I believe the highest temp recored in a rocket stove was 2446 f, this was found about 6” up from the bottom of the riser.
A more average temperature, found in a more assessable place… ie … top of the riser would be half of that.
Hi Dewy, expansion is a real issue when building rocket stoves and you really need to think hard to avoid bigger issues.
Different materials expand at different rates ie … placing steel inside concrete will never work as the steel will expand at a faster rate and cause the concrete to crack.
That is why dry staking fire brick can be an easy option, most fire bricks are very regularised with accurate and smooth surfaces, so it is possible to dry stack them together.
The big issue with a batch box is the metal door to brick interface as it is difficult to get a rigid fixing and also allow movement!
One way is to avoid fixing anything to the actual core but, instead attach the door to whatever is containing the core ie .. the bell.
As the bell wont be getting as hot as the core it wont expand as much and can be made more rigid by cementing the bricks togher.
As an example … you could dry stack a core, cover the core with ceramic fibre mat and build another brick box around the ceramic fibre.
In the outer brick box you can now fit the door frame with a flexible gasket and keep your fingers crossed it will work.
Once you have a design and show us a picture of what you want, it will be easier to discuss and problem solve any issues.
The more dense the mass, the more effective it will be at storing heat, ie … solid heavy rock with no air voids would be a solid dense mass.
Lots of particles mounded together will contain, or trap lots of air and would not be dense at all.
So a bucket of concrete would be very dense, a bucket full of gravel and sand would not be dense because there will be air separating the partials.
If you want to use gravel, it would be far more effective to cement the particles together.
However some folk do use pebbles or gravel, just don't expect it to be as effective as a solid mass.
Hi Dewy, Whatever size you build, you must stick to Peters spec's, bricks are cut to size with a wet cutter so they can be used in any size box.
You need a buffer between different materials ie … bricks, CFM, steel.
My thoughts are ….. it seems like you want to build a batch box in the same way as the majority are built?
Perhaps the most popular method is dry stacking full fire bricks on edge but split bricks with a supporting backing would be fine.
I have lit a few fires now and the paint is working very well, it has gone very hard and most defenatily protects the vermiculite.
The overall performance of the fire has changed as it now just takes a few minutes to catch and very quickly gets up to speed.
However there is a noticeable difference in the hot plate temperature at max burn, the temperatures are well down on the previous model with the vortex and preheated air but, it does not not need feeding so regularly and the gasifier mode is really handy and really attractive to look at.
In gasifier mode, the fire does not like to be reloaded as it will show smoke out from the chimney, so I have to expose the main air supply to allow fresh wood to catch and then close it down again.
I have only had four fires, so time will tell how it works for me long term.
Hi Thomas, there is a series of 3-4 videos on my You tube channel that show how I built the last version of my J tube.
It was quite a complex design that worked extremely well, there has been a few ‘nay sayers’ but from my perspective I built a J tube that looked good and produced an awful lot of heat without any visible smoke!
In fact, I am in fact sort of detuning the design for this winter season as it was simply to hot to enjoy the ambience of the continuous burning fire.
The previous design with the vortex working was regularly reaching 950f making to top glow red and the barrel sides far to hot to approach for cooking purposes?
So this time there wont be a vortex or a preheated air supply, if I ever re build, I would try my previous design in a 5” format to cool things down a little.
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.
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.
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 …..
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 …
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.
You may well be able to develop the concept to your own design as there are already a couple of New Zealand down draft designs available.
The idea has been discussed on the forum before and we have had an active member who had a commercially available down draft stove in his NZ home.
I seem to remember they were quite complicated to start up because you have to warm the chimney via a by pass to get it going.
I have found a wire brush attachment on the end of a mini grinder is my preferred way to get the paint off a barrel.
Takes a full hour and a half and quite a lot of physical work but, you get a very clean and shiny end result.
I have done the burning method but it ruined my lawn!
You could save a lot of time and weight by using a 5 minute riser, however you need to consider the pros and cons as ceramic fibre can potentially become a hazard to your health!
The 5 minute riser has been a very popular method for the last 10 years but just recently it has been bought to our attention that the fibres may be more of a heath risk that was previously thought.
However it can still be used if you are aware of potential dangers of disturbing loose fibres when you maintain your stove.
The 5 minute riser is basically a roll of mat inside a metal tube that replaces the brick riser, it is light and super efficient and has been a favourite method for many years but when the fibres get super heated they can become dangerous if inhaled.
So the only real danger would come when you do your annual clean out maintenance and clean the inside of the bell.
Note this does not necessarily apply to other applications of ceramic fibre as the fibres only become hazardous when exposed to the flame path.
However you should try to buy only the body soluble forms of ceramic fibre!
You would need to wear a mask and carefully hover up the dust.
Eric, I think that twin wall stainless chimney pipe is a thing of beauty but you dont have to use it, virtually any steel or cast pipe will work, you can insulate the pipe by winding titanium cloth around the pipe (car exhaust wrap).
Because this is in you smart house then twin wall will be lovely but not essential.
Re the vermiculite, you can see in my latest vortex stove video how my own vermiculite is lasting, although your batch box will have more power!
Hopefully someone who owns a 5” batchbox (or is it 6”) will help you but, I don't think there are single answers to your questions.
