Bench for a rocket stove

Hello everyone. First time posting here. I have some work ahead of me and wanted to get some good constructive comments.

I have a straw bale house built on concrete piers and have built a concrete foundations for the stove and the bench that would go along with it. First plan was to build a brick batch box rocket heater but unfortunately I am not able to do that now. It would have been a 165 mm size with a 180 mm stainless steel chimney that I already bought. Since the batch box is not an option now, I would like to replace it with a metal 150 mm Gamera 7 rocket stove, connect that stove to the bench and the chimney and maybe one day, if  there will be an opportunity, build the brick batch box rocket stove.

I'd like to describe all the parts of the system and if anybody has any kind of comments, it would be very helpful since I am basically a complete beginner regarding stoves.

So:

1. the metal rocket stove (blue circle in the drawing) is nominated at 14 KWh, 150 mm diameter exhaust.

2. The bench would be around 2,25 meters long, around 50-55 cm deep, and around 40-45 cm high. The duct pipes would also be 150 or 160 mm diameter. The outer wall of the bench would be build with bricks 6,5 cm thick and then plastered outside with ~1-2 cm clay plaster. The inside space between the duct pipes and the wall of the bench would be filled with cob.

3. the chimney is 180 mm in diameter. I already purchased this so hope the size isn't too big. It's stainless steel, 316. It was a good discount. 2,5 cm insulation around it. 6 meters high.

4. The green square underneath the metal stove would be the place where in the future a possible batch box stove would be built.

My main questions are: are the stove, the bench and the chimney compatible to get a good functioning system? And also, will I be able to build a masonry batch box in the future, if I chose to? Is the 93 x 80 cm enough space for it? What kind of material should I use for the duct pipes? Stainless steel or galvanized steel? Something else?
Also, what kind of floor insulation should I put underneath the bench and stove? Between the concrete foundations and the heating system itself. Currently, I have poured the concrete as a base of foundation, but since my house is 50-60 cm above the ground I still have opportunity to make plans while building everything up to the floor level where the stove and bench start. Am definitely putting bitumen hydroinsulation before laying concrete blocks to get to the adequate height.

I didn't draw any cleaning holes for the duct pipes. I guess at the other end of the bench, opposite to the stove, would be the best place? And I don't have any bypass from the stove to the chimney. Is that necessary to start the fire? I see there are stainless steel valves available to buy, with a simple on/off function.

BTW I live in continental Croatia on a hill, almost at 300 m. The border with Slovenia and Hungary is few km from my place.

Thank you everyone for your time. I included a drawing of the whole system + a photo of my house, the wall, where the stove would be. I think I would also have around 15-20 cm space between the wall and the stove. Hope everything is clear.

I can post a link to the stove I just purchased, if it's permitted. I added the photo for now.

Luka

Hi Luka.

93 cm pad length will be almost enough to squeeze fi150 BBR there, but not more. Firebox length would be around 700 mm Bricks in Europe are usually 120 mm wide so 700+240 would already put you over your 93 cm. You also need to add 10 mm gaps between firebox and the bell. I would suggest extending the slab while you can. For fi 165 you want 1100 mm and another 200 mm so the exhaust gases will have unobstructed path to the chimney exit. So 1300 mm deep, assuming you will keep 800 mm width.
At this footprint, the bell size will be around 2 m tall assuming a system with no bench.

Nowadays benches are just open bells without any piping. Cheaper, easier and better following the principle of free gas movement.

Cristobal Cristo wrote:Hi Luka.

93 cm pad length will be almost enough to squeeze fi150 BBR there, but not more. Firebox length would be around 700 mm Bricks in Europe are usually 120 mm wide so 700+240 would already put you over your 93 cm. You also need to add 10 mm gaps between firebox and the bell. I would suggest extending the slab while you can. For fi 165 you want 1100 mm and another 200 mm so the exhaust gases will have unobstructed path to the chimney exit. So 1300 mm deep, assuming you will keep 800 mm width.
At this footprint, the bell size will be around 2 m tall assuming a system with no bench.

Nowadays benches are just open bells without any piping. Cheaper, easier and better following the principle of free gas movement.

Cristobal, tnx for your input. Regarding the pipes in the bench, I will draw a version without them. I understand what you mean.. How do you comment on the size of the surface area? I know it's a bit too small, but will it be a big problem or just make the bench hotter and the exhaust air a bit hotter?

As far as the future plans go, the bench would stay even if I decide to build a batch box. So, maybe 2 meters height won't be necessary. I will definitely build a bigger base now so that I have enough of it. Regarding the width,  80 cm, I could make that 90 cm. The depth I can make 130 cm, yes..

Hi Luka,

Welcome to Permies!
And congratulations on your beautiful straw bale house build, looks very nice!
I've understood that the diameter of the system should be the same from fuel feed to chimney.
But maybe someone else with more experience can tell you if the transition from 150mm stove to 180mm chimney is ok or not.

Hi Luka;
Yes, you can use your insulated  7" chimney pipe, with a 150mm RMH (150mm-200mm), which is allowed beyond the riser.
If you build a piped bench, it would need a minimum of 8" (200mm) piping to support the output of a 6" (150mm) Batchbox.
If you build a stratification chamber, you would be ok using a 7" (180mm) final chimney.

Yes, I agree with Nina, your new home is looking great!

