6" rocket stove in straw bale building

Hi everybody!

We want to build a rocket stove with a small mass bench, in our straw bale building (that sits on a tire foundation). Our walls contain wood beams and our floor is OSB covered with adobe tiles, so we really need to take care of both weight and shielding against the heat.


It's a BBR, 150mm stove pipe. I have modified the BBR from Peter van den Berg, with a smaller riser. I read that the riser can be a bit smaller? I did that to reduce weight. I chose ~2,5x the firebox height which should be enough?


The stove is placed in a corner and we want a small mass bench in the other room. We will go through the strawbale wall with insulated pipes (double wall - 150mm). The path of the smoke will be redirected through the bench, or can be sent directly outside through a metal plate.
The normal smoke path would be from the bell down into the left tube through the wall, heating the bench and going back into the right pipe going directly outside.

We had the idea to put a cooking plate above the firebox. Is that okay or does that have negative effects on the burn? Or does it burn too hot to cook on? I was thinking of 8mm plate steel for this. Maybe we put a layer of ceramic insulation if it is too hot?

What I had in mind for fire safety:
* Placement of the stove ~25cm from the walls
* Layer of (open) bricks underneath the stove.

Questions I still have:
* What should the distance between the OSB flooring and the firebox be?
* Path riser to bench, will it work?
* Insulating the riser, with 3cm / 5cm / 6cm of ceramic blanket?
* Are there any other safety concerns I need to take into consideration?

Thank you all in advance!

Erik Slagter wrote:It's a BBR, 150mm stove pipe. I have modified the BBR from Peter van den Berg, with a smaller riser. I read that the riser can be a bit smaller? I did that to reduce weight. I chose ~2,5x the firebox height which should be enough?

The riser could be shorter, yes. Almost every aspect of the core's figures are related to the base number, represented by a capital B. So the height of your riser is confirming to 8B, which is correct. Proportion-wise, the rest of your projected core seems to be according to the recommended values, so the mentioned width of the firebox seems to be incorrect, this should be close to 217 mm.

It seems to me you have to try again in order to save weight on the core. For example, leave out the full firebricks around the riser, those aren't necessarily.
I can't see how you are projecting the smoke path from the riser to the bench. I understand your desire to keep weight down but it looks much to cramped, including the 150 mm insulated duct through the wall. The transition from riser to bench need to be much more spaciously otherwise the thing won't work. This core design is notoriously picky about friction in the smoke path, which need to be avoided at all cost. So one of the two: you'll need a much wider bell around the core or use a barrel plus a wide manifold to get the gases to the bench. All in all, I'm inclined to think you projected a core that isn't matching the small bell and bench, this core should be smaller in my opinion. And/or larger bell/bench combination. Please don't use a piped bench!

Erik Slagter wrote:We had the idea to put a cooking plate above the firebox. Is that okay or does that have negative effects on the burn? Or does it burn too hot to cook on? I was thinking of 8mm plate steel for this. Maybe we put a layer of ceramic insulation if it is too hot?

A cooking plate on the firebox' top is OK, no negative effects on combustion efficiency. Although, a steel sheet of 8 mm is far too thin, it'll warp in minutes.

Erik Slagter wrote:What I had in mind for fire safety:
* Placement of the stove ~25cm from the walls
* Layer of (open) bricks underneath the stove. 

The heater could be closer to the walls when a heat shield is used. This is a corrugated metal sheet mounted on spacers on the wall so air is able to stream behind it. I've used it myself and it works wonders. It would be best to lift the core from the floor to begin with, 50 cm is a good distance. Even then, it would be beneficial to have air space below the heater's construction. Keep in mind: a mass heater cannot be lightweight, it will stop being what it says to be.

Erik Slagter wrote:Questions I still have:
* What should the distance between the OSB flooring and the firebox be?
* Path riser to bench, will it work?
* Insulating the riser, with 3cm / 5cm / 6cm of ceramic blanket?
* Are there any other safety concerns I need to take into consideration?

