Granite batch box problems.

Attempting to build my 6 inch batch box system. Teething problems a plenty. Been a serious learning curve. From outset I think I took on too much information as the build progressed from different sources and kept taking little bits of information from here and there and not sticking to a definite plan that was tried and tested. Lesson learnt.  I will attach photos.
Not drawing very well for a start. Attached an inline fan I had lying round to the chimney stack as short term problem solver. Turned my stove into a blast furnace.
Issue that resulted is my square 5mm steel plate on the top to seal the small hole left from my top construction, (which is made of angle steel, vermiculite brick and cob) the steel warped and resulted in gaps all around the steel plate opening up. Could see the flames coming out the riser. Keeping the fan on stopped the smoke coming out in the wrong place but not ideal.
With the fan on and disregarding the steel issue the system warms up but the second I turn the fan off I'm getting smoke coming back out the door I am thinking this is an issue with the shape of my firebox, low and wide but who knows at this point.
I should have bought red bricks as they aren't that expensive and would've made the whole process easier and more reliably air tight, however had lots of granite lying around in the garden so thought I would use that. Not being a mason and my build essentially being dictated by the size of the granite blocks it's ended up as a wide squat looking thing which I am also wondering if this is affecting performance.
Insulation around the firebox and around the riser.
Just wondering is it a lost cause, top off and figure out another method of sealing the top. Being quite wide I found this a puzzle. Anyway any help or advice met with serious gratitude, I'm pulling my hair out. Should've fitted a wood burner!

Hi Martin, that was a good experiment so now you need to build one that works!
I just cant see any way around your issues apart from a re build to known specifications..

I agree, it needs to be rebuilt to published dimensions. Also, granite may be strong and dense which makes it excellent for a mass enclosure, but for the firebox, and the riser especially, you need a material that is more insulative. Granite will suck heat out of the fire and transfer it to the other side or just into itself, until the whole system is really hot. This is bad for the fire getting up to proper complete combustion temperature.

The answer I expected, if you were a guessing man what would you suggest to be the main issues? I realise this is difficult to answer. I really thought I was working within the bounds of the design, 26 inch riser, 6 inch cross sectional system. Nothing under 6 inches throughout.
My guess was the shape of the batch box itself is the foundation of the issue.
Looking like tearing it down is likely.
Gutted to say the least.
Thanks.

Firebox and riser are built out of firebrick with ceramic fibre wool as the insulation material. Thanks for the reply.

I expect the shape of the batch box is a factor. A 26" riser in a 6" system is much shorter than spec and will weaken the draft right there, plus if the riser is granite it will take a lot of heating up (even wrapped in insulation) before it could approach a good operating temperature. You need at least a couple of inches clear above the riser top, three inches would be better. What is the overall airflow path? Does it go through the bench in channels, or is there a bell, and where is the exit to the chimney? If a bell, what are the internal dimensions?

Ah, you replied while I was writing So the core materials are not bad even if not ideal for heating quickly. I think just reconfiguring the core to spec (which obviously requires increasing the height of the enclosure around it) will improve your situation, depending on the rest of the system being okay.


I don't think you need to change the main enclosure, or the bench unless some dimension there is wonky. Actual dimensions of internal parts will help giving good advice.

It is a bell type bench, 10 inches wide 12 inches deep and the flue is 6 inch twin wall insulated flue, exits horizontally then makes a 90 to vertical. Hypothetically could a brick bell be added above the riser a smaller version of the granite box but made of clay brick? Then allowing the riser to be extended. More than likely a sledgehammer job but just exploring options.

Internal main chamber is 800mm x 800 mm 1m high. I got myself confused with the whole sidewinder cook stove design I think and got lost along the way in some in between build that achieves not a lot. Unless the fan is running. Thanks again for your input

It does look nice with the granite facing.
Where is the flue exit from the bench? I notice your flue outside does not extend much above the eaves, so may not have the best draw under some conditions. Many systems appear to need a bit of warming up to start. Does it draw at all when warm, or only when fanned? I'm wondering if a bypass would help with the draw.

