8" rocket mass heater 3d mock-up and questions

Hi,

I will build an 8" rocket heater soon , but, I don't have the bricks yet, so just made a 3d mock-up to see how it all fits,
i'm new to this so any suggestions will be appreciated.

the mock up is made with 9x4.5x2.5 bricks, and the U-shaped tunnel is all 7x7"

a few questions,
-Is the burn tunnel distance ok like that?
- i can't find perlite or vermiculite here, except for the gardening type, i was told that volcanic sand works well as insulant, is it a good alternative?

thanks

Leonard Allen wrote:-Is the burn tunnel distance ok like that?

Leonard,
The burn tunnel should be as short as possible, so this could be 9" shorter at least.

- i can't find perlite or vermiculite here, except for the gardening type, i was told that volcanic sand works well as insulant, is it a good alternative?

The gardening type is perfect, in essence it's the same mineral. When mixed with clay slip I would prefer perlite, because the aborption of water is not as massive as compared to vermiculite.

Peter Berg wrote:

Leonard Allen wrote:-Is the burn tunnel distance ok like that?

Leonard,
The burn tunnel should be as short as possible, so this could be 9" shorter at least.

- i can't find perlite or vermiculite here, except for the gardening type, i was told that volcanic sand works well as insulant, is it a good alternative?

The gardening type is perfect, in essence it's the same mineral. When mixed with clay slip I would prefer perlite, because the aborption of water is not as massive as compared to vermiculite.

Peter

thanks ,
I understand the idea of a short burn tunnel but I'm confused about the optimal length, as i read it has to be about half of the heatriser ,which would be 16" as the heat riser is 32.5 high.
Can it then be about a third of the heatriser height and less too? 9" shorter would be 10.5" long for the burn tunnel, about a third of the 32.5" heatriser.

Leonard Allen wrote:I understand the idea of a short burn tunnel but I'm confused about the optimal length, as i read it has to be about half of the heatriser ,which would be 16" as the heat riser is 32.5 high. Can it then be about a third of the heatriser height and less too? 9" shorter would be 10.5" long for the burn tunnel, about a third of the 32.5" heatriser.

Try to see it a slightly different way. The tunnel half of the riser lenght is a minimum value, the longer the riser, the stronger the draw. Lengthening the riser would enlarge the mass as well when built out of dense firebrick. That would call for an insulating material, if at all possible and a maximum riser length, not found out yet. So, you ought to define the length of the burn tunnel first, according to that the minimum length of the riser. The feed should not be any longer than half the tunnel and/or a third of the riser. The riser is creating the draw, the tunnel and feed are working against it. Do not stick to the minimum values, I would think. The same goes for the top and side gaps, the top gap (the distance between the riser and the top of the barrel) should not be any less than 2 inch. However it will work fine with 4 inch and even better with one foot!

There is something funny to that top gap, some people followed meticously the recommended values and ended up with a stove that refused to run properly. It turned out to be the top of the barrel became hollow during heating up, thereby restricting the top gap to less than the minimum value.

Peter Berg wrote:

Leonard Allen wrote:I understand the idea of a short burn tunnel but I'm confused about the optimal length, as i read it has to be about half of the heatriser ,which would be 16" as the heat riser is 32.5 high. Can it then be about a third of the heatriser height and less too? 9" shorter would be 10.5" long for the burn tunnel, about a third of the 32.5" heatriser.

Try to see it a slightly different way. The tunnel half of the riser lenght is a minimum value, the longer the riser, the stronger the draw. Lengthening the riser would enlarge the mass as well when built out of dense firebrick. That would call for an insulating material, if at all possible and a maximum riser length, not found out yet. So, you ought to define the length of the burn tunnel first, according to that the minimum length of the riser. The feed should not be any longer than half the tunnel and/or a third of the riser. The riser is creating the draw, the tunnel and feed are working against it. Do not stick to the minimum values, I would think. The same goes for the top and side gaps, the top gap (the distance between the riser and the top of the barrel) should not be any less than 2 inch. However it will work fine with 4 inch and even better with one foot!

There is something funny to that top gap, some people followed meticously the recommended values and ended up with a stove that refused to run properly. It turned out to be the top of the barrel became hollow during heating up, thereby restricting the top gap to less than the minimum value.