* 50 cm, as mentioned.
* No, much too cramped.
* 25 mm would be adequate.
* A big hole through the strawbale wall and a much larger pipe.
 Concerns for weight: bite the bullet and open up your floor and make a concrete pad. Somewhat larger than the heater itself, preferably on undisturbed ground and in any case without the tire foundation in that spot.

Peter covered most of everything I was thinking. I will just add that I did a cooktop on the firebox of an 8" batch. Used a cast iron top which has had no problem after 4 winters, but it tends too get too hot for anything but a kettle. Also at a terrible height for people to get burned. I have a three year old and have subsequently insulated and cobbed over the cast iron.

Daniel;
Excellent news the cast iron cook griddle is holding up so well!

Hi Peter. Thanks for the great input. We will regroup and look at our options We are now talking with the architect that designed our house and he has some ideas that we might apply, i.e. using a conventional stove and a small brick wall as a mass.

@Daniel, I was also thinking it might get too hot to cook anything. Maybe with a layer of insulation?!

"the mentioned width of the firebox seems to be incorrect, this should be close to 217 mm."

It looks to me like the  Sketchup dimension got snapped to the wrong edge of one of the bricks, so automatically read too large by the width of a brick.

This week we took some time to rethink our stove and we have some new ideas and new questions





1) metal drum bell - 20 gallon would be ideal I think (if we can find those). 55 gallon would be _big_ in our house :p
2) we are placing the rocket stove on a insulating pad
 a) first layer of perlite / cement mix - 10cm
 b) layer of bricks on side - 11cm with plenty of air between
 c) layer of 1,5" brick (open core)
 d) install a temperature sensor in the base of the pad and another underneath the stove in the centre
3) the bench is connected through a 15cm double walled pipe to our drum
4) insulate the riser and firebox with 3cm ceramic fiber cloth
5) top of the firebox 2 layers of 3cm vermiculite board
6) walls of the riser / firebox 5cm fire brick
7) cook stove / oven will be an improvement later on...

We are curious why not to use a piped bench, Peter? I see many examples of this on youtube, but maybe it's an old bad idea? We would have only around 1.5m pipe for the bench and I want to make sure that the gasses will always go upward through the bench, to prevent bad draft.


Smoke path:



Many systems have a bypass for when the stove is cold. I have an idea how to do this, with some extra pipe y's etc but would that be necessary in our small system?

Another thing about cleaning it, this is our idea:
* one inspection hatch at the bench
* make the barrels removable, so we can clean it every yearly

We went to a metal recycler today and he said that is a big difference in quality of metal drums. Oil drums are much better compared to drums that are used for silicone or other food related drums. He said that the latter ones will not last long... Anybody has experience with this?

So I’ll second what Peter had mentioned just to make sure it is noted. The pipe transitioning the barrels to the bench is too small in diameter. I believe this should be 1.5x the diameter of the stove or bigger. I’d shoot for this transition to be  around 10” in diameter or else it will restrict the flow. I’m on my phone so didn’t open the sketch up but this transition should be at least 4” from the bottom of the barrel to allow the gases to exit on all sides.

Piped masses can be done but add friction to the gases. Much easier to stratify and get all the heat without running it through a pipe. Also it is harder to total the ISA of a pipe versus bell. The pipe doesn’t stratify so only the top 2/3 actually count. Peter’s math on the brmh site was done for bells. Again, not impossible just adds a lot of unknowns.

Erik Slagter wrote:We are curious why not to use a piped bench, Peter? I see many examples of this on youtube, but maybe it's an old bad idea?

A piped bench poses a lot of friction in the smoke path, especially the bends. Every bend is equivalent to 1.2 meter of straight pipe. I counted at least two bends, so your small bench is equivalent to a straight piped bench of 3.9 meters.
A bell bench on the other hand, is virtually frictionless. It's based on slowing down the gases up to a point where gravity can do its thing, the hotter gases are rising to the top. By doing so, the cooler gases are exhausted first since the exit is just above floor level.

