Batch box heater with bench bell and some doubtful optimistic design features

Hi all,

I have been feverishly reading and studying and thinking about rocket mass heaters for a couple of years and I'm now ready to build one in my house in southern spain...but I would love some opinions before I finalise my design. I plan to build a 150mm batchbox as close as possible to the the famous Peter's design with a masonry bell that has a metal plate on top and a secondary bench bell. But I'm not sure whether to make the bench a mass with a tube or a hollow bell. In the pics you can see I plan the bench bell to push it's exhaust down through the floor and pipe it over to the existing chimney that starts downstairs. You will see behind the bell there are 2 holes (outside the combustion system). I plan to suck air in one and out the other with a little 12w fan to have hot air downstairs. The core and firebox are directly cut and pasted from peter's 150mm brick core model that he so kindly offers to all to use.

These are my main doubts:
Can I really pipe the exhaust as I propose downstairs and to the chimney, or should I perforate the wall and go straight out (meaning the bench is not a bell as the air will go straight though)?
I know you don't usually calculate the bell floor area, but seeing as I go down, do I include the floor in my Calcs?
Should I insulate that downstairs pipe to ensure good draught, or can i calculate it's ISA into the bell size?
I propose to insulate the firebox and make the riser from rigid fibre ceramic 25mm board. It is quite pricey and I have to pay transport. More locally I can buy vermiculite rigid board...is it adequate?
Is the p-channel design inferior to the floor type channel I see on newer designs?
I can buy locally a high temperature weber mortar. I imagine it's easier to use than clay and sand, but is it better or worse technically?

That's all for now, please have a look at the pics and tell me if I am missing something....

Thanks!!

Robbie

Hi Robbie;  Welcome to Permies! And welcome to the wonderful world of rocket science!

I'll try to answer some of your questions.
I'll start with an easy one.   Use a floor channel for secondary air.  It works as well as the upper P channel but is super easy to remove when the stub needs changing.
The upper P channel requires disassembly of the batchbox to repair. Even Peter uses a floor channel now.

Using Weber mortar might seem easier but it is permanent. Clay and sand mortar works great AND it pops apart if you need to make changes. Also the "clay mortar" can be saved and reconstituted to use again.  It is also very cheap!

Your idea of getting hot air to travel down pipes and over to the chimney is overly optimistic "in my opinion"  I don't think it will work well at all.  But I could be wrong.
You should leave the bell and go UP to a chimney.

I think your bench should be a bell rather than a piped mass. Air flow thru a bell is much better than a piped mass. No ash to remove from pipes either!
No your floor is not part of the ISA "heat rises".

Blowing warm air about with fans is not very effective. Air cools rapidly away from the heat source. It won't hurt anything to try but I would not expect great results.

I will say your sketchup pictures are sort of confusing.  A hand drawn picture might be easier to understand.

Hey Robbie;
I forgot about your ceramic /vermiculite board question.
If you were building a J tube I would give vermiculite board a try.
But as you are building a batchbox. I think they would fail quickly.

I recently attempted to build a batchbox using brand new insulated fire bricks.  
Rated for 1750 F they lasted five burns before failing completely!
A properly built batchbox creates an inferno of flame!  It takes some serious materials to withstand that.

I think spending extra now will save you doing a rebuild sooner than you think.

Back again Robbie;
I've been studying your sketchup.    Not that confusing after all.

I would give up your thought's of going thru the floor.
I would extend the bench around the corner and install your outlet from the top of the bell and into the existing chimney.

Thanks for that very detailed reply, in so short a time...

I will definitely take your advice about both the mortar and the ceramic board, I had a feeling I was on the wrong path on both accounts. I also had seen that the floor channel appeared on later designs and wondered if it could be some kind of evolution of the p channel.

Going through the floor does seem weird, it just handy because higher up the chimney, on the main floor, I have a kitchen in the way so can't link into it high up. I think I will just go straight out and adjust the design to have my recommended 5.3m2 of surface inside the bells, and put a new tube-type chimney up the wall outside. Do you think that chimney would require insulating to draw correctly? Or take it up the wall (from the bottom of bench bell) and out the wall higher up (roof not an option) with a couple of elbows? would the tube thinkness be  something like 150mm? the existing chimney has a 300mm tube inside a brick outer wall, no insulation, so maybe it's too fat to draw anyway.

Does the steel plate bolted down onto ceramic fibre rope look reasonable to give some instant heat? I'm also doubtful about sealing the bench bell, I imagine it has to be very tight, correct? My idea is to build it with brick and the top covered in these 90x30 cm ceramic pieces called "rasillones" they use here in spanish roofs. Can this part be built of normal bricks and cement/sand mortar, seeing as it is less hot, or should it be clay/sand with refractory brick.

