6" rocket stove in straw bale building

So a 30x30x2mm stub with 2 30x30x2mm feed would work for us. I will also recreate the Mallorca door with same dimension steel, just change the lengths of the steel. I don't need to reinvent the wheel

oops double...

We've made some progress with the build. We made the cut-out in the wall and put the sandwich of CS board / brick / CS board. (on this picture we've yet to install the sides).



We're now going to continue with the firebox and the riser. Our rental tile saw is not cutting super straight.. So we are battling that and the clay tiles have a 'lip' on them forcing us to 'shave' a the lip of every brick...

We've chosen to use a stub of 30x30x2mm and the feed 2x 30x30x2mm. The door from the Mallorca build we adjust to fit out smaller firebox, we use the same material, just shorter lenghts.

We are not sure if we need to partition the air path going to the bench. Otherwise the air might just directly go outside, through the pipe, right? We want the cooler air, that has been through the bench, to go outside the pipe.




We've not glued the firebricks, they are just dry fitted. I was thinking of using "Quick Fire" Masilla refractario , or do need to use actual refractory mortar? Or just clay /sand mix? Is this 'refractory paste' ?

Erik Slagter wrote:We are not sure if we need to partition the air path going to the bench. Otherwise the air might just directly go outside, through the pipe, right? We want the cooler air, that has been through the bench, to go outside the pipe.

Right then, the hot gases are coming down to the level of the bench. Once there, they tend to stay as high in the bench as possible, due to the effect of warm air will rise relative to colder air. So the hot air tend to stay against the ceiling of the bench, as long as there is colder air beneath it.
The chimney pipe need to reach down to lower than half height of the bench' inside. No shortcutting will occur unless the air that's coming down has the same temperature as the air that's already there.
I assume you have seen some of my designs employing this phenomenon? The Mallorca build is a good example, the blind ended bench is going to the right and the chimney exit is at the left. The end of that bench was getting warm, despite the exit was much closer by. The trick was that the exit opening was wide and low, lower than halfway up the bench' height. That's it, nothing more to it.

In order to work well, the opening between bell and bench need to be full width and height, exactly as the inside of the bench. No ridge or lintel at all that's lower than the bench' ceiling.

Of course you are free to do what you see fit, build a separation wall inside or whatever. The only sure thing is that in that case the heat extraction part is posing friction in the smoke path which will in some way hamper this in my opinion already cramped heater

Erik Slagter wrote:We've not glued the firebricks, they are just dry fitted. I was thinking of using "Quick Fire" Masilla refractario , or do need to use actual refractory mortar? Or just clay /sand mix? Is this 'refractory paste' ?

I am unable to recommend any of the refractory mortar you are able to buy there. Have a look at the text on the bag or bucket, when it says aluminium cement it's quick drying mortar. So quick, that it dries on the brick as soon as you apply it. My advise: buy the smallest amount of all the brands available and try all of those.

Peter van den Berg wrote:
The trick was that the exit opening was wide and low, lower than halfway up the bench' height. That's it, nothing more to it.

For us, we could not understand how it works looking at the Mallorca build. It's magic to me I guess you have experience and you get it

Peter van den Berg wrote:
In order to work well, the opening between bell and bench need to be full width and height, exactly as the inside of the bench. No ridge or lintel at all that's lower than the bench' ceiling.

Of course you are free to do what you see fit, build a separation wall inside or whatever. The only sure thing is that in that case the heat extraction part is posing friction in the smoke path which will in some way hamper this in my opinion already cramped heater

We are going to remove the partition for the exit pipe. But we need some way to support the bricks on top,  is it ok to have the bricks in the middle in order to achieve that or will it that pose friction?

Peter van den Berg wrote:
I am unable to recommend any of the refractory mortar you are able to buy there. Have a look at the text on the bag or bucket, when it says aluminium cement it's quick drying mortar. So quick, that it dries on the brick as soon as you apply it. My advise: buy the smallest amount of all the brands available and try all of those.

We are going to see what we can get. Not sure: do you recommend the aluminium cement or not? We notice that even clay&sand dries almost instantly with the firebricks. Could we use clay&sand instead of refractory mortar?

Erik Slagter wrote:For us, we could not understand how it works looking at the Mallorca build. It's magic to me I guess you have experience and you get it

No magic at all, just plain old-fashioned physics, which I absolutely love. Laws of physics are really, really reliable, one can't get around it.

Erik Slagter wrote:We are going to remove the partition for the exit pipe. But we need some way to support the bricks on top,  is it ok to have the bricks in the middle in order to achieve that or will it that pose friction?