Another remark: the total internal surface area is quite small in this system. You are aware of the fact that a batchrocket will run hot enough to serve 5.3 m²? For reference, see https://batchrocket.eu/en/building#belltheory

A well-designed batchrocket system (with low internal friction) won't need a bypass, in my opinion.
Another point: your exit to the bench is pinched because of the proximity of the riser base. Try to eliminate the bricks that are in the way. Best to exit the barrel with a 200 mm to 150 mm reducer, and lift it above the bottom, as Daniel Ray mentions, in order to avoid fine ash accumulation right in the exit hole. Otherwise you do need something like a manifold, a wider area that functions as ash collector and transition from barrel to bench. As an aside, in a bell bench collection of ash isn't a problem, in a piped system, it is.

Erik Slagter wrote:We went to a metal recycler today and he said that is a big difference in quality of metal drums. Oil drums are much better compared to drums that are used for silicone or other food related drums. He said that the latter ones will not last long... Anybody has experience with this?

Oil drums are oily inside, those won't rust through easily. Food grade drums are painted inside, you have to strip it bare, one way or the other.
But an open top barrel (with lid) is much easier to work with. An open top barrel that has a smooth outside, without reinforcing ridges, would be ideal. Those tend to be made from thicker material as well. I am using a very thin and shitty barrel for experiments purposes, it's there for 5 years now and it's driven very hard at times, I tell you.

First of all we would like to thank you for taking the time to reply to our questions and giving us advice, we really appreciate it!!

Daniel Ray wrote:
Piped masses can be done but add friction to the gases. Much easier to stratify and get all the heat without running it through a pipe. Also it is harder to total the ISA of a pipe versus bell. The pipe doesn’t stratify so only the top 2/3 actually count. Peter’s math on the brmh site was done for bells. Again, not impossible just adds a lot of unknowns.

I always thought that brick benches had more friction, because the pipe is smooth --> less friction. But I guess it makes sense. I just don't know how we can safely go through the wall without insulated pipes. A insulated 200mm pipe won't work because of lack of space...

Peter van den Berg wrote:
A piped bench poses a lot of friction in the smoke path, especially the bends. Every bend is equivalent to 1.2 meter of straight pipe. I counted at least two bends, so your small bench is equivalent to a straight piped bench of 3.9 meters.
A bell bench on the other hand, is virtually frictionless. It's based on slowing down the gases up to a point where gravity can do its thing, the hotter gases are rising to the top. By doing so, the cooler gases are exhausted first since the exit is just above floor level.

Oh wow, I know bends have impact, but I did not know it was so much!

Peter van den Berg wrote:
Another remark: the total internal surface area is quite small in this system. You are aware of the fact that a batchrocket will run hot enough to serve 5.3 m²? For reference, see https://batchrocket.eu/en/building#belltheory

A well-designed batchrocket system (with low internal friction) won't need a bypass, in my opinion.

Our drum is tiny, compared to all the power the BBR has. But I chose a small barrel because of space constraints.. The stove will be installed in our living / bed room, which is only 4x5m and we'd like to also heat the room on the other side of the wall, which is also around 4x4m. It's sort of a tiny house and last year we heated the space with a small wood stove (similar to the one bellow), but that did not have enough power to heat the other room...

So I am just thinking if the BBR would be a workable solution here, or are we just pressing the BBR into a wrong format (small mass, small bell, piped bench). Or should be maybe look at alternative stove solutions!? We'd like something clean burning, that is for us most important. And we have free wood, so we are not so worried about that. last year we burned 1 m3, Spanish winters (Valencia) are not long nor cold.

My wife really likes to have a cook stove and or oven, maybe a vortex is better?, or just our small stove with a mass?

This the type of stove we have now, it has a 80mm stove pipe.

Peter van den Berg wrote:
Another point: your exit to the bench is pinched because of the proximity of the riser base. Try to eliminate the bricks that are in the way. Best to exit the barrel with a 200 mm to 150 mm reducer, and lift it above the bottom, as Daniel Ray mentions, in order to avoid fine ash accumulation right in the exit hole. Otherwise you do need something like a manifold, a wider area that functions as ash collector and transition from barrel to bench. As an aside, in a bell bench collection of ash isn't a problem, in a piped system, it is.