Luka,

Regarding the bench size please read here:

https://permies.com/t/240864/Double-bell-rocket-mass-heater

The bench size will be limited by the human preferable seating height.
If you want to incorporate the bench into the future bell I would also suggest to widen the pad to 1050 mm (4 bricks + joints). It will be easier to have larger footprint for future connection to the bench. It will be less busy in the bell resulting in better gas flow. Having a larger footprint and incorporating the bench will allow you to lower the main bell height. So 1050x1300.

P.S. Please share more images of your good looking house if you can/want.

Thank you all for your messages ! I will post some photos of my house here. It's around 60 m2 floor level and two galleries that make 40 m2. So, 100 m2 all together. Been building it for 8,5 years. A lot of time, money, patience... Natural building is not what some people think it is you really have to have will to do all this.. I use some basic tools and just take more time. I did my whole gallery wood floor with a cheap jointer/planner and a hand circular saw. But it took months..

And for the bench, I made some changes now. Made it bigger. Correct me if anything I say is wrong... By surface of the bench we mean the surface of the inner walls? That would then here be around 3,25 m2. Would that be a good airflow with these dimensions and the 150 mm J-tube metal rocket? And in the future, if am able, I would build around 1,3 meters high bell with a batch box inside, 160 mm size. That would all fit later on also? As far as the complete surface area of the whole system? The batch box is a wish for the future, maybe after few years and some workshops. I honestly don't have the knowledge now..

Until then, I will make good use of the Gamera metal stove. I think it will work very well. It's a certified and ready stove. Looking forward to start with that.

I included some new drawings. Only thing that's missing is the plaster on the bench. I drew the cleaning openings.

Oh yeah, I wanted to ask If the height of the input and the height of the exhaust should be the same or as I drew here? Higher for input, and lower for the colder output? tnx!

Hi Luka,
what a beautiful house!

Yes, with a bell we're talking about the interior surface area. Without the bottom surface. Side walls and the top.

I don't think you need the interior wall, unless you need it to rest the top slab of the bell on it.

Did you consider going through the roof with the chimney? Or what's your solution for running the chimney through the straw bale wall without catching it on fire?

Your time has been well spent Luka, you have a gorgeous abode.  Respect!

Croatia is such a fabulous country with a vibrant culture. I had a long summer on the islands around Korçula decades ago, and the beauty is still etched in my mind, the inland areas are also amazing

For fi 160 you need 6 m2 ISA. The bell + bench ISA may be larger than ISA of a single bell  When the bell is heated, you open the gate to the bench and it starts absorbing more heat. The inner surface area is calculated by adding the area of ceiling and all walls above the cold air exit.

I wonder what is the exit temperature of the Gamera stove. If it's quite high then you will have to cover the ceiling of the bench with firebricks on T-bars or on the middle row of brick columns . If it's low you can use stone slabs like Benjamin did.

Congratulations for beatifully built house. 8.5 years - I know exactly  what it means and how much persistance, headache and sacrifice it takes.

Cristobal Cristo wrote:For fi 160 you need 6 m2 ISA. The bell + bench ISA may be larger than ISA of a single bell  When the bell is heated, you open the gate to the bench and it starts absorbing more heat. The inner surface area is calculated by adding the area of ceiling and all walls above the cold air exit.

I wonder what is the exit temperature of the Gamera stove. If it's quite high then you will have to cover the ceiling of the bench with firebricks on T-bars or on the middle row of brick columns . If it's low you can use stone slabs like Benjamin did.

Congratulations for beatifully built house. 8.5 years - I know exactly  what it means and how much persistance, headache and sacrifice it takes.

Thank you, yes, many years and sacrifices.. But beats living in a cramped up apartment, that's what I say

I got some info about the exhaust temperature of the stove itself. They are around 200-250 °C max. The builder of the stove says firebricks would not be necessary for the top of the bench. What would you say now?
btw I added valves to the system (1&2 would block the bench and number 3  would be a bypass). Is this functional?

Benjamin Dinkel wrote:Hi Luka,
what a beautiful house!
Yes, with a bell we're talking about the interior surface area. Without the bottom surface. Side walls and the top.
I don't think you need the interior wall, unless you need it to rest the top slab of the bell on it.
Did you consider going through the roof with the chimney? Or what's your solution for running the chimney through the straw bale wall without catching it on fire?

Thank you very much!

Well, I have straw bales in the roof also, so wanted to avoid cutting and going through it. It's always a risk because of water. That's why I thought the duct through the wall would be OK. And of course, to highly insulate it. I would only consider putting the chimney inside if it is not possible to make it work on the outside... Cleaning is also a bit easier on the outside..

Rico Loma wrote:Your time has been well spent Luka, you have a gorgeous abode.  Respect!

Croatia is such a fabulous country with a vibrant culture. I had a long summer on the islands around Korçula decades ago, and the beauty is still etched in my mind, the inland areas are also amazing

Thank you , ,., I am glad you liked it here. We have very different scenery in Croatia every 200-300 km. I live in the continental part, but in 3 -4 hours I get down at the coast. Stone is the traditional building material there. So different. ANd the islands are amazing, I hope to go there again soon..

You wouldn't need to completely seal off the bench space; when the direct path to the chimney is open, most of the gas will flow through that. For positive control, I would put a flap hinged on the angle between 2 and 3, so it could direct flow either into the bench or straight to the chimney. One control instead of three is much better.

Glenn Herbert wrote:You wouldn't need to completely seal off the bench space; when the direct path to the chimney is open, most of the gas will flow through that. For positive control, I would put a flap hinged on the angle between 2 and 3, so it could direct flow either into the bench or straight to the chimney. One control instead of three is much better.

Yeah, the stove builder just said the same thing. Will try to find some good valve for this purpose. It would make things easier.