The more open the bench is inside, the better. Small, separated pillars could be done,  just to support the bricks on top. You'll need two layers of those at least, the top layer overlapping the seams of the first one. By the way, isn't there something in Spain like sidewalk pavers? When long enouigh, those could span the width of the bench, in two layers again. We've found sort of pavers in Mallorca and used two layers. This could have been three or four and even then the bench would be warm enough.

Erik Slagter wrote:We are going to see what we can get. Not sure: do you recommend the aluminium cement or not? We notice that even clay&sand dries almost instantly with the firebricks. Could we use clay&sand instead of refractory mortar?

Sorry, I wasn't clear. I don't recommend aluminium cement-based just because of this instantly drying. You need to water the bricks beforehand and try the mortar on that. Make the mortar yoghurt-thickness and apply brick by brick.

You dont have a huge choice when it comes to bonding the bricks together, fire clay mixed with two parts sand is a very popular method but can be very messy and does not add a strong structural bond however it will seal the bricks together and is easy to take apart should you wish to rebuild.
As I say this is what most people seem to use on mass heaters.

Refractory cement (Fondue cement) mixed with four parts fine sand will offer a strong bond if used correctly but, requires soaking the bricks beforehand and can only be mixed in small batches because it sets very fast.
Fondue cement mix is not easy to use unless you have experience with brick laying and understand it cant be manipulated like a Portland cement mix can be!

Home brew is a good alternative and can be used in medium heat areas up to 400c
This is a mix of fireclay, lime, portland cement and silica sand the ratio is 3 sand 1 cement 1 lime 1 fireclay powder.
Home brew is very user friendly and easy to clean up, making a neat job.
Home brew is the choice of brick oven (pizza ovens) builders so it is very proven.

In all cases the bricks need to be wetted with water just before being used and with the Fondue cement, soaked for 5-10 minutes.

A lot depends on your  ability, confidence and especially …. how you want the finish to look.

I do not mean to add a twist to this discussion, but much of the talk is about the bonding and setting of the fire bricks,  thus my question

Can the Cordierite type of shelves be used in various areas requiring spans beyond the length of fire brick  (almost always the tops of each design) with fire brick placed over top?  These are often advertised as Kiln shelves and such.

Perhaps if any cutting is done to these shelves, they are no longer strong? Just wondering.

Scott Weinberg wrote:I do not mean to add a twist to this discussion, but much of the talk is about the bonding and setting of the fire bricks,  thus my question

Can the Cordierite type of shelves be used in various areas requiring spans beyond the length of fire brick  (almost always the tops of each design) with fire brick placed over top?  These are often advertised as Kiln shelves and such.

Perhaps if any cutting is done to these shelves, they are no longer strong? Just wondering.

I have not found cordite here in the DIY stores. Here in Spain, they only have these firebricks, that they normally use for the bbq / paellero (to make Paella on wood fire).

Fox James wrote:You dont have a huge choice when it comes to bonding the bricks together, fire clay mixed with two parts sand is a very popular method but can be very messy and does not add a strong structural bond however it will seal the bricks together and is easy to take apart should you wish to rebuild.
As I say this is what most people seem to use on mass heaters.

We've found a supplier for Fire clay, we are going there to ask what they also recommend. They have loads of experience building large bread / pizza ovens & firepits. The BBR might burn a bit hotter though
https://corcaroli.es/en/refractory-products/refractory-clay/

Fox James wrote:
Refractory cement (Fondue cement) mixed with four parts fine sand will offer a strong bond if used correctly but, requires soaking the bricks beforehand and can only be mixed in small batches because it sets very fast.
Fondue cement mix is not easy to use unless you have experience with brick laying and understand it cant be manipulated like a Portland cement mix can be!

👌

Fox James wrote:
Home brew is a good alternative and can be used in medium heat areas up to 400c
This is a mix of fireclay, lime, portland cement and silica sand the ratio is 3 sand 1 cement 1 lime 1 fireclay powder.
Home brew is very user friendly and easy to clean up, making a neat job.
Home brew is the choice of brick oven (pizza ovens) builders so it is very proven.

In all cases the bricks need to be wetted with water just before being used and with the Fondue cement, soaked for 5-10 minutes.

A lot depends on your  ability, confidence and especially …. how you want the finish to look.

Good to know! Those firebricks really soak any moisture, like crazy!

The floor level where the stove will be is slightly lower than the floor level of the bench. We are now realising that maybe we should fill up the bench with vermiculite mix so that it will be at the same level as level of the stove? (which is actually even higher than the floor level - see pic)
Is that correct? I think we need to have the bench 2 layers higher so that the air inside the bench is well above the stove pipe exit.