Yeah that is because I was lazy in Sketchup And I did not draw a manifold for the pipe a bit higher I realise the blocks of the base of the riser should not 'pinch' the pipe there.

Peter van den Berg wrote:
Oil drums are oily inside, those won't rust through easily. Food grade drums are painted inside, you have to strip it bare, one way or the other.
But an open top barrel (with lid) is much easier to work with. An open top barrel that has a smooth outside, without reinforcing ridges, would be ideal. Those tend to be made from thicker material as well. I am using a very thin and shitty barrel for experiments purposes, it's there for 5 years now and it's driven very hard at times, I tell you.

Very interesting! He had some nice smooth food-grade 200L barrels with removable lid for cheap.

You could scale down to a 125mm or even a 100mm system?

Fox James wrote:You could scale down to a 125mm or even a 100mm system?

I'll second that. The whole of the batchrocket/bell system is fully scalable. If you want a small heater, just calculate the internal surface area and pick the right size of core for that. This system scales up and down very quickly, from 150 mm down to 120 mm the power delivered is halved already.

So, calculate the internal surface area of the barrel and the (bell) bench together, excluding the floor. And pick one size from the table or calculate it by means of the simple way Benen Huntley came up with. https://batchrocket.eu/en/building#bellsizing

Okay I think we can work with a 125mm system. Seeing the kwh's it produces, it is also a better fit for our house we believe. 100mm would also work, but we can't really buy pipes that size here in our region. It's either 80mm (for pellet stoves) or 120 / 125 / 150 or 200. For 100mm I have not see any insulated pipes online.

Back to the drawing board!

You could use a 120mm chimney or similar on a 100mm system, it is OK to go a bit bigger just not any smaller.

Working on the last design and looking for building materials, we came up with some questions:
1) What is the approximate temperature in the firebox? We are looking for the vermiculite board (for the ceiling of the firebox) and it can handle maximum 1100 C. I was reading that the firebox can get much higher!?
2) What is the approximate temperature at the bench? We are now thinking about going through the wall with bricks, so that there is less friction and like this we will not use any pipes. But we need to have some insulation between the bench and the strawbale wall. We found some mineral wool insulation boards, but they go up to 450C.
3) What should be the distance between the top of the riser and the top of the bell? Would 20cm suffice?

Would calcium 30mm sillicate boards work in the ceiling of the bell and in the ceiling of the firebox?

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 we can try first with 2 burns a day, because our house is very well insulated and pretty small. Also it doesn't get below 0 here often.

So I think we are going to try with vermiculite boards for the top of the firebox, as we can't source large firebricks... Good idea to keep it accessible for replacing Fox James! It's more expensive compared to CaSi boards, but we hope it will keep Casting the ceiling of the firebox would be an option, but would complicate everything (and be expensive)

Sourcing 125mm insulated pipe seems very complicated at this moment. DIY stores don't stock them, because they prefer 150mm / 200mm... We are now going to order the pieces online.

We would like to go through the wall with bricks (instead of the pipes) and continue with the little bench. Like this we have about 3 m2 ISA with the bell and the bench together.
We are going to put insulation around the bricks that go through the wall.
Would this design be ok now?

The design is getting better, through the wall with bricks is OK. How hot it will be is hard to say, I would go for double layer over the whole of the bench. inside the wall 50 mm insulation in the shape of calcium silicate board, another layer of bricks and a second layer of CS board. Double separated, as it were. The small brick bell will get very hot, I'd recommend everything above the riser's top in refractory material.

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!

Thanks Peter and Fox James

Good idea to double insulate the blocks. I want to be sure not to burn our house down We will add some temperature sensors in the wall, just in case.

