2024 Neapolitan oven build

[daidensacha]

Formwork for vent casting

I decided on bending and laminating 9mm ply to create the outer formwork. Added cuts in ply, wet the ply to allow it to bend, glued and clamped them together in a press knocked up out of MDF and other scraps.



The shape worked out as planned.



The following image shows the vent mould sitting upside down on the custom top designed to fit an adapting plate for the flue to sit in. The other images show the sides cut and fitted together, easily disassembled by removing screws.

[daidensacha]

Making a Custom Vent Gallery Mould out of Vermiculite and Casting Plaster

I took inspiration from David’s custom made vent gallery and started by making a form, and casting a mould out of Vermiculite and casting plaster.

My first attempt was a failure. It was 8:1 Vermiculite and casting plaster. After a day in the form, I removed one side of the form and it was not set, totally crumbly and easily fell apart into crumbs. Initially I thought 8:1 was not enough plaster, so I mixed in more casting plaster to the crumbs increasing it to a 4:1 mix. Adding this time more water, I think now my initial failure was due to insufficient water. It was just to dry.

I cracked open the second attempt this morning, and put in in front of a fire as it is really heavy, very wet and I want to try to speed up the drying process. On David’s instruction, I started to carve it back to the shape I want for my flu gallery. I’m pretty impressed, it was super easy with a sharp knife, and smoothing the surface with a spatula. Now I just need to let it dry. I’m knocking up a second outer wall form to put around this mould, so I can fill the space between it and the outer form with refractory concrete.

Some images of the form and mould. Bear in mind it’s upside down. The rectangular end matches the planned size of my arch vent opening, the round end is 200mm diameter and matches the flue size.

[daidensacha]

Giovanni Rossi Thanks Giovanni.

Yeah, the closest I found so far are comments saying it doesn’t get that hot. It doesn’t really put my mind at ease. I do trust the German oven accessory suppliers though, so I have asked them for advise, and they came back with a list of items that they say will do the job.

I had read something similar, that the double walled flue by design is supposed to heat as this does facilitate the draw for the flow of the smoke. The flue has 30mm of insulation, although I don’t know yet exactly what sort of insulation it is. There is supposed to be 60mm clearance from combustibles according to what I read here in Germany, so the hole in the sheeting needs to be big enough to allow for that.

This is what I have been recommended to use. WIth this fitted, the flu doesn’t touch the sheeting it passes through, and there is air space under the top cap to allow for heat dissipation. I will ask the supplier if it’s also worth filling that space with insulation, or better to leave it unfilled.




I’m still curious about that flue temp though, gonna find out eventually. I’m trying to plan for every thing to avoid unexpected issues, problems, and unwanted costs.

[Giovanni Rossi]

I agree that info is hard to find and I was unsuccessful when I went through this same exercise because the first design of my pavilion had a polycarbonate sheet roof. I was confident the roof wouldn't combust if I maintained the proper clearances, but would it start to melt? The Class A double wall flue pipe I decided to use was rated for continuous operation at 1000^oF and required 2" clearance from combustibles. Ultimately, I decided to go with an asphalt shingle roof. I've shot the flue where it penetrates the roof with an IR gun. Sorry, I can't recall the numbers, but it didn't alarm me at the time.

I found this on a UK flue suppliers site today:

Twin Wall Flue FAQ's (picture shows double wall with insulation but doesn't state Class A - however, the same 50mm clearance is recommended)

How hot does twin wall flue get?
Twin wall flues will heat up when in use as they transport the hot stove gases away from the appliance, the outer part of the flue can be over 90^o C or 194^o F. The temperature of the twin wall flue will vary depending on:

• KW of the stove.
• What fuel is being burnt.
• Length of the flue (the base of the flue will be hotter than the top).
• Temperature outside.

[daidensacha]

Double wall stainless flu though Polycarbonate roof

I‘m working on my outdoor kitchen floor plan. I will pave a 9 x 5 meter terrace, in which I will also lay a foundation for the oven. This will be covered with a polycarbonate roof.

I want to run a 1 meter section of double walled flue through the polycarbonate roof, and have been researching the details. I’m quite a way off from it, but planning it out is a good process, and I when I understand what is required, I can plan it out better. There is a lot of info on the the clearance required between the flue and flammable materials. In Germany with the climate, there is an abundance of suppliers and info.

What I haven’t been able to find out is what temperature I can expect the outside of the double walled flue to get to. There is 30mm between the inner and outer layers, and this is filled with insulation. I gather from what I have read it will still get warm, but I can’t get even a rough idea of what temp to expect.

