16cm Batchbox with water heating in second bell - Advice?!

[rocketyurts]

Hello stove enthusiasts,

It's been a while since i last posted in this forum but I've been trying to follow what's going on. Many thanks for all the information that you've all gathered here and for all the people who shared their builds and ideas!
For my next build I'm looking for some comments and advise as it's going to be a more permanent build that ideally will last a long time in the new house - and also because I want to get feedback on this idea of adding a water tank into the second bell (similar to this design: www.permaculturenews.org/2012/11/23/rocket-stove-hot-water/ but with a masonry bell)

Short background to this new build:
After living in a yurt for around 8 years heated by this 16cm J-Tube with a huge piped-bench submerged in the floor ( donkey32.proboards.com/thread/3614/winter-rocket-stove-review-feedback ) I've decided to turn the yurt into a strawbale roundhouse:

Before:


Now (you can still see the bench in the middle):


Now the new floor will be 20cm below the level of the bench so a new build is in order. Since this first J-Tube i've built two batch box rockets for our open wood and ceramics workshops wich perform well so this one is going to be a batch-box as well.

I've used the ressources on www.batchbox.eu **THANK YOU FOR THIS TREASUREBOX** to calculate the 16cm system (the wood load per fire for the 160cm system is an estimate):

So I started sketching a few ideas how the stove could look like, I'm going for a single skin design for the bells plastered with clay from the outside (just above the riser, double skin insulated with split fire bricks):

Questions:
1. In our last BB-rocket stove builds we've insulated the firebox and the bottom half of the riser (all just regular ol' bricks) with a 25mm Calcium Silicate board (double on top and bottom of firebox). The riser was insulated with a ceramic wool blanket.
See pictures here:
jonglirium.com/wp-content/uploads/2024/11/BB-Heater-Core.jpg
jonglirium.com/wp-content/uploads/2024/11/BB-Heater-riser.jpg
Would you recommend this as well for this build?
I plan on using fire bricks for the core and riser and lining the top of the bell above the riser with split firebrick. Should I change anything with the insulation that was used before (calcium silicate board, ceramic wool)?

2. I would base the mortar on our local clay and mix the mortar for the firebox and riser and other heat sensitive areas like the top of the bells from clay and grog (made from broken fire bricks) replacing the sand fully with grog (1 part clay - 1,5 parts grog). Is this the tested and tried approach?

3. Do you think the door to from the first to second bell is large enough in oder not to restrict air flow?

4. The water heater has a volume of about 40l - it's meant to pre-heat the water going through it (a pressurized stainless steel coil going through an unpressurized tank) a small electric water heater will be installed behind it.

Do you think it makes sense to build in the water heater like this? What kind of output in terms of hot water is to be expected - any thoughts on this are welcome!
If the water tank turns out to make not so much sense i would build the system as a single bell without the floor channel and without the door betweeb Bell I and Bell II

Thanks a lot for your time and thoughts!
I'm looking forward to this build and a bussy wintertime.

[peterberg]

In my opinion, you are overthinking this greatly. Much too complicated and mostly cramped for the functions.

Don't do a triple bell that's too large so you'll need valves and so on. Ditch this design and start a new one with a single bell, large enough to house the combustion core and the hot water heater. The boiler in a second bell won't get hot enough, anyway.

[rocketyurts]

Hey there Peter and thanks for the super quick reply!

The coin dropped very quickly after reading you: Man I made it complicated - Thanks for pointing out obvious!
I'll sit down and do a new draft asap - for now water installations want to be tackled before frost arrives.

All the best & thanks a buch!

[rocketyurts]

So here we go.

I simplipfied the design. Just from the top for speed. The whole thing is now just all the same height similar to the first bell in the first design (1,40m internal and about 1,60m total height).

Questions:
1. In our last BB-rocket stove builds we've insulated the firebox and the bottom half of the riser (all just regular ol' bricks) with a 25mm Calcium Silicate board (double on top and bottom of firebox). The riser was insulated with a ceramic wool blanket.
See pictures here:
jonglirium.com/wp-content/uploads/2024/11/BB-Heater-Core.jpg
jonglirium.com/wp-content/uploads/2024/11/BB-Heater-riser.jpg
Would you recommend this as well for this build?
I plan on using fire bricks for the core and riser and lining the top of the bell above the riser with split firebrick. Should I change anything with the insulation that was used before (calcium silicate board, ceramic wool)?

2. I would base the mortar on our local clay and mix the mortar for the firebox and riser and other heat sensitive areas like the top of the bells from clay and grog (made from broken fire bricks) replacing the sand fully with grog (1 part clay - 1,5 parts grog). Is this the tested and tried approach?

3. Do you think the gases will be able to flow unrestricted? The Water tank will be placed in the top 2/3 of the bell so there should be around 60-70cm unrestricted (safe for the mounting of the tank) air space underneath it before the air is channeled into the chimney pipe

4. The water heater has a volume of about 40l - it's meant to pre-heat the water going through it (a pressurized stainless steel coil going through an unpressurized tank) a small electric water heater will be installed behind it.
Do you think it makes sense to build in the water heater like this? What kind of output in terms of hot water is to be expected - any thoughts on this are welcome!

5. ISA. If i play it safe i arrive with an ISA of 5,73m² (out of max. 6,01m² recommended ISA). If I add half a brick more I'm at 6,1m². I didn't substract the firebox door CSA from the ISA and wonder if I should cut it that close.

6. In the floor plan of the rounhouse it makes a lot of sense to attach the shower here using the oven as one wall (as the roundhouse does't have too many corners) - is that a very stupid idea? Anyone tried that before and can recommend a finish / tile application technique?

Thanks in advance for reading & puting time and thought into this project!!