Looking for advice on materials: Ytong

[robbie]

Hi!

I'm about to start building my first batchbox stove with bench mass or maybe secondary bell bench, I've received some great advice on the permies forum, but thought I'd start fishing around here for more advice. I've been working on the design for a couple of years (!!) and have done some experiments outside that were quite illuminating!

Here is what I want to know: I planned to use ceramic fibre rigid board as both the riser and around the refractory brick combustion box, but just found out today that it is really quite expensive, at least in spain where I live. I'll spend the cash if necessary but, I thought about using ytong instead which is an aerated autoclaved block rated to 1200 C....anyone had experience with it? or maybe just around the box and not as the riser? I'm planning a 200mm batchbox and from what i read the riser is as good square as it is round.

Also, do I need to account for the internal surface area of the pipes in the mass bench when sizing my masonry-with-steel-top bell?

Thanks for any help you can offer!

Robbie

[peterberg]

Hi Robbie, welcome to the boards.
Regarding using autoclaved aerated concrete: don't use it anywhere near flame, it will be eaten in a racing pace, especially in the riser. I've done a demonstration years ago, a 6" version core built out of those blocks. After four times running it very hard it was literally falling apart, crumbling and collapsing in slow motion. It worked, but this material isn't up to the rigours of a hot fire for any length of time. I am even doubtful about using it as back insulation behind firebricks.

In case you are planning to have a piped bench with some 90º or 180º corners in it coupled to a batch box: please don't! This design is very picky about friction in the smoke path, although a larger diameter pipe than the system diameter seems to work quite good. Go planning for a bell bench instead, and yes, all the heat extraction area need to be included in the bell calculation.
I'll have a look at the permies forum, there are more things to reckon with in this systems.

[fasardi]

Hi Robbie, someone told me in this great forum than you can use ytong and cover it with refractory bricks or half bricks.
Ytong in Argentina is expensive too, and in a facebook group here (called Mardelfuego) I saw a BBR made with hollow bricks and covered with refractory half bricks. I tried that, and build a DSR-II that way, so far is working well, but I only used for a winter.
I bet you know it, but if you don't, I recommend you to check batchrocket.eu, it's peterberg page and it's full of well organized info regarding bells.

[fuegos]

Hello Robbie what about using insulated fire bricks & a 6 minute riser ? insulate firebrick splits were about 0.23€ each from my local building supplies in Aragon . Where about are you in Spain ? Maybe a riserless core might be an alternative ?

[robbie]

Thanks for the replies, I will abandon my penny-pinching idea of the ytong blocks!! And yes, I was planning one 90 degree bend in the bench...I can do it in a straight run, but then my bench will be only 3m long and then go straight out the wall. I like the idea of a bell-bench, but I worry about sealing it adequately and also I want the mass to heat the floor which is on a concrete slab above my kid's room which I guess wont happen with bell. I suppose that using a tube larger than the 200mm system is a bit like bell, no? I will refine my sketchup model and post some images here for your viewing pleasure!

I looked into insulating fire bricks, but cant find a good price, and I get the impression that the ceramic fibre rigid board is, while expensive, is going to be the cleanest and neatest to work with. I'll keep looking for the insulaing bricks...

Also, it seems that everyone is more keen on clay/sand mortar rather than (quite expensive) refractory mortar due to its ability to be modified. But will the refractory mortar allow me to use the stove sooner? How long will I have to wait if I use sand and clay? It's getting cold here!

Thanks again for the help, nice to know you are all out there, and a reply from the man himself, mr peterberg, gave me a real thrill!!

[robbie]

Hi again!

I would really appreciate some more feedback on this latest offering. I am very close to start building and luckily learned that the batchbox dislikes bends from you guys last week.

I have a few pics of my design, the main doubt I have is, have the bench as shown, as a bell, or piped mass. I'm guessing the inside surface of the pipe has to be included in the calcs for the bell size, no? If it's a bell, can I then go round the corner?

The white stuff in the pic is 25mm ceramic fibre board.

The metal plate on the top above the riser is bolted onto the metal insert that I will have to somehow connect to the brick sandwiching some ceramic rope as a seal.