We've found some refractory tiles, used in pizza ovens normally: refractory tiles 30x30x2,5. I think we could use that as the top of the firebox, with the 'nice side' on the interior of the firebox. Would this work?
Would this tile also be sufficient for above the riser? Or should we go with a metal T's and 5cm refractory bricks like others have done

Another option would be refractory brick 28x14cm and double layer these?

Or the vermiculite panels, that would be a bit more expensive in total..

We've been 'dry building' today. We think we've also solved our problem with the firebox ceiling; we can just put a layer of 5cm refractory bricks into place. They are properly supported (we think). What do you guys think? Is it okay just to glue the bricks with clay/sand mix or should we get refractory mortar / glue


Peter: is this your idea with putting all refractory bricks above the riser?


This week we are going to rent a wet saw, so we make all the cuts safely. Exciting!

Now we are getting somewhere. Yes, above the level of the riser a liner of firebricks.
The firebox built out of 5 cm firebricks on edge and closed off with the same as flat is good. A refractory paste seems to be in order here, you don't want the bricks wander due to the thermal cycling.
The riser built of thinner firebricks on edge but with a proper running bond, not stacked like in the dry build.
The small bell surrounding the riser out of normal bricks on flat and above the riser the red bricks on its side as outer layer. The inner layer out of thinner firebricks on edge with a layer of 12 mm superwool between.
Now, you need to bridge the bell above the firebox level at the front. I'd suggest to do that with a couple of steel rods or something like that. Between the deck of the firebox and the bridged bell some superwool to allow the firebox room for expansion.
The deck of the bell with steel T's and firebricks as you mentioned and more superwool again.

As such, it's grossly the same configuration as in the Mallorca build of 2017, the first one done like this. Have a close look at the video, more questions might arise from that.
It still isn't finished with stucco as was the original plan but running well all the same.

Peter van den Berg wrote:Now we are getting somewhere. Yes, above the level of the riser a liner of firebricks.
The firebox built out of 5 cm firebricks on edge and closed off with the same as flat is good. A refractory paste seems to be in order here, you don't want the bricks wander due to the thermal cycling.

Added to the shopping list :-)

Peter van den Berg wrote:
The small bell surrounding the riser out of normal bricks on flat and above the riser the red bricks on its side as outer layer. The inner layer out of thinner firebricks on edge with a layer of 12 mm superwool between.

We hoped to put all bricks on edge and reinforce with steel wire between each layer (steel wire in the joint (voeg in Dutch :P ). But not sure if that would cause issues with expansion / contraction over time.. And we also don't want the bricks to start glowing red hot :) Our motivation is to reduce the total weight of the bell.

Peter van den Berg wrote:
Now, you need to bridge the bell above the firebox level at the front. I'd suggest to do that with a couple of steel rods or something like that. Between the deck of the firebox and the bridged bell some superwool to allow the firebox room for expansion.
The deck of the bell with steel T's and firebricks as you mentioned and more superwool again.

Yes, that was our idea as well. The bricks can't just float. OR we could add a 10mm steel plate and replace the bricks of the deck of the firebox all together, so we can also cook on it?! Would that work?

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.

We are going to use 3cm refractory brick for the riser, so that is already pretty light weight. The outer shell of the bell weighs the most.

Please, don't use steel wire in the seams. This will expand much more than the bricks. All steel structural parts need to be embedded in superwool in order to provide expansion room.

Overall: it's hard to reduce weight in a mass heater. Those refractory materials tend to be heavy, no escaping from that.

Okay, yes makes sense. Brick and steel are different. We did the calculations and the difference between laying the clay bricks on edge and on side is 90kg. We are talking with our architect as well about this.

Our idea for the stove door. The idea is 'to hang' it on the top of the firefox and also to add metal straps around it so it will stay in place. We can put superwool between the metal and the metal plate.


Primary air intake is 20% of the riser CSA.
The p-channel should be around 5%, I have to talk with the fabricator what dimensions he has available... Also what he recommends for the thickness of the plate steel. 4cm wide would be nice I think.