Thanks for your feedback ,

Since it's my first build i want to make the most proven setup, also winter is looming here as i am in the southern hemisphere , i have only 2 or 3 weeks to build it and i must get it right or i'll freeze! . I will have time to experiment on the spring, i'd like to make one with a small oven for the next build

So i could either reduce the burn tunnel from 19.5" to about 10" as you suggest or make the heatriser taller?, or both?
Would 10.5" long burn tunnel with a 32.5" heatriser(1 barrel) be ok?
would these options work well too?
- 10.5" long burn tunnel with a 48" heatriser(1 barrel and a half)
- 17" long burn tunnel with a 48" heatriser(1 barrel and a half)

As for the top gap, not sure i understand, if the gap between the top of the heatriser and the top of the barrel is too big doesn't this create a bottleneck when the gases try to go down the barrel walls into the exhaust? or on that place there is enough draft to push ?

Leonard Allen wrote:So i could either reduce the burn tunnel from 19.5" to about 10" as you suggest or make the heatriser taller?, or both?
Would 10.5" long burn tunnel with a 32.5" heatriser(1 barrel) be ok?
would these options work well too?
- 10.5" long burn tunnel with a 48" heatriser(1 barrel and a half)
- 17" long burn tunnel with a 48" heatriser(1 barrel and a half)

As for the top gap, not sure i understand, if the gap between the top of the heatriser and the top of the barrel is too big doesn't this create a bottleneck when the gases try to go down the barrel walls into the exhaust? or on that place there is enough draft to push?

Questions, questions...

Together with the system size, there is a barrel diameter. A 6 inch size could do with a 25 or 30 gallon drum, an 8 inch system size will fit into a 55 gallon drum.
As I wrote before, define the minimum length of the tunnel and proceed from that, not the other way around. In case you have to lengthen the tunnel more than 30% of minimum size, you have to enlarge the system size as well to keep everything in proportion, I would say. The only way to know whether a given deviation would work or not, is to try it yourself.

Since you are in a hurry, concentrate on a design as close as possible to the recommended sizes in the book.

As I wrote before, a too cramped top gap will create friction, a larger one will not, rather the contrary. One of my experiments, a 6" system, did sport a top "gap" of 3.5 foot, and burned like a true rocket. This one provided a large proportion of the produced heat as direct radiation due to the greatly enlarged barrel surface area. In case this isn't your goal, use a top gap of 3" or something like that.

In deed! With every answer more questions come up ,thanks again for your help.
I'll try with the burn tunnel length you suggested, with enough insulation .
I can lengthen the riser later on if needed, by using a sliced barrel. Also to try a bigger top gap

Bricks are here, they are used firebricks, here is a mock up of the combustion unit only, still need to make the insulation and support layers below it.

It is 7x7" csa throughout, with an 8" tube, not sure how long the riser will last as it's thin sheet of metal but at least it's easy to change, i can make the tower from bricks later on.

It has 12.5" burn tunnel, and a 35" riser, which means the 55 gal drum would have to be extended a bit at least 3" , or cut the riser to 32".

what do you think?

Leonard Allen wrote:Bricks are here, they are used firebricks, here is a mock up of the combustion unit only, still need to make the insulation and support layers below it. It is 7x7" csa throughout, with an 8" tube, not sure how long the riser will last as it's thin sheet of metal but at least it's easy to change, i can make the tower from bricks later on.

There's two potential problems concerning that riser. Burning out will be very quick, at the most inconvenient moment namely at full burn. Paired with the risk of the riser insulation collapsing into the burn tunnel. Please, don't do this, you may regret it later on.
The second problem is the conversion from square to round. Despite both having the same cross sectional area, there is a restriction in that transition which will absolutely hamper the gas stream at the spot, a bad thing.

Now and then there's some debate of how to measure the feed, tunnel and riser. The best method I've come across is doing the measurement in the heart of the complete J-tube. So the feed is measured from the top down to halfway height of the tunnel. The tunnel from the heart of the feed to the middle of the riser, and the riser from halfway the height of the tunnel to the top.

The J-tube on its own will burn like crazy, not hampered by any barrel, downstream path or convoluted bench channels. The point is, you have to stay as close as possible to the specifications. So you'd best to skip the stove pipe riser, build it out of light fire brick is possible and use as much insulation material as you can. That's a good starting point, best not to cut corners here.

