"Heat Engine" recommendation request

Hello great people here.
I have been reading some of the topics on permies on and off for awhile, but this is my first post. I live in the mountains and heat our brick house with wood. I'm tired though of all the mess resulting from bringing the wood indoors, and especially the smoke given off by the two metal stoves we currently use, one in each of the rooms we have.
I dream of building a small annex in the back of the house, directly on the outside of the rooms I want heated. I wish to build an efficient, hot burning wood stove there, some sort of rocket stove I imagine, I'm open to suggestions. That would be the heat producing "heat engine".
I wish to pass the hot gases inside the house thru an new opening in the brick wall, where they would heat a thermal mass (bell or bench, not decided yet). The cold gases would exit either thru the existing brick chimney that already exists built into the house structure, or thru another opening in the outside brick wall.
I envision burning wood as needed during the day or afternoon and thereby heating up the thermal mass for the remainder of the evening and night.
The question I have for you is which type of "heat engine" would you recommend?
I wish for something that would be able to burn unattended for longer periods, I'm not too keen on tending the fire every few minutes as some of the J tube rockets seem to require, and I wish to be able to burn both thinner wood/branches but also bigger pieces of split wood, hoping I can insert a larger quantity of wood fuel at a time and leaving it alone for a longer period (minimum 45 minutes or longer if possible). A longer burn time is important to me, if it could be achieved, not necessarily burning all the wood very quickly and releasing lots of heat at once.

I contacted Matt at Walker stoves since I was hoping his design would work, but he pointed out that his burner cores exhaust the hot gases on the entire top surface of the stove, great for a cook top, but not easy to channel into a hole in the wall into the house.
I don't wish for a tall riser though, as I need to channel the hot gases into a bench, I wish for the exhaust to be lower in height.
Looking thru the topics on this forum I see many designs that look potentially very good, but me being new to this, I wish for your recommendation for design that is known to be reliable, fairly easy to build out of fire brick as high tech foamboard might be difficult to source in my country.
I see many designs discussed: shorty, sidewinder, batch rocket, etc, etc. Which design do you recommend I should start studying first, I'm a bit confused and feeling like taking a drink from a fire hydrant at this time. I like tinkering and learning new diy things, but I don't want to make it into a new career, and I wish for something that would work reliably so that other family members would learn to operate easily if I'm not home.
Thank you very much for your suggestions and ideas.

Hi and welcome to Permies!
Usually it’s best to build the whole rocket in the room you want to heat. But I can understand that you want the dirt in a separate building.
I think building a batch box rocket (anything that’s not a J tube) in that separate room could work.
The confusing names are all different types of cores for batch box rockets (BBR).
Best case most of the core and the bell or bench would be in the room you want to heat and only the opening with an insulated door would be in the separate room.
If you want large intervals of feeding you need to build a large batch box. I would go for 8” (200mm).
I’m skeptical about using the existing brick chimney. Could you run a metal pipe through it?
Can you provide us with a simple sketch of your house?

The size and condition of your existing chimney would be important information, as well as its location relative to your proposed addition. Your climate would also help us give the best advice.

I agree with Benjamin that you could put the feed for a batch box in the new room and the rest passing through the wall into a bell/bench in the existing rooms. I think a first generation batch box (with floor channel) would be easiest for a beginner to build. The tall riser is not an issue for feeding a bench. The latest Shorty core depends on a custom fabricated and welded doorframe which needs to be built exactly to spec. A batch box system sized correctly for the load would only need to be fired once or twice a day, not kept burning continuously. Your addition would only need to be big enough to hold a stack of wood for a few days' use and room to comfortably feed the fire. The bell/bench inside the house would likely not take up more space than the safety clearance for your two existing stoves, though you may need to reorganize the interior layout. Again, a sketch would help us give the best advice.

I have read, here on the forum about folks doing what you want to do with rocket stoves.  I did not know this is called a heat engine.

The stove is built outside with tubes running into the house.

Like the others have suggested building a RMH inside might be more efficient.

"Heat engine" is not a regularly used term, though it does describe a RMH.

A whole batch box outside with some mechanism for getting heat indoors is always going to sacrifice a lot of the heating potential. Far better to have the system inside, even if it is fed from a separate space outside the heated rooms. (Not practical for a J-tube, but can work for a batch box which needs to be loaded less often.)

I think Glenn is right here, having the batchrocket core in the annex and a bell in the living room is asking for problems.  
May I suggest this: a hole in the wall between living and annex, big enough to have the entire core sticking through, walls, floor and deck of the firebox. Plus some space extra to be filled with superwool, acting as an expansion joint. Depending on the thickness of the wall, this core could be a reversed Shorty or a sidewinder. A straight Shorty that is exhausting in the normal way back to the wall isn't advisable, almost certainly too cramped.
The bell on the other side of the deviding wall can be of any shape, as long as the end of the core fits in it. The only real requirement for building such a core is the specific door frame which is also acting as an air duct.

A Shorty is the most user-friendly of the whole batchrocket family. It's a casual burner, easy to refill and no fiddling with the air controls is required.

Anne Miller wrote:I have read, here on the forum about folks doing what you want to do with rocket stoves.  I did not know this is called a heat engine.

I would translate “heat engine” to the core of a rocket mass heater. That’s where the rocket part happens.
Then there’s the bench or bell that puts the mass in the rocket mass heater.
I think among the RMH-enthusiasts we use the word stove if we want to talk about something to cook on. Specifically to make a difference between a rocket stove (often just sole metal pipes) and a rocket mass heater.

Thanks to all for your replies and suggestions.
I heard from several of you that I'm asking for trouble by placing the heater core in a separate room from the thermal mass, and I trust your judgements, so I'll reconsider the project at least until I get some more hands on experience. I plan on studying the different variants of the batch box and building a simplev one first.
For those that asked, I live in the mountains of eastern Europe, we get low temps into the negative 10 C up to minus 18 sometimes (down to zero degrees F), so basically zone 7a
Thanks for your input, I suspect I'll be back with more questions now and then as I discover the joys of making heat with (hopefully) no smoke.
Thanks again
Costi R.

For a very quick and easy demonstration of a batch box, you could try the no brick cutting 4" batch box mockup that I built to test the idea. Just a small basic core with no surroundings, but it gives the sense of how a batch box works, and you can reuse the bricks for a full-size build. The core can be built using the same brick layout, no matter the size of your local bricks.

Thank you Glenn for the link, this is what I need at this stage, experimenting with different mockups to see what works reliably before committing to a design.
I prefer building with (fire) bricks rather than high temperature fiberboard (I think that's the name), as it is difficult to source in my country. So I thank you for plans building heater cores out of brick
I will try to explore Mr Peter van den Berg's site or postings to learn the different variants of batch boxes he invented, if they can be built of bricks I will try some of them and report back.
Thanks again
Costi R