Anyone give me a rough estimate?

[david s]

1. As the oven floor is always cooler near the oven mouth, insulating under the entry bricks will also help reduce this temperature difference and heat being drawn from the floor to the supporting slab. Some builders include a small gap between the floor bricks of the oven and the floor of the entry to reduce the difference and the heat sink effect.

2/3. I’m not really sure what you plan with the flue gallery when you say you plan to cast it, yet the drawing shows brick construction. The area that gathers the smoke should be at least double the area of the cross sectional area of the 6” pipe. Ie wide but shallow like the head of a vacuum cleaner. This allows you to make the gallery quite shallow to make the oven easy to work.
Regarding using standard bricks on the front I can’t see that being a problem as there will be no flame impingement there.

"The area that gathers the smoke should be at least double the area of the cross sectional area"

A 6” diameter pipe is sufficient for your oven. 42” and larger are recommended to use an 8” pipe. As yours is at the upper end of the range of the recommended 6” pipes you could consider a 7” pipe, but they are not generally manufactured, you’d need a custom made pipe. Because you are going higher through a roof the extra height provides extra draw so 6” should be quite adequate. At 305cm2 for the gallery vent against 176 cm2 area for the 6” pipe, it should work ok. As the smoke is only an issue at start up you can just make sure that you start with a really small fire at light up, feed it sparsely for the first 10 mins, then you’re away no problems.

A drawing probably explains this better. In my build the bottom of the gallery where it sits against the oven mouth, has an area of 550cm2, which is more than 4 times the flue pipe area of 123cm3. I have also managed to keep the gallery very shallow by placing the pipe slightly behind the the smoke entry which has the added advantage of increasing the volume of the gallery at its base.
I don't have smoke issues as you can see from my 17 year old oven. You are correct in recognising that the flue pipe and gallery suck from both the oven chamber and from the other side. That is why WFOs require such big flue pipes compared to pot bellied stoves.

The diameter of the pipe is the primary determinant of draw, but the smooth flow of the hot gases is also a primary consideration, just as valve ports, manifold (often replaced by headers) and bigger diameter exhaust pipe in cars help the engine to breathe more efficiently. My oven is 17 years old as as you can see has no smoke issues. I don't know what is the ideal ratio of gallery area/angle to pipe area, but I've seen lots of ovens that have no funnelling that do have smoke issues.

[daidensacha]

Yes the perlite/plaster mix was merely to create a weak and fast setting mould over which to create the gallery casting.Sanded smooth and painted with a thin oil makes for a smooth surface easy to remove.

As previously explained the base of the flue gallery area should be greater than the cross sectional area of the pipe , for smooth air flow and efficient smoke extraction. A restriction at the base is likely to cause smoke out the front , particularly at start up. Once the oven and flue pipe are hot it should draw perfectly well. If it doesn’t bother you it’s fine. The flue and gallery only serve to put any smoke well out of the cook’s way.

Thanks David.

Ok, 8:1 vermiculite and casting plaster. Was it easy to remove then? I haven’t yet worked with the vermiculite, only conceptually. Yet to get hands on with it. It’s got me thinking.

Re: the vent opening size. I’m beginning to grasp it, I think. I guess the reason it needs to be at least equal in size as a cross section area of the flu is that the volume of smoke would then flow better. If it were to be much bigger, for instance, flu cross section area of 314cm2 vs vent opening area of 425cm2, would that then create a possible issue of there being to much smoke at times in the vent gallery to pass easily through the flu? A vent area with size of 373 x 114mm would be 1.5 times the size of a flu with diameter of 200mm.

[david s]

Yes the perlite/plaster mix was merely to create a weak and fast setting mould over which to create the gallery casting.Sanded smooth and painted with a thin oil makes for a smooth surface easy to remove.

As previously explained the base of the flue gallery area should be greater than the cross sectional area of the pipe , for smooth air flow and efficient smoke extraction. A restriction at the base is likely to cause smoke out the front , particularly at start up. Once the oven and flue pipe are hot it should draw perfectly well. If it doesn’t bother you it’s fine. The flue and gallery only serve to put any smoke well out of the cook’s way.

[daidensacha]

That is an out of the box way of creating the flu gallery. It’s not really cleaner for me how you created the chamber void. In the first photos, you used casting plaster to build it up, then added the flu on top to create the hole. I’m guessing you then chipped the casting plaster out?