Thanks very much for resolving these doubts, it's great to have some company on this slightly intimidating project!!! If you give me your advice on the tube I will adjust my model and run it by you again if that is not an abuse of your helpfullness!

Hi Robbie;
You are not alone. There are plenty of rocket scientists here, would will gladly give you options and moral support!

I suggest coming out of the bench bell out of the bottom side.  Go straight up in the room it is in and use a 90 to take it thru the wall.
Your new chimney, It should be insulated or you will have draft problems.
If your batch is a 150 then your chimney should be that size as well .
Yes the steel plate will give you instant heat.  If one could be found, cast iron would be better.
Sealing the bell.   You should do the best you can to seal things.

Now I will tell you that there is a fair chance when you go to light your stove off cold... you will have little smokes....   I promise that as soon as it heats up all the small smokes will disappear.
Any tiny leak, will pull air in rather than let smoke out once things are burning.  A bucket of cob mortar is kept on hand to seal any leaks you do see.

The bell bench can be all plain clay brick. If you wanted, it could be cement mortared. I suggest clay sand mortar.
Fire brick is only needed in the core and in the tall bell at and above riser level.

Ok, great, things are getting clearer for me now!!

If my idea of hot air being piped around is not that good, I might make the bench into a piped mass as it will heat the ceiling (the separating floor slab is concrete) of the lower floor and I can use a simple wall mounted fan to push the hot air down with no pipes...

When you say there might be a little smoke on lighting, do you mean smoke from the chimney outside, or do you mean smoke trying to leak out of the door into the house?

For the plate I plan to a square plate that they sell here for use as an anchor plate for steel frame buildings, 1cm thick, so it should last, and if I need to replace it in a few years, it's an easy job.

I'm about to order my ceramic fibre boards and tubes etc and get started once I have finalised the design!

I will be sure to shae my experiences and photos here for the amusement and information of the other users!

Thanks again!

Hi Robbie;
Well some smoke had better start coming out your chimney or you plumbed something wrong!   The chimney smoke will disappear as soon as your rocket gets hot.
The smoke I was referring to  would be coming from small leaks in your mass.   This is very common!  It will stop as soon as that smoke finds the outside chimney.
A bucket with cob mortar is kept available to seal them up.

Looking forward to your adventures & photos !

brilliant, thanks for the advice, I'm going to order my material and when I get strated I'll post some pics on this thread...

Hi again,

I've just found out that ceramic fibre board is very very expensive! I'll pay for it if I have to, but I'm now thinking of using ytong autoclaved bocks, that are insulative and rated to 1200 celcius...any thoughts?

Hi Robbie,   If they really are rated for such a high temperature they sound like a perfect replacement for the cf board.

I did notice on Wikipedia that they are also called aircrete which often contains regular portland cement which cannot stand the high temperatures of a rmh over a period of time.

Hi again, I am almost ready to build, please have another look at this latest iteration and help me decide...does the bench benefit from being a bell, requiring sealing etc, or will a piped mass bench be just as good?


thanks again!

Hi Robbie;
Both are good.
A piped system with a solid mass will take much longer to heat up, but then will hold that heat longer.
A bell system will give you heat in your bench much sooner but will cool off quicker after your fire is out.
I don't know what part of Spain you are in. If you are in the mountains where it gets cold, than a solid piped bench might be more what you need.
If you are lower altitude near the coast than a bell system might be more to your liking.

thanks, decision made, it'll be piped mass as I live continuosly in the house and it seems tecnically easier to build rather than sealing the bench.

a burning question: how long before I can fire up the stove with clay/sand mortar?
thanks again for your help

Clay/Sand mortar can be fired up as soon as you apply it. The heat actually helps the whole stove to start drying out and is one of its many perks rather than using a one way refractory mortar which requires a more delicate first burn.  

thanks again, I've been away the last few days, so I didn't get a chance to check the boards. I Appreciate the input!

[img]Here is my almost finished design!!!
it's peterbergs sidewinder core with an oven / door insert

Can I make the pipe in the mass thicker to get the required ISA?? making it kind of like a bell?

Robbie Mendelssohn wrote:Can I make the pipe in the mass thicker to get the required ISA?? making it kind of like a bell?

Not sure if I'm understanding you correctly but thick or thin, a pipe of the same diameter will have the same ISA.
A bell is an open chamber allowing gases to flow freely and stratify, whereas a pipe mostly just funnels the gasses all in one confined space, so I'm not sure how your pipe will act like a bell?

The idea seems similar to mine!

https://permies.com/t/44806/Cobbling-workshop-heater-cooktop-oven

Read till the end!

I say it would be a bit like a bell in that it has extra volume, being of larger diameter and the exhaust would still be moving slowly compared to when it narrows at the vertical chimney. I bought the tubes today so I hope I'm right...