But it's your project, you aren't obliged to follow my recommendations of course.

I could use that bit of pipe as part of the flue system , but If the pipe lasts one season as a riser i don't mind, i can change it in the spring, but if you say it lasts less and i'll be freezing in july, then i'd do it with bricks.


As for the measurements, to make sure i understand, is this what you mean?, measuring from the axis of each tube:



If i have 4 bricks on edge as bridge between feed and riser (10.5") plus 3.5" on each side,it's a 17.5" long burn tunnel.
Then the feed tube could be about 8.5"?, and the riser at least 35" but could be more as mentioned before?

Leonard Allen wrote:As for the measurements, to make sure i understand, is this what you mean?, measuring from the axis of each tube:

Yes, that's the right picture.

Leonard Allen wrote:If i have 4 bricks on edge as bridge between feed and riser (10.5") plus 3.5" on each side,it's a 17.5" long burn tunnel.
Then the feed tube could be about 8.5"?, and the riser at least 35" but could be more as mentioned before?

Yes, and affirmative. To my knowledge these are the right proportions, comes out roughly as a:b:c = 1:2:4 following this method. It's best to build this core outside first and try it, to get the hang of it. Can be done in one afternoon, remember to seal leaks though.

Leonard A. : Basically, I agree with everything Peter B. has told you. A couple of assumptions to start, you have Ianto Evans' great book - 'Rocket Mass Heaters', You have been to 'ernieanderica.info', and looked at their designs, [ these are proven designs, and E&E W. are both moderators on this site as they have Paul W.s' trust ! ]
I also want your confirmation that you are building on a nonflammable surface .

Picture 1 : The voids created by the 1st layer of bricks can be filled with just perlite, or the local volcanic ash you showed us on an other thread .

Unless you want to make a trade-off allowing much of the heat produced in the Feed Tube, Burn Tunnel, and the Base of the Heat Riser, penetrating into, and being absorbed by the thermal mass of the floor I would apply a layer of heavy duty aluminum foil shiny side up under the 1st layer of bricks, and directly onto the floor, a little clay slip should be the only glue/sealer needed !.
In the area surrounding the outside of the bricks your perlite should be mixed with enough clay to make a golf ball / tennis ball sized mass that while still moist will pop apart if squeezed. [ there will be areas with-in the footprint of your base where the Clay/Sand mix used here should be reinforced with straw as needed to accept weight-bearing from cob used to seal around the base of the metal drum you are going to use! ]
The surface of this layer of Perlite/Cob should be roughly textured to allow for the next layer of P/C to lock together! This pattern should be repeated with all following layers.
If you want to maximize the transfer of the heat at the Feed Tube, Burn Tunnel, and base of the Heat Riser, to the metal barrel, or the Bench/Thermal Mass - for maximum efficiency - then a layer of Aluminum Foil, shiny side up under the 2nd layer of bricks could be used here !

2nd Layer of bricks : The "Cross" formed at the Burn Tunnel end of the 1st layer of bricks is there to help support the metal barrel, and is to be continued up through the next several layers. Much has been written about whether a ash pit is needed, I haven't made up my mind how I will proceed on my next build, which will be The 1st one I build for myself !

I am not familiar with what your volcanic sand/ash is, or how best to make it work for you, good luck !

If you are lucky enough to have a Potter for a close friend, learning how to use clay slip as your only binder to hold your bricks together, and also as an aid to hold/join/seal your cob layers will be easier.

Ideally, Your bucket of clay slurry should form a dimple on its surface when you test it by pushing a couple of fingers down into it disappearing slowly but disappearing well before the clay starts to settle out, leaving a opaque puddle of water forming on top, - there will be a noticeable difference in the rate at which the clay in your clay slurry will settle out,compared to another clay, hence the need to learn by trail and error or from your local friendly Potter !
The idea here is to dip your bricks 1 at a time into the clay slurry, and then quickly set the bricks into place giving you a monolithic structure. [re-enforced and shaped with your structural cob ! ]

A quick note on the end of your barrel deforming as it is heated, it can happen, more so with a used barrel, and that with that type of barrel that has both ends intact/sealed to the drum, the type of drum you want is the one made with one end removable, held in place by a clamping band !
Again a barrel with one open end, recycled after one us is unlikely to deform with heating, especially, if it has been properly burned 1st to clean it out to make it safe for indoor use !