Behind the mass/bell I want to have a gap through which I plan to blow air passed the bell or mass and send downstairs to heat a bathroom/bedroom. I have heard that this might not work very well...any ideas?

Iwanted to make the bench L-shaped but have been put off, meaning I can go out if the wall and have a clean-out cap...but would I be better bringing the chimney up inside? and going out with 2 45 elbows?

Once I get some of your well-informed views I'll get building!

Thanks in advance!

Robbie

[fasardi]

Robbie, your pics aren't showing!
With clay/sand mortar you can start the stove right after you finished it, all you have to do is start your fires with less wood, but you can start right away. If you use refractory mortar you will have to wait at least a week to start it. I think there's a post from Peterberg about this matter, i'll look for it and get you a link.

[fasardi]

Now than i checked, those recommendations where to fire a casted core, so really don't know if they apply. I used refractory cement only to paste the refractory half bricks in the core, and before fire it the first time waited a week and use this recommendations. I read than here in Argentina ( in the facebook group than i mentioned before) if you used only clay you can start the stove right away with strong fires to dry it.


Heater cast batchbox 18 cm, bell, bench

[robbie]

thanks, i'll try the pics again, they show in the preview, but not in the published post. here are the urls:

guejar.com/rocketstoves/sideview.png

guejar.com/rocketstoves/outside.png

guejar.com/rocketstoves/yopview.png

the last one is where you see the gap behind the mass/bell bench with the heat tubes of doubtful design!

[fasardi]

Well, with the pictures is all clearer to me.
I guess the top metal plate will work for some time, but given the temperatures expected there you will have to replace it from time to time, depending on the thickness. If it is stainless steel will be way more durable, but also more expensive.
About the bell bench, don't be afraid about the sealing, you will see in the first fires if the seal works, and if it doesn't, you can allways have clay/sand mortar prepeared and apply it when you see some smoke. I did it with my single skin bell and it works like a charm. Always try to put the bricks with a very thin layer of mortar, like 5mm or so. Even if in time some cracks appear, inside a bell there's negative pressure, so it will suck air in, and don't let smoke out.
You can make an L shaped bell, and the exit can be in any place of the bell you want, just make sure than the exit to the chimney is the lowest part of your bell. Think of it as it will fill from upside down, and keep in mind than your flue have to be always at least so big as your system section in cm2. But to function as a bell it will have to be bigger than that to allow stratification (I don't remember how much bigger)
About how to get the heat downstairs, I really didn't tried it, i think it can work but with some fan or something like that.
Can your floor hold the extra weight? Your stove will weight more than 1200 kg, keep that in mind.

[fuegos]

"Also, it seems that everyone is more keen on clay/sand mortar rather than (quite expensive) refractory mortar due to its ability to be modified"
not just it's ability to be modified .IMO it's a better material , more suitable as it isn't as rigid as refractory mortar & i think copes with the thermal shock better.Iv'e had some cracking in my firebox on the refractory bricks but the sand/clay mortar is intact.

[Karl L]

Oct 5, 2020 11:22:53 GMT -8 fuegos said:

"Also, it seems that everyone is more keen on clay/sand mortar rather than (quite expensive) refractory mortar due to its ability to be modified"
not just it's ability to be modified .IMO it's a better material , more suitable as it isn't as rigid as refractory mortar & i think copes with the thermal shock better.Iv'e had some cracking in my firebox on the refractory bricks but the sand/clay mortar is intact.

I've often wondered how to make the sand/clay mortar. How do you get the sand and clay to mix?

[fuegos]

Make a slip , combining dry or powdered clay and water and then add sand.I used sharp sand in a 3:1 mix for mortar and a 2:1 mix with arlite for casting the oven roof slab.

[belgiangulch]

Using powdered fireclay. I put one scoop of clay and 3 scoops of builders sand and add water, mix by hand.
The size of your mixing container determines how much to work with.

[Karl L]

Oct 6, 2020 1:03:48 GMT -8 fuegos said:

a 2:1 mix with arlite for casting the oven roof slab.

Do you mean 2 sand, 1 clay powder, and use this mortar to bond arlite aggregate, making a kind of concrete?