I have not designed the door yet, but I guess I can let the fabricator do his magic

Did someone design a stove door that keeps the window clean? Or does the BBR not have this problem? friends have a pellet stove and after 30minutes you can barely see the flame because of the soot.

What you are referring to is about two different ways to provide secondary air. The P-channel is an overhead channel, that comes down into the firebox above the port. This one is about 5% of the riser's cross section area, and is separated from the main air inlet.

The floor channel, which is what you have drawn, has two different cross section areas. The one that's in front of the port (called stub) is square with an internal cross section area of 5% or a bit more. The horizontal part (called feed) however, need to be twice as large. One way to achieve that is to double the same duct as the vertical part, with the walls between those ducts cut away over 100 mm (4") at the end closest to the stub. As drawn out at the batchrocket website. The reason for this is that this way the short stub is acting as a venturi in itself, air speed inside it goes up, cooling this part better. In my arrangement, it's very rare to see the stub glowing red.

Furthermore, the feed start end doesn't need to stick out of the door, in fact it could be about 50 mm (2") shorter. The main inlet situated lower, right in front of the secondary air feed, and an angle iron on top of the feed so the primary air need to go up in order to reach the fuel load. This combined inlet is a little larger then, 25% of riser's csa.

The overall effect of this arrangement: there's a "pool" of cold air in front of the feed at all times. Due to the primary air is forced to go up first, the sooting of the door glass is minimal. Due to the "pool" in front of the secondary feed this will suck in more air while the stub heats up. So during the burn the emphasize is shifted from primary air to secondary air, exactly what is needed in the batchrocket. The 2" space between the fire and the door need to be maintained in order for this to work, of course. So in this arrangement the firebox is somewhat 3" deeper than the longest pieces of fuel.

My own heater is using this arrangement for the past 7 winters and it works very well. I'll clean the door glass just once a year, typically at the start of the season when the heater accumulated some moist during summer and the glass is sooting up a bit with first fire of the season.

Regarding the door as a plate that hangs down from the firebox: this will curl up like nothing else when heated from the inside. Unless it's reinforced with angle irons and so on. The same goes for a steel plate on top of the firebox, it inevitably will lead to disappointment. What would work is a cast iron plate, this is much more dimensionally stable when subjected to heat.

You are totally right, I am mixing p-channel and floor channel. We are going to use a floor channel.

I am dissembling your Sketchup designs and they have so many interesting details! Tonight i will revise my designs ;-)

Hi Erik,
A while back, Peter coached me on the same combined air inlet and can say for me it worked very well for keeping the glass clean and providing the correct airflow.
batch box upgrade

Gerry Parent wrote:Hi Erik,
A while back, Peter coached me on the same combined air inlet and can say for me it worked very well for keeping the glass clean and providing the correct airflow.
batch box upgrade

Your stove is looking slick!

So using the Peter's designs from the door - Amayuelas - I came to the following `detailed` design. I think the metal guy can create this and will be plenty sturdy

The vertical tube, with air inlet and are outlet


The door parts from the front


A detail for the little plate with the airslot


From behind


I wish I could just weld things myself....

@Erik,

The door frame design you've found is for the DSR2 specifically. It's essentially untested for use on the straight batchrocket. So you are about to spend money on a door frame you don't need.

The drawing of the floor channel shows the complicated way of how to make this. Have a look at the batchrocket update of Gerry's, somewhat down that same page there's a description and pictures of how it van be done.
Furthermore, I seem to remember that you were planning a 5" batchrocket. The horizontal feed of this looks a bit cramped, it would be better to use the same duct as the stub side by side. The stub looks quite short, remember that the air need to be delivered in the top half of the port. It seems that the whole of the stub is as high as half the port's, so this should be higher than you have drawn.

Peter van den Berg wrote:@Erik,

The door frame design you've found is for the DSR2 specifically. It's essentially untested for use on the straight batchrocket. So you are about to spend money on a door frame you don't need.

Okay, would it work? Or does the BBR run hotter? The material costs are not that high actually - 12 euro in steel + the work for the welder guy.