I did read the post to the end as recommended and was thoroughly entertained, I like the science fair interlude.

Is that ytong block wrapping the firebox? I was considering using it until I dug deep and spend a staggering quantity on ceramic fibreboard.

Here's a question.... To make a slab for the top instead of steel, can I cast one like someone said here with clay and sand and arlite? I can't find a refractory slab around here.... And, why arlite? Isn't it insulating, rather than accumulating,,? Also,  I'd hate to see it collapse and crush my fibreboard riser :-p

Robbie Mendelssohn wrote:I say it would be a bit like a bell in that it has extra volume, being of larger diameter and the exhaust would still be moving slowly compared to when it narrows at the vertical chimney. I bought the tubes today so I hope I'm right...

I did read the post to the end as recommended and was thoroughly entertained, I like the science fair interlude.

Is that ytong block wrapping the firebox? I was considering using it until I dug deep and spend a staggering quantity on ceramic fibreboard.

Here's a question.... To make a slab for the top instead of steel, can I cast one like someone said here with clay and sand and arlite? I can't find a refractory slab around here.... And, why arlite? Isn't it insulating, rather than accumulating,,? Also,  I'd hate to see it collapse and crush my fibreboard riser :-p

Yep it is ytong. I had to support it, because it cracked. But so far, the core is holding. The flue elements behind, for the riser, not so well thought.

Clay perlite sand for the bell's top? This is a joke? Even if expensive, buy the proper stuff.  refractory of good quality. Or make a composite bell top. T bar steel. And firebricks, gas proofed by superwool and in a criss cross two layers.

By the way, your white oven won't heat enough.

Thanks for the input satamax, I had a feeling that the arlite slab was a bad route to go down....

And the oven won't heat enough.... What kind of temperature can I expect in that area? I'd be disappointed to make an oven and not eat any baked spuds.

Any images of how a composite top might look?
Some t section steel and then bricks laid dry on top with next later mortared onto them? I was thinking a thick steel plate that might last a few years.... Here they sell thick square plates that are bases for steel columns. I would corbel the bricks to close the bell top to 60x60cm, the biggest plate size.

Robbie, with a black oven, i don't seem to reach more than 120 celcius at the bottom of the oven, at the first burn. 180/200 C° at the second burn, 300C° at the third. And my rocket is a 220mm batch. About 9 inches.

Go corbel and steel plate if you want. How thick are those steel plates? But on top of a bell, avoid anything which could crack and collapse. Like homemade clay bricks.  

What i was trying to say, in France, and Italy, they used to do this, reversed T bar, and hollow bricks laid on top, to hold the concrete above. Not for big spans thought.



I was thinking you could do this with firebricks, and with another layer on top, even of tiles. The only thing is not to mortar the two layers. But rather put a bed of sand, because of the heat expansion.

I see what you mean, sounds good. My thinking is that maybe a steel plate (1 cm thick) will spall and fail and need replaced, but if those t-sections fail the collapse will be quite spectacular! Also, the firebricks I can get are rated to 900c which is maybe not as hot as the top of the bell, who knows. I think I might leave the oven out and just have a flat faceplate instead. If the plates heats up enough this winter, I'll cut a door and weld a box into the back. Thanks agin for the feedback!

Robbie, my nine incher"s bell goes to 550 600C°.

So, that's no spalling territory yet.

A 60cm square steel base plate would give fast heat due to conductivity, plus considerable mass. Good plan.

I would advise a bell rather than piped mass; there is no reason a bell can't have thick sides and top giving significant mass. My bell has sides of 2 1/2" brick plus 6" of cob, takes hours to warm through, but keeps warm for 24 hours. The steel access plate in the side gives instant heat.

Thanks again to all who have helped out, I'm now constructing the bell...I have a technical question: I have scaled the core to a 175mm system as that is the available tube. But now my floor channel is 67mm wide. I can only get 70x30 or 60x30 tube, so should I go bigger, or smaller? or do I need to fabricate my own tube, which will be tricky....? Thanks guys!!!

Robbie Mendelssohn wrote:Thanks again to all who have helped out, I'm now constructing the bell...I have a technical question: I have scaled the core to a 175mm system as that is the available tube. But now my floor channel is 67mm wide. I can only get 70x30 or 60x30 tube, so should I go bigger, or smaller? or do I need to fabricate my own tube, which will be tricky....? Thanks guys!!!

60x3 good at lower altitudes, bellow 800m elevation. 70x30 above.

brilliant, thanks satamax!!! I'm at 1100m, so 70mm it shall be!

Thanks to all who helped me here! My heater is fired up and rocketing! I have put pictures up in this blog article:
Link to my article, click here!

hi Robbie,

What did you use as the ceiling of your fire box in the end? And how has is it holding up over the years? We are also in Spain and looking for something that can withstand all this heat.