Advice batch rocket heater

Hi there, this is my first ever post on premise despite having had the forums for a few years.  At long last  I am planning on building a batch rocket heater based on a design shown at batch rocket.eu. This will be my first ever build. Basically it is a bell with two benches, one bench either side.

My question is: in the description it suggests the bell should be built completely from fire brick but at the workshop where a demo was built only the top has was built with firebrick.  It also mentions that in the US the bell would be built with a double skin, the inner fire brick the outer brick.

The less fire brick used the cheaper the build.  A double skin would again be more expensive to make, but would have greater mass.  Can anyone with experience make any suggestions?  At the moment I ma thinking of making the bell with a single skin of fire brick, but in Ireland this is expensive, well everything is expensive.

I am possibly worrying about nothing and should just get on with it!

Thanks for taking the time to read and any advice greatly appreciated.

Peter

Without a basic diagram we can only guess but just assuming you are going to build a 6” system, then red clay brick may be ok too.
It depends on how big the bell is and how high above the riser the roof will be, anything close to the riser should be fire brick but clay brick will be fine for the majority.
However if you are building a 10” system things might be different.
I would build a single skin as you can always add an outer skin….

Hi Peter; Welcome to Permies!
Also congratulations on becoming an apprentice rocket scientist!
After you have successfully built your first working batch box you will become a true Rocket Scientist!
Your certificate, lab coat, and pocket protector will arrive as soon as supplies are back in stock. (Don't hold your breath)

So let's try to direct you to an economical build, or at least as cost-effective as your local supplier will allow!
To start you do not need to build your bell completely out of firebrick.
Only above the riser is firebrick needed, that screaming hot air that your batch produces starts cooling instantly after leaving the riser.
Regular solid clay brick is all you need for 80% of your build.

Your batch box core is of course all firebrick.  Everything else is clay brick.
In the US, some states require a full double skin.
Although making it more resistant to any (leakage) in your home it is certainly not a requirement! (except in those states)
A warmed-up rocket stove will have negative pressure, any tiny air leak will pull air into your system, and not leak it into your home.
So in my opinion skip building a double skin.
What you do want, is dry bagged clay and bunches of clean medium-grade sand. Any rocks are a big problem in your clay mortar, if using (free) sand it must be screened first.

It is hard to do, but try not to overthink your build.  (most everyone does on a first build!)
This is easy easy, playing like a child in a mud bog.  Very messy while building, be prepared to make a clay mess in your home.
Just take it one step at a time, figure out your next step when you get to it.  If you try to plan everything out perfectly ahead of time A) it will take way too long and B) you may have no hair left after ripping it all out!

It sounds complicated and scary but believe me it is not!
If you're not sure how to proceed then ask us for help, we are doing our best to switch every wood burner in the world over to an RMH... But it takes time!
Everyone like yourself, who successfully builds and LOVES their new Rocket becomes the best advertising we can have.
After you are a happy successful rocket builder you will be telling complete strangers all about RMHs and offering to display yours and maybe even offering to help build!

Again Congratulations on the first step to becoming a ROCKET SCIENTIST!

Thanks so much for the advice.  Unfortunately I have habit of overthinking pretty much everything I do! I appreciate the positivity immensely.  I'll let you know how it goes ,though I won't be staring the build for a couple of months. When I run into trouble I'll know where to go. Thanks again.

Peter

Hi so my first real question.  I sourcing the firebricks.  They are available if 1", 2" or 3" thicknesses but all cost the asme amount (?). $5.50 each. I have attached a PDF of tech specs for the bricks. Would there be any reason to go for one thickness over another for the burn chamber, riser or the bell contraction above the riser?  Was going to go for insulated fire brick for the riser at least, but they are only rated to 1000C (1892 F) which the designer suggests is not enough.  I will now insulate with refractory blanket.

For more information I am planning on basing the build on this resource :Batchrocket.eu

It is the Bell with two benchs.

Thanks for any thoughts

Peter

Peter McDonnell wrote:Would there be any reason to go for one thickness over another for the burn chamber, riser or the bell contraction above the riser?

Total n00b to RMH/RBH tech but I saw in one video recently that you want the least mass possible for the chamber in order to keep the fire as hot as possible - https://www.youtube.com/watch?v=phrz7VjQdGo 30 seconds in...don't know if this helps...

Hi Pete;
So your bricks are very close to the US size of 9"x 4.5"
The thicknesses are different.
Here we only have a full-size brick of 2.5" or a "split" brick of 1.25"


Although the heat-up time for the core is longer with thicker brick, the resistance to cracking is much better, either from heat stress or simply bouncing a piece of wood inside the box.
I strongly recommend buying the 3" bricks. If there was a significant price difference I would opt for using the 2".
I would stay away from the 1" completely.
For your riser, again I suggest Morgan Superwool to create a five-minute riser.

Tom's statement is true.
But for my builds, I want a long-lived RMH that might take a bit longer to come up to full temperature but lasts and can absorb inexperienced operator abuse.