I hope i have answered a couple of your questions from this and other threads on this site, without adding more confusion ! Be Safe, Keep Warm, Pyro magically , Big Al

i made a quick test and the draw is very strong! without the barrel though but still it's very impressive to see. and the gaps are barely covered.

You can see the sideways burn on the images and video



video

allen lumley wrote:Leonard A. : Basically, I agree with everything Peter B. has told you. A couple of assumptions to start, you have Ianto Evans' great book - 'Rocket Mass Heaters', You have been to 'ernieanderica.info', and looked at their designs, [ these are proven designs, and E&E W. are both moderators on this site as they have Paul W.s' trust ! ]
I also want your confirmation that you are building on a nonflammable surface .

Allen
Thanks alot, much appreciated feedback .I bought the book but wasn't sure about the burn tunnel length as they use 7 bricks on edge there,
but on text it says the burn tunnel is half of the heatriser, the difference was that it's measured differently, in the book it's measured from inner edge to inner edge i think , but measuring from the axis gives a shorter burn tunnel as Peter explained above.

I'm building it on a ceramic floor , with the ashpit as the first layer , so the whole structure up to the feed tube will be about 2' high, and the barrel will be about 2' away from a wood board wall.

Picture 1 : The voids created by the 1st layer of bricks can be filled with just perlite, or the local volcanic ash you showed us on an other thread .

Unless you want to make a trade-off allowing much of the heat produced in the Feed Tube, Burn Tunnel, and the Base of the Heat Riser, penetrating into, and being absorbed by the thermal mass of the floor I would apply a layer of heavy duty aluminum foil shiny side up under the 1st layer of bricks, and directly onto the floor, a little clay slip should be the only glue/sealer needed !.
In the area surrounding the outside of the bricks your perlite should be mixed with enough clay to make a golf ball / tennis ball sized mass that while still moist will pop apart if squeezed. [ there will be areas with-in the footprint of your base where the Clay/Sand mix used here should be reinforced with straw as needed to accept weight-bearing from cob used to seal around the base of the metal drum you are going to use! ]
The surface of this layer of Perlite/Cob should be roughly textured to allow for the next layer of P/C to lock together! This pattern should be repeated with all following layers.
If you want to maximize the transfer of the heat at the Feed Tube, Burn Tunnel, and base of the Heat Riser, to the metal barrel, or the Bench/Thermal Mass - for maximum efficiency - then a layer of Aluminum Foil, shiny side up under the 2nd layer of bricks could be used here !

2nd Layer of bricks : The "Cross" formed at the Burn Tunnel end of the 1st layer of bricks is there to help support the metal barrel, and is to be continued up through the next several layers. Much has been written about whether a ash pit is needed, I haven't made up my mind how I will proceed on my next build, which will be The 1st one I build for myself !

This height gives me room to add more perlite, so in the pictures there are about 2 brick layers worth of perlite/clay, and there is volcanic ash below that for 2 more layers.
I think i still have time to add some aluminum under part of the first layer as you suggest.
The ashpit is 7" high, but i can always reduce it or cover it completely if needed,by adding perlite/clay with thin firebricks

I didn't do the cross layout , i'm using a brick ring instead, we're just improvising as we go along here as you can see.

here are some images of the progress, or lack there of!

perlite under the ashpit








this is the barrel which i already burned, i'll have to use that one unless i can find one with a removable end.

more pics, the combustion unit is ready,

The whole support structure is quite messy as you can see, i hope it won't crumble ..

we had to make everything higher because the ashpit is above the floor, i made another thread to ask about the flue system height because of this.

more parts, and questions


i was advised not to use metal and round shaped heatriser, i take the advice seriously, and ideally i would make it from refractory bricks, but i'm going over budget already so i might use a cast iron pipe, if it lasts through one winter

I got this cast iron pipe from a neighbour who works with metal, it is 8" in diameter and 1/3 " thick, 8mm.
My question is , how long do you think this would last as a heatriser? months , weeks? if it lasts 5 or 6 months it would be fine as i can build another heatriser for the next winter, just not sure how fast it will burn and collapse.