I was just wondering if you filled the bricks you used to make the bell with cement as you went along, as otherwise they wont be very dense with all the air in the holes
I cant say for sure but I dont know how well the bricks will radiate or store heat if the holes have not been filled?
If the holes line up then perhaps you could still fill them or perhaps it wont make much difference …

I filled the holes in my bricks with fine sand or cob if it was handy.
The empty hole will act as an insulator.  

Matt Walker on an episode of Stove Chat talks a little bit about filling holes in bricks:
Bricks with Holes in a Rocket Mass Heater Build?

We did not fill in the holes...

At some point, describing how a bell bench would work, I mentioned this: the opening between the bell and the bench need to be as high and wide as the inside of the bench in which it's opening to.

Now, you have a lifted opening, and also not as wide and high as the inside of the bench. In fact there's a threshold between the bell and bench, on top and at floor level.
Could be enough to hamper the workings of this setup.

looking at it, the hole in the wall looks cramped. But it's > 3 times the diameter of the tube, so I guess we just have to see how this is going to work and if not, adjust and see again... The width is the same, just not the same height as the bell.

We are doing something that is not very conventional

Eric, I wonder if you can fill the holes by in pouring in self levelling  cement, this a product I use a lot and is a very low viscosity product that might just find its way down.
You should be able to find it very easily, you can mix it in a jug and pour it down,  it goes hard in 20 minutes.
Whatever you decide, you most definitely need a hight mass material for the top!
PS i do feel for you as you are obviously trying hard to get it right …..

Fox James wrote:Eric, I wonder if you can fill the holes by in pouring in self levelling  cement, this a product I use a lot and is a very low viscosity product that might just find its way down.
You should be able to find it very easily, you can mix it in a jug and pour it down,  it goes hard in 20 minutes.
Whatever you decide, you most definitely need a hight mass material for the top!
PS i do feel for you as you are obviously trying hard to get it right …..

Not possible anymore, as there is mortar in between the bricks, it would only fill the top layer. Should I really take the bench apart and fill the holes?
Thanks, Fox James! We are making lots of mistakes..

It is a difficult call but you cant expect full performance from air filled brick bell walls!
However it may still work enough to satisfy  your needs ?

I also made a fair few  fundamental mistakes at my first  attempts as it not only took studying but also practical experience before it all came together for me!

If you follow Peters designs to the letter you are guaranteed success but you could soldier on and see if your efforts work to match your needs and possibly adapt at a later date…. or you could adjust your design and cut your losses !

One other thing that concerns me a little is your riser design made from brick, do you intend to insulate the outside?
A more up to date approach it to use insulating material to form the riser ie the ‘5 minute riser’ or use insulating bricks rather than use dense fire brick  and then insulate the outside.

Wifey convinced me to redo the hole in the wall (add an extra 10 cm - 4") and we are going to redo the bench as well (fill up the holes).

The riser will be insulated with 1" ceramic fiber wool.

Did anyone use natural stones in a rocket mass heater? Can they withstand such high temperatures? We are thinking about using one here, in the back, so that we don't need to put any partition wall underneath to support the bricks.

That is a difficult question without knowing what the temperatures will be….. any  porous stone that carries moisture is likely to crack if the temperatures goes over boiling point, the moisture will turn to steam and expand at an alarming rate causing some stone to literally explode. (Granite is a good example)

If the stone is completely moisture free but has air pockets the same thing can happen.
So material like fire brick is very uniform and dense with no air pockets however, even fire brick can crack if exposed to rapid heating and the brick is wet!

For the same reasons made made materials like refractory cement or pizza stones, need to be cast carefully and vibrated to remove all chance of trapped air.
Adding steel is a common mistake as the metal will expend at a different rates to the surrounding material and almost always cause cracks.
However,  a properly cast refractory slab will be a very safe bet, up to date methods use burn out fibres that melt at low temperature and allow moisture to escape and also use high temp fibres like carbon, that add strength.

Unfortunately this type on refractory is not easy to DIY.

In you case, living in Spain, I would look to pizza oven supplies as there should be able to at least recommend something suitable, if not supply you.

We've received the metal yesterday and today I spent sometime creating the floor channel.

Fox James wrote:That is a difficult question without knowing what the temperatures will be….. any  porous stone that carries moisture is likely to crack if the temperatures goes over boiling point, the moisture will turn to steam and expand at an alarming rate causing some stone to literally explode. (Granite is a good example)

If the stone is completely moisture free but has air pockets the same thing can happen.
So material like fire brick is very uniform and dense with no air pockets however, even fire brick can crack if exposed to rapid heating and the brick is wet!