Thanks Tom and Thomas,
This morning I was looking to source Morgan Superwool.  I have found one company that seems to stock their products in Ireland.  If this is th same stuff it comes in either 8mm or 9mm thickness.  This is the link to the product:
Morgan Superwool Blanket

Is this the correct supercool blanket for insulation and making a 5 minute riser?

Peter

Pete,  I might of missed the bit about where you live?
If you are in the Uk there are a few outlets that sell ceramic fibre blanket but make sure it is EU approved and even then just be aware that this product can be bad for your health  under certain circumstances!
That is not to say you cant or should not use it but please evaluate if the relatively small risk is within your boundaries.

Some folk actually use ceramic fibre board to build their  heaters, this type of constrution will provide  almost instant heat but has many disadvantages.
Fire brick is a far more durable and safer material to use but it will take time to get up to speed and can be difficult to start up depending on your design and chimney.
Personally I prefer to use 1” fire brick but it is far more difficult to work with as you need some type of form to hold it together.
3” bricks are easy to build with but will take a lot longer  to get up to temperature.
A lot will depend on your personal use for the stove…. ie … is it to run all day in a workshop or is it for use in a house with good insulation or one of many other possibilities ….?

Hi  Pete;
Superwool HT is the correct product.
However, 25mm is the thickness you are looking for.
Several wraps of the 9mm would do it.

Hi Peter,

In order to create a good 5-minutes riser, don't do several layers of a thinner blanket than 1". The innermost layer tend to shrink back quite a bit and becoming loose in the riser, not a good idea.

The build with the two benches was done differently because of firebrick shortage at the time. How to use the best bell build-up is greatly depending on where you live and what the local frost levels are. There's a range of ways to build a bell: from single skin in red brick, double skin both in bricks on edge, double skin with the outer layer bricks on flat, or both layers on flat.
Materials of the inner and outer skin could be very different, outer skin is often done in red brick, compressed loam blocks, even concrete street clinkers. Inner skin could be done in red bricks up to the height of the riser's end, in firebricks higher up.

Af far as I am aware you didn't specify the size of the heater, the drawing represents a 200 mm or 8" system size. Building this on a wooden floor is very, very tricky, please build a fundation from the solid ground up through the floor which would support the heater. Keep in mind, this particular heater could be weighing in at several tons, ISO tons or Imperial likewise.

The combustion core of the thing could be built out of 2" or 3" firebrick thickness, on edge of course.

Thanks again to all who have given advice to me here.  I live in the far SW of Ireland  a few hundred metres from the Atlantic ocean - so it has an Atlantic climate.  Frosts are rare enough possibly 10 -15 during the winter but sometimes we get snow that lasts for several days - who really knows these days!  
The main house is more than 200 years old - 700mm + rubble built walls ( fieldstone and  subsoil ) with hemp lime plastering inside. Many of the internal features are original - where we have been able to add insulation we have, but the main part of the house is draughty even with all the windows and doors shut.  This is due to the internal ceiling construction, these are original and we don't really want to change.  There is a wood burning stove in the living area which is on more or less continuously when we are home and central heating running form a heat pump (sparingly) in the winter but the house is cold (obviously) by morning.  

Not sure if any go that helps but it gives some context.  

Sorry Peter, the floors are stone over Portland cement based concrete over subsoil.  Originally the floors were just earth.

I think I am going to give up on the Superwool 5 minute riser at least temporarily, I cannot source the correct thickness in Ireland.  I can from the UK but the unit cost is high plus shipping and then there will be customs duty on top of that. Having looked at the Rocket Mass Heater Riser chapter again I am now thinking of insulated firebrick but slightly adapted to the hexagonal shape shown on batch rocket.eu Other possibilities I am considering are vermiculite board or perlite and clay.  I spent months trying to source a vacuum moulded ceramic fibre tube - but no one would supply to me form Europe.  Cost would probably have been crazy anyway as a one off.  I know I can get firebricks!

Hi Peter;
Be sure to get insulated bricks of no less than 2500F  (apx 1400C)
I attempted to use 1800F insulated bricks in a 7" batch.
They lasted 5 burns before failing.

A perlite-clay riser is a good substitute.
I have made several, they last several years or more if made properly.

I used a 16 gal barrel as an outer form and a round cardboard cement tube (sonitube) as the inner form.
The inner form burns out with the first fire, leaving a smooth clay finish.
They are heavy but can be lifted off in the event you need to work on the core.

You still have not said what size batchbox you want to build but anyway you may want to jump onboard here https://donkey32.proboards.com/thread/703/vortex-stove as Trev lives in Ireland and is a very helpful chap.

Hi Fox James, I am planning on an 8 inch riser but this may change. I did provide a link to the plans though nothing is set in stone.  Interesting to encounter another Channel Islander on here,  I was actually amazed.  I was born and raised on the slightly bigger rock.

Peter

Yes that is interesting, not many Jersey folk left in Jersey anymore and in fact we are only just a majority in Guernsey with 47% being non locally born.
I forget exactly where Trev lives but it is in a stone house near the sea, he will have info on materials and best size to choose. Trev often talks about the chimney and how the strong winter wind can effect draw.
Unfortunately he does not post on this forum but there is quite a lot of cross over between the forums.