What do you think?


the ash cleanout door frame maybe

insulation container extended from a water heater

leonard A. : how long the metal Heat Riser lasts depends on your location, climate, and comfort issues and how much area you feel you have to have warm ,
saying that, expect it will actually go through a couple of heating seasons, when it fails it will fail from the bottom up, as the upper reaches of the Heat Riser
seem to be a slightly oxygen deficient environment retarding rusting. This is proved by the fact that there are barrels out there that have been the sole barrel
used through 20+ years of Rocket Stove use w/out failure/replacement !

On my next build I hope to use a sacrificial light gauge piece of 8" stovepipe, or sonotube, on the inside of the heat Riser I make with packed Clay Slip and
Perlite, I'm hoping to get 2-3 yrs. before I need a rebuild, by that time I am hoping to get or make D.I.Y. replacement ceramic castable parts.

You need to start looking for the barrel with one end removable that is held in place with a clamp band as an upgrade to the type of barrel you have now.
After you remove the only end left in that barrel you can install it with the ' cut ' end down and the clamp banded removable end up so that you have a way
to check the Heat Riser and all other internal parts by detaching the removable end and clamp band even performing checks between individual heating cycles.
Being able to check it will give you a great feeling of comfort and it will make rebuilds a little easier !

I don't have any handle on how long your floor tiles directly under the Rocket Stove will last, I believe you have insulated the base of your Rocket Stove well.
This means that you are not trying to use the floor under your Rocket as a thermal sink, as a positive your rocket stove should come up to temp faster this way!

Be Safe, keep Warm, PYRO Magically - Big Al

Leonard,
The steel riser will burn through at the hottest spot, in effect, the end which is closest to the tunnel.
Edit:
Allen wrote the same but used a different terminology, sorry.

Apart from that, I've spotted a bottleneck, it's the transition from firebrick to steel duct. The round riser, projected over the square brick foundation is smaller than the round duct itself. This restriction shouldn't be there, it will hamper the gas velocity downstream of the tunnel.

allen lumley wrote:leonard A. : how long the metal Heat Riser lasts depends on your location, climate, and comfort issues and how much area you feel you have to have warm ,
saying that, expect it will actually go through a couple of heating seasons, when it fails it will fail from the bottom up, as the upper reaches of the Heat Riser
seem to be a slightly oxygen deficient environment retarding rusting. This is proved by the fact that there are barrels out there that have been the sole barrel
used through 20+ years of Rocket Stove use w/out failure/replacement !

if it lasts two seasons then is more than enough, the area to heat is about 400 sq feet, and i'm in 42deg latitude in Patagonia, so it's cold but not extreme.

allen lumley wrote:
On my next build I hope to use a sacrificial light gauge piece of 8" stovepipe, or sonotube, on the inside of the heat Riser I make with packed Clay Slip and
Perlite, I'm hoping to get 2-3 yrs. before I need a rebuild, by that time I am hoping to get or make D.I.Y. replacement ceramic castable parts.

You need to start looking for the barrel with one end removable that is held in place with a clamp band as an upgrade to the type of barrel you have now.
After you remove the only end left in that barrel you can install it with the ' cut ' end down and the clamp banded removable end up so that you have a way
to check the Heat Riser and all other internal parts by detaching the removable end and clamp band even performing checks between individual heating cycles.
Being able to check it will give you a great feeling of comfort and it will make rebuilds a little easier !

i couldn't find that barrel yet but it would be ideal specially if i use the iron heatriser i showed above.

Ceramic parts would be great, if i use the iron heatriser i'd like to make the heatriser insulation castable, i've read someone uses paper, cat litter and clay, not sure how that is prepared though.

allen lumley wrote:
I don't have any handle on how long your floor tiles directly under the Rocket Stove will last, I believe you have insulated the base of your Rocket Stove well.
This means that you are not trying to use the floor under your Rocket as a thermal sink, as a positive your rocket stove should come up to temp faster this way!

Be Safe, keep Warm, PYRO Magically - Big Al

excellent! , that's the idea

Peter Berg wrote:
Leonard,
The steel riser will burn through at the hottest spot, in effect, the end which is closest to the tunnel.