It will depend on what you burn and for how long ..ie … one batch will not heat your bells outside, beyond hand warm but, perhaps if you kept it running for 8 hours the brick will fully saturate and maybe get to 200c but I am just guessing!
Vermiculite is not very popular and there are plenty of early post on this forum where it has failed in riser constrution for example, however there may have been improvements in recent years and perhaps other manufactures have appeared because I have had good results using vermiculite board in my vortex stoves running around 750c.
Having said that if you did choose to use vermiculite for the roof I would make it accessible for easy replacement should it need replacing.
calcium silicate boards are not good in the flame path.
I think you need to insulate the fire box in order to generate maximum heat and combustion, if you dont insulate the heat will travel through the brick and into the soil rather than radiate back into the firebox.
If that happens you wont get much draw and the system will fail.
Perhaps the most efficient rocket stoves are made purely from insulating material, with no brick at all, but they have a few disadvantages, mainly in that most available insulating products are soft and get damaged, or they are not good for your health.
So a more usual method would be to use thin fire brick backed with insulation.
Electric oven doors are often good quality ceramic glass, the big range ones can offer a large piece of glass for free.
However not all ovens have ceramic glass!
Of course even the good ones can be badly marked and dirty and be aware it is normally also only the inner glass of double glazed doors that will be high temp ceramic glass.
Ceramic glass is easy to cut with a decent hand held wheel glass cutter, a quick wipe with turpentine, a decent straight edge and ‘one’ firm draw and the glass will snap very cleanly.
Making the door is anther matter!
Ceramic glass does not flex very well if at all, so any warping of a metal door will crack the glass unless you allow for movement.
Luckily most rocket stove, fire box doors, wont get that hot but never the less mounting the glass too rigidly will crack the glass, that is why most wood fired oven doors are made from cast iron.
Well I guess there is only one man on this earth who can really help with that!.
Just bare in mind that when you work with CFB is very soon becomes unstable and the fibres can loosed up around the cuts, so it is not easy to get tight accurate joints.
You could use a form, 150mm pipe, and build the tube over the form, then clinch the components together with stainless wire and pull out the form.
You could, as has been suggested, cut out rings and try to glue the rings together but from my own experience with glue it will form a rock hard joint that only really glues the first mm of fibres and can be easily split away from the main body.
You could make rings and send some stainless wire dont the length at 50mm spacings and bend over the ends.
Maybe a cardboard form could be coated with zircon or similar and the pieces fitted on top while wet, then the cardboard could be burnt out.
Hi April, how is it going?
Have you made the afterburner?
Why an octagon and not a more round design using more pieces?
Ceramic fibreboard is not the easiest stuff to work if you plan to make the half laps joints that you show, I dont think that will work very well?
It depends on what you classify as spiral pipe, the corrugated, expandable and flexible pipe is not a good option but the spiral air con pipe is pretty clean inside?
Air con pipe is wound on a machine from flat galvanised sheet.
I have read that galvanised pipe can gas off some nasty stuff at certain temperatures but I dont know the details.
Piped mass is not so popular anymore as the Bell system has become more popular.
The type of pipe that has been used might depend on other factors like…. are you going to load tons of mass over the pipe because you will then have to use a strong pipe.
There is another thread going on about using hollow concrete blocks instead of expensive pipe.
I am assuming you want the clay to fix your bricks together, the standard grey potters clay will be more than adequate as it is normally around 25-28% alumina and fired to around 2200f in a standard kiln.
I mix mine with 50% silica sand and water to make a nice mix, the thing about clay is it doesn't really stick anything together very well at all!
When it is wet the clay will offer a pretty good strong bond but when it drys out you can just break the seal very easily so you dont actually need a high quality, high alumina fire clay to bed bricks together.
Basically the higher the alumina content the higher temp it will take before it turns into a brick!
So the brown ‘earthenware ’ clay is fired to around 1700f and standard grey potters clay is fired to around 2000-2300f
So most folk building a rocket stove, use the clay to seal and stabilise the bricks together, to fill gaps and level out courses.
However once it has been heated and full dried, you can quite easily dismantle what ever you have built and clean up the bricks.
Personally I am not a big fan of using clay because it is very messy and can mark the brick face making your work look scruffy! ( i build pizza ovens for a living)
If I am using new clean fire brick then I use fire cement in a tube fitted in a mastic gun, if I am using red clay brick I use what is called ‘home brew’ that is a mix of lime, cement, sand and powdered clay.
Home brew is very easy to use but has an upper limit of around 600c .
If you are not really bothered about very neat brickwork joints then virtually any clay will do but, if you want super neat brickwork then the mix is more important.
Oh nice shinny (expensive) chimney!
I think you could improve the overall performance buy making the exhaust outlet lower, as low as you can.
Maybe an easy way would be to fix metal around the support frame and exit the chimney lower.
That way a lot more of the hot gas will stay under the water barrel.
Possibly the best long term would be a bigger outside barrel going right to the floor, larger diameter and longer in length?
No it is spiral pipe made by twisting flat sheet and is pretty standard for air con in large buildings hotels etc….
You really want a smooth inside to the pipe not flexible ribbed pipe.
I have been using air con pipe with car exhaust wrap for many years but it is thin wall and wont last forever.
Some air con pipe is galvanised and some folk worry about that if it gets to hot, I am not sure if that is a real issue or not?