Peter van den Berg wrote:
Furthermore, I seem to remember that you were planning a 5" batchrocket. The horizontal feed of this looks a bit cramped, it would be better to use the same duct as the stub side by side. The stub looks quite short, remember that the air need to be delivered in the top half of the port. It seems that the whole of the stub is as high as half the port's, so this should be higher than you have drawn.

I've decided to prepare the work for the welder myself, so I don't have to explain all the intricate design features to him :-) So I chose 30x20mm x 2 for the floor channel and 25x25mm for the stub, are you suggesting to use 25x25 for the floor channel as well? 25 * 25 * 2= 1250mm2 and 20 * 30 * 2 = 1200mm2, so that should be okay, seeing 5% of the riser CSA is 960mm2

I will make the vertical stub longer. I could change the floor channel to 35x35mm, seeing that would be closer to 1,5x 25x25mm.

The internal cross section area of the larger duct is about 1.5 times as large as the smaller one

Thanks for the explanation!

Erik Slagter wrote:Okay, would it work? Or does the BBR run hotter? The material costs are not that high actually - 12 euro in steel + the work for the welder guy.

I simply don't know whether or not it would work. As I said, it is untested in combination with a straight BBR. If you want your heater to work as intended, just follow the recipe, don't combine parts from different designs.

Erik Slagter wrote:I've decided to prepare the work for the welder myself, so I don't have to explain all the intricate design features to him :-) So I chose 30x20mm x 2 for the floor channel and 25x25mm for the stub, are you suggesting to use 25x25 for the floor channel as well? 25 * 25 * 2= 1250mm2 and 20 * 30 * 2 = 1200mm2, so that should be okay, seeing 5% of the riser CSA is 960mm2

Ahhh, there's the discrepancy. This way, your secondary air provision would be woefully cramped, sorry. The calculation should be based on the inside csa, that's where the air transport capacity is.

For a 150 mm system, the csa of the riser is 17671 mm², the internal csa of the vertical stub should be 5% of that, equals 883 mm², or larger, and the horizontally feed part twice the csa of the stub.

My calculation: a square duct of 35x35x2 mm has an inside of 31x31 mm = 961 mm², so that is what is nearest for the vertical stub, a 30x30x2 duct is already too small. The feed part should be twice the internal (again) csa of the stub according to latest insights, one way is to use double the duct size of the stub, side by side. Or what I did for my own heater, a 60x40x2 mm duct, which has an inside csa of 56x36 mm =2016 mm², somewhat more than the 1766 mm² that's minimally required. Is this clear now?

Thanks Peter, I really hope I can still change the order i made at the metal supplier  

I will send some time tomorrow take scale down the `Mallorca door`. I think they are pretty similar to the DSR door though.

DSR door has the air intake as part of the frame

DSR door has slots in the side of frame, to disperse the air

Erik Slagter wrote:I will send some time tomorrow take scale down the `Mallorca door`. I think they are pretty similar to the DSR door though.

DSR door has the air intake as part of the frame

DSR door has slots in the side of frame, to disperse the air

Obviously, we are differing in what we regard as similar to what. For example, the Mallorca door frame is made of steel duct because it is mounted at the outside of the brick. The DSR2's air system differs in the sense that this duct is used for optimizing the air supply in a quite different core, i.e. a riserless one.
May I point you to the fact that the Mallorca build is a 150 mm BBR stock system? What size of system are you planning to build then?

We're building a 120mm system, but our pipes are 125mm. I see I am mixing around the numbers a lot..

Thanks Peter

That changes the calculation, a lot. For a 120 mm system the csa of the riser is 11300 mm² then, 5% of this is 565 mm². The stub size could be root 565 = 23.8 mm square inside. A 25x25x2 mm size is a tad too small, 30x30x2 mm is much better. So the feed could be two of the same ducts side by side.

The difference between this and a 125 mm system seems rather small. Without calculation, I would use the same ducts for the floor channel.
Best to put the steel works on hold until the kinks are ironed out.