Are you out of granite? Extending the main box in that would look better than changing to brick. I would keep the external dimensions the same as you raise the walls of the main box.

The bell dimensions sound a bit tight, but with good layout of in and outflow is probably okay. Where does the chimney connect to the bench bell interior? Height, lateral location?


A bypass to help warming up the system may be good, as well as extending the chimney a bit. I would try the chimney after reconfiguring the core, to see what effect each change has.

I agree with Glen and Fox.
Rebuild a stock size 6" batch inside your current granite enclosure.
Insulate between the original granite and your new firebrick batch.
Use a taller riser, I suggest a ceramic blanket five minute riser or 2500F insulated firebrick.
Increase your bell height to accommodate a taller riser.
With a batch 8"-12" top gap is recommended but certainly no less than 3-5".
Your door may need to be changed.

Your build is very good-looking!
A few minor modifications (build to specifications) and it will rocket.
You will be bragging about it to your friends and neighbors in no time!

I had played with the idea of cutting a small hatch in the top of the bench to accommodate a small cast iron door for pre warming the flue. Will look at doing this. I bought the flue thinking I could add a metre at a time until it reaches the desired draw if you see what I mean.
Seems I need to take the core out. Build the walls up higher and re evaluate. The flue connects to the bench on the far right hand side at the same height as the inlet to the bench. Both are just slightly higher than the firebox but it's marginal. Both inlet and out of the bench are at the highest point I could fit them in the bench, just under the bench seat.
I have around 500 pieces of granite left. Being in the NE of Scotland the stuff if everywhere. My house is a Victorian train station so stone is all over the place. I had planned on lime plastering the whole thing to hide my shoddy stone work I'm possibly just fed up of looking at it as it's been problematic (of my own making).
Thanks so far everyone for the input. Really helping.

Martin;
Your home sounds very cool!
Any chance of a photo or two?

The picture of your chimney seems to show it going down pretty low so I assume you have a horizontal section and a 90 degree elbow?

Yes it goes horizontal from the bench out the wall, then a 90 degree elbow. Had wanted to avoid a flue inside, added to the catalogue of errors!

photos of course
It's randomly on Wikipedia also.
It's called Wartle Station.
Great house, lots to do as ever with old houses.

You mention that the exit flue is set as high in the bench cavity as you could get it; this is not the best configuration. You want the exit to come from as low in the bell cavity as possible, so that the hottest air (at the top) does not exit until it cools and naturally falls to the bottom of the space. With a fairly low and long space, this is not such a big deal, but you would get better heat extraction (once the core is working well) by adding a baffle near the exit so that only air near the bottom of the space can reach the exit. Of course, maintain full cross sectional flow area, or a bit more if the gap is a slender rectangle.

@Martin, maybe you didn't know, but the batchrocket is a tight design. Making the firebox wider and lower to fit the door that you have lying around won't work. I can't see in the pictures how high the port is, but it might be as high as the firebox. I.e., there isn't any piece of rear wall above the port. That's a capital mistake, tried by lots of people including me.
Judging by the total height of the bell, the riser is probably too short as well.
Of course you could keep the fan on the short chimney, but it's better to extend it well above the highest point of the roof.
Conclusion might be that a major rebuild is in order.

Stick to the tried and tested design, stay as close to the recommended values as you possibly can. Then, and only then, the thing will do what it says on the tin. Open it up, make pictures and post those here, there might be other discrepancies. The chimney exit or the space around the core, to name just a few.

Thanks everyone for all the Input. Going to spend a week or 2 not thinking about it and then start dismantling the main bell. Would a rebuild to walker's design as below be suitable for retaining the bench? New door in order and going to go out and buy bricks this time, I feel it will be much easier to get it air tight, the uneven granite was quite difficult to work with.
I'm a joiner I should have bought drawings I know better.

I do feel for you but many of us have had to learn the hard way, on the plus side you will end up with a great heater!
A lot  of these designs require a good straight, vertical run of insulated chimney, so bare that in mind when you re build.
Anyway as long as you keep posting, there will be plenty of help available.