For the same reasons made made materials like refractory cement or pizza stones, need to be cast carefully and vibrated to remove all chance of trapped air.
Adding steel is a common mistake as the metal will expend at a different rates to the surrounding material and almost always cause cracks.
However,  a properly cast refractory slab will be a very safe bet, up to date methods use burn out fibres that melt at low temperature and allow moisture to escape and also use high temp fibres like carbon, that add strength.

Unfortunately this type on refractory is not easy to DIY.

In you case, living in Spain, I would look to pizza oven supplies as there should be able to at least recommend something suitable, if not supply you.

Thank you for the thorough explanation, Fox James! We are now thinking that we should probably use some metal T (like Peter is using in his Palma de Mallorca video on top of the riser https://youtu.be/Yo5lBIcx_8U?t=174) to support the bricks. Then there is no need for other complications. Although the metal is not recommended, I suppose that if it is insulated, it should be ok?

My wife made some progress. I can't lay brick, she's alright It isn't perfect but it'll do!

We bit the bullet and bought a 120A inverter welder and I spent some time learning 'the stick'




I am pleased!

Another progress post:



I thought a 5cm spacing between the riser and the ceiling of the bell was the ideal, but we're not sure anymore. My wife thinks about 20cm (2 layers of bricks on the side). Can someone refresh us? I can't find the source anymore.

Hey Erik;
Your wife is way closer than you.
20cm would be a bare minimum, 30cm would be much better
5cm is very tight for a J-tube, a batch needs room to breathe.

Erik Slagter wrote:I thought a 5cm spacing between the riser and the ceiling of the bell was the ideal, but we're not sure anymore. My wife thinks about 20cm (2 layers of bricks on the side). Can someone refresh us? I can't find the source anymore.

Your wife is right in this respect, the recommended minimum top gap for a batchrocket is equal to the system size, in your case 15 cm. Personally, I'd prefer more, something like 30 cm.

That 5 cm spacing is used often for a 15 cm j-tube with barrel, although it runs much smoother with a larger top gap.

Now I see Thomas already answered this question along the same lines...

we went for a spacing of 27cm, that worked out with the material we had.

I am close to finishing my door build. Does anyone have good (alternative) ideas for the treatment? Do you use fireproof paint (rattle can paint), or are there other options that work as well?My welds are not great, so i need something to hide all the flaws, haha!

We just had our first burn. This time just a small hand of branches, because we have to wait until the refractory caulk of the door dries completely.

It's 17 degrees Celsius in the house (64F), and we burnt an hour. We could see that the stove pipe barely gets warm, 21 degrees C. The front panel gets warmer - 35 degrees C and the bell gets around 25 degrees C

Will post some pics tomorrow

That is great news, take you time drying out all the materials, the stove will not work at max capacity until everything is completely dry and that can takes sometime but dont rush it  to avoid cracking any bricks!

yeah, we take it slow.

Another thing was that we had a bad batch of bricks. Loads of them already had defects in them. I hope they won't explode haha

Today we had a bigger load in the chamber and it sort of failed horribly...

We are getting smoke from various places of the wall, it's impossible to take a picture of it but we can see small plumes of smoke between some bricks, the joint at the wall and also at the inspection port. We are not sure if this is because of the wood, or lack of draft? The wood are orange tree cuttings (from last spring), not sure if that is dry enough...

Also when we open the door, we get a lot of smoke back into the room. And it burns pretty nasty... A lot of soot.
https://youtube.com/shorts/IQwEjQq6s0A

Does anyone have a good idea what we should do in this case?
* should we plaster over the bricks to stop these plumes?
* is our wood giving us problems or is this because of other things?

The beeps in the video is our Carbon monoxide alarm....

Hi Eric;
Well, you are not dry yet and it takes time to warm it all up.
Smoke leaks are very common.  some you may be able to easily cob shut, others might be best to ignore for now.
As soon as your bricks heat up and you get a draft going those seeps will disperse as the pressure changes from positive to negative.
All those little tiny smoke seeps will become little tiny fresh air intakes.

Your wood could be part of the problem.  
If you can locate some very dry wood to burn that will help warm things up.
Is there a bypass built in to assist in drafting?
A handheld propane torch can heat the outgoing chimney to assist the draft.

Starting up a brand-new stove is almost always a smoky surprise.
As long as they are built properly, they will heat up and stop smoking.

Listen to Thomas and just keep lighting small fires.
What about the chimney, how tall is it, does it go straight up, is it insulated?

ok, let's keep doing small fires until it dries