH, 8" version. I'm now seeing the batch box and frankly the higher heat of the batch box at the heat riser barrel kind of scares me. While I really like The idea of the fire bring enclosed behind a sealed door, and the more efficient burn, the temps are crazy. The ceiling would only be above 36 to 40 inches above the barrel top for a RMH and about 23 inches above a bathbox double barrel. I fear the ceiling would dry, become brittle and catch on fire.
I'm in central Indiana where the temps remain in 20° to 30° range from late November to early March with a week or two of temps down to -20°. I live in a walk out basement. Half of my living quarters are below ground. I am completely surrounded by tall timber.
An a-frame house with loft sits above me, (partitioned off and rented out).
The house is pretty much and open floor plan. The portion mainly underground is 616sq ft, the add on is 416 sq ft for a total of 1032 sq ft. The addition has R19 insulation in walls, R30 in the ceiling. It has a new insulated metal roof. Windows are double pane low e, so are the doors. Foundation is a concrete slab.
The walls and ceiling are 3/8 headed panel pine. The roof slope is 2" in 12".
I plan on the exhaust pipe/chimney being 12ft from the a-frame roof which has a slope of 16" in 12". Would 5ft above the roof be sufficient? 2ft to clear roof slope plus 3 ft. It would be impractical to make a chimney that extends 3 feet above the a-frame roof line. (28 feet a-frame roof plus 2 feet for slope of addition plus 3 feet clearance above a-frame ridge would be a 33ft chimney! Quite expensive as well with triple wall stainless pipe being $145 for 3 feet.

Sherry Johnson : Welcome to Permies.com, our sister site Richsoil.com, and a BIG Welcome to the Rocket and Wood stoves Forum Threads !

With >35,000 Fellow (sister?) Members World wide you can gome here 24 / 7 to talk to someone who wants to talk about what you want to talk about !

Have You been to Rocketstoves.com to Download your copy of the 3rd Edition of 'Rocket Mass Heaters'? The download puts the book into your
hands today with the ability to make all the copies you need, no worries about loosing a couple pages !

With your location a 6'' system should easily handle all your heating needs and supplement your primary cooking source (?) Many people are in love with
the ''Batch box'' Rocket Mass Heaters RMHs,They are a lot harder to build and can be more finicky - I consider them to be an advanced build, something to
attempt after having successfully built and lived with a 'Regular' RMH, The increased height requirements can also be an issue, especially if using the barrel
top as a cooking surface.

Generally any of several options are available to turn the top of the Regular RMH into an oven, and can be as simple as a beehive shaped out of Sheets of
aluminum foil, or something bought commercially.

Just talking about the Thermal Mass regardless of type of RMH, The Exhaust Gas Temperature at the last clean out / base of the Final Vertical chimney should
be close to 200 ºƒ if it is not going through the interior of your A-frame !

This opens up a few possibilities and even ethics! Because of the low Exhaust temperatures you would not need triple wall pipe !

This is something you need to work out with your Local Code Enforcement Officer ! Will he allow you to use chimney requirements for masonry heaters as a
best practice rule- if so the cost of your piping would be greatly reduced ! Warning a future owner resident might want a more conventional wood stove with
higher operating Exhaust Temps and burn the a-frame down trying to use that chimney !

I had to guess a little on layout -a sketch would help a great deal ! For the good of the Craft ! Big AL

Late note : While the 1st video is of an8'' system and goes through triple wall here exhaust temps are closer to 150ºƒ link below :


https://permies.com/t/25435/rocket-stoves/video-great-rocket-mass-heaters

Your chimney 12' from the A-frame technically meets code for ending 5' above your low roof, but the A-frame is likely to induce some unpredictable turbulence for a good distance. What side of the house will your chimney be on (N-S-E-W)? Where do prevailing winter winds come from? In your case, being on the upwind side may be an advantage to avoid possible downdraft gusts in storms. You would want a very good chimney cap in any case, maybe the "H" style.

As far as efficiency, those who have done tests find that the J-tube and batch box are similar enough that exact details of construction would make the difference. The J-tube may not be enclosed, but the top opening and relatively small fuel load at any one time make problems unlikely (as long as loading recommendations are followed - no tall sticks rising way out of the feed tube!)
The batch box would be more suited to a situation where you can't or don't want to tend it regularly during the burn. You would not necessarily need a double barrel for a batch box, just an appropriately sized mass to take the heat dumped by the fire.