Apart from that, I've spotted a bottleneck, it's the transition from firebrick to steel duct. The round riser, projected over the square brick foundation is smaller than the round duct itself. This restriction shouldn't be there, it will hamper the gas velocity downstream of the tunnel.

Yea, maybe grinding the bricks inside to a curved shape can lessen this problem, or filling the corner walls in curved shape too, not sure.

Leonard Allen wrote:Yea, maybe grinding the bricks inside to a curved shape can lessen this problem, or filling the corner walls in curved shape too, not sure.

That's the idea. The shaped bricks would eliminate most of the problem, filling in corners could only been done reliably in the two corners opposite the tunnel.

Peter Berg wrote:

Leonard Allen wrote:Yea, maybe grinding the bricks inside to a curved shape can lessen this problem, or filling the corner walls in curved shape too, not sure.

That's the idea. The shaped bricks would eliminate most of the problem, filling in corners could only been done reliably in the two corners opposite the tunnel.

I wonder though , how much does it affect the flow? enough to create backdraft?, Could i compensate for the bottleneck by making the riser a bit higher to create more draft?

----
Here is the bell with the position for the flue pipe and the ash cleanout door, i read it has to be bigger than system size, not sure we made it too big, small, or if it's too low compared to the barrel.
That bell box measures about 15' x 15' and 20' down from barrel to ashpit

i'll try to do a test by sealing the bell with a temporary roof.

Leonard Allen wrote:I wonder though , how much does it affect the flow? enough to create backdraft?, Could i compensate for the bottleneck by making the riser a bit higher to create more draft?

In my experience: the fiercer the fire, the more this bottleneck will work against the draft. Compensating won't help, the moment the stove start to hold back is shifted further down the timeline, that's all.

Leonard Allen wrote:Here is the bell with the position for the flue pipe and the ash cleanout door, i read it has to be bigger than system size, not sure we made it too big, small, or if it's too low compared to the barrel.

I would think this "bell" do count as an ashpit. Looks like it is large enough, this transition will work like a funnel from barrel to flue pipe and this is how it should be done.

Leonard : I looked at the last picture for two days before I realized I wasn't seeing any clean out ! I just woke up and sent this out !

Big Al

We 're lighting it up as we build it,i'll post photos later ,the insulation is still wet, .so far it worked well but at times it smokes, in bursts, or when you just start a fire and it goes out then it smokes alot, because the system is cold i guess, but it shouldn't do that i'm sure.
, we'll test making the chimney outside higher before anything else. but maybe we got the flue system wrong. or could be the whole thing is still wet?

The system has a 1 1/2" gap between barrel and insulation, a little offset. i hope this is not the problem as taking the insulation apart to make it smaller would be alot of work!
the top gap is 5" just not to have to cut the barrel.

one problem was that i couldn't find 8" pipes and elbows in stock so we made the pipes from metal sheets, and the elbows are made with bricks,so because of the square cross section they are larger than the pipes, i guess this should be ok since all is kept at 8" or larger but never below that.

well, it works! like a true rocket!
we added the outside chimney , seems that did it, no smoke when starting a fire, no backdraft, no smoke when the fire goes out, no smoke at all!.Also we were impatient as we should have waited for it to dry a little.

before adding the chimney we pulled the barrel and the insulation had swollen like an inch above the tower as you can see in the pictures.

you can also see the brick box "elbows" which could be a problem because the condensation is getting in the clay mortar, so we'll have to do something about that.

other than that all good so far, we should start covering the bench!

https://www.dropbox.com/sh/ry3vwwpp9ivvrc8/HrsHd7AZYX

Leonard A : congratulations,a good job carried through to a great result ! AND a fantastic clean out ! Keep coming back here, mostly we need to have a
'few more of them that has done, to those that are doing !! " Stick around and collect some apples ! For the good of the Craft ! b.s.,k.w., PYRO-AL

allen lumley wrote:Leonard A : congratulations,a good job carried through to a great result ! AND a fantastic clean out ! Keep coming back here, mostly we need to have a
'few more of them that has done, to those that are doing !! " Stick around and collect some apples ! For the good of the Craft ! b.s.,k.w., PYRO-AL

Thanks Allen, this was a newbie, rookie first time build, and it would have failed miserably without this forum ,thanks specially for your help and Peter Berg.

i hope it still works after covering all up with the bench!