I’m not sure it’s how I want to build my vent, and I want to build up around the front of the oven in a particular shape with bricks.

G oing back to the vent opening size, could you clarify something for me? In talking about the vent opening, I use the world’s width and depth. THe cross section of my 200mm flu pipe has an area of 314cm2.

To create the vent opening of a similar size I can play with the width and depth, here is three examples in this image.



In this arch plan it has a vent opening of 373 x 82mm, with an area of 305.82cm2. In the image I calculated 3 vent sizes with varying widths and depths, all come to around 317cm2. In my mind, I imaging the narrower the width, it has less coverage fo the top of the arch to catch smoke. The greater the volume of smoke the greater the chance the smoke will come out the front of the arch. I imagine there would be an inflow through the front of the arch, and a properly designed vent with flu would create sufficient draw that the outflow of smoke would go up the flu and chimney. This is purely conceptual, and I have absolutely no direct experience of it though. With a view to creating a vent opening that catches the maximum possible smoke, I imagine the two wider vent examples would be best. The 85mm deep example with a width of 373mm would be more per your previous description of the vacuum cleaner head.

You clearly have a lot of experience with these ovens, so I would be interested in knowing your opinion of the optimal size for the arch I want to build.



In this plan of the arch, with a vent of 373 x 85mm, i can keep the vent landing to a size that gives me easy access to the oven, and also is big enough that there is a staging area big enough to use as an area to keep items warm. The part of the vent landing inside the outer arch would be 242mm, and would include a 12mm heat break in the floor and between the outer and inner arch.

I would build the outer arch in this plan from “reichsformat ziegelsteine”, which are old format German clay bricks very close in size to the firebricks that I am using. I found 250 which are cleaned and in really good condition, and pick them up today. They are perfect to use as the decorative front of the oven, and also for other small touches like around the wood storage opening under the oven.

I bought the 350mm wet cutting saw for this project, so it will make cutting easier. The unknown for me at this point is the optimal vent opening size and how it meets the flu to create sufficient draw so that smoke doesn’t pour out the front of the arch.

[david s]

I think you are creating a lot of extra work for yourself by building a brick gallery then casting another adapter casting on top of it.
check this approach for a similar solution.
(sorry it won’t paste on mobile)
Go to “The Brick Arch” thread post #10 under “Design Styles, Chimneys and Finish “

[daidensacha]

david s This is an image of the anticipated vent chamber from the inside bottom of the arch up to where the flue plate would fit. The vent opening as stated in my previous post would be 373x82mm, to match the area in size of a cross section of the 200mm flue.

The lower 116mm high edition with vertical sides would be the part inside the brick arch. The upper chamber with angled sides that transition to a 255mm square section would be above the brick arch.

The concept you gave of the wider shallower opening like the head of a vacuum cleaner is why I changed the dimensions to 373x82mm.
Alteranive possibilities would be 245x128mm (which would include the 12mm heat break.

Question is if the wider would be better to catch smoke coming out of the arch. My arch has a radius of 383mm.



In this image I have calculated the section of the inner arch that would relate to the width of the vent opening. Yellow if it were to be 245mm wide, and green if it were to be 373mm wide. Obviously the 373 wide would be more efficient when the fire is fresh and smoking more.

[daidensacha]

1. As the oven floor is always cooler near the oven mouth, insulating under the entry bricks will also help reduce this temperature difference and heat being drawn from the floor to the supporting slab. Some builders include a small gap between the floor bricks of the oven and the floor of the entry to reduce the difference and the heat sink effect.

2/3. I’m not really sure what you plan with the flue gallery when you say you plan to cast it, yet the drawing shows brick construction. The area that gathers the smoke should be at least double the area of the cross sectional area of the 6” pipe. Ie wide but shallow like the head of a vacuum cleaner. This allows you to make the gallery quite shallow to make the oven easy to work.
Regarding using standard bricks on the front I can’t see that being a problem as there will be no flame impingement there.

Thanks David. I was reading about putting a heat break in the floor as well. Although I haven’t come across any details yet how to do it, namely whether to leave it as a gap, or to fill it with vermicrete or such. Per your previous advice, I was planning on filling the heat break gap between the oven and vent arch with vermicrete.

WIth regards to casting the gallery, i’ve attached some image which as a visual aid make it more clear. I need to transition from the vent gallery opening in the arch (387x128mm) to a cast flu with diameter of 200mm. To do this, I will cast directly on the arch a piece that will transition from the “387x128mm” to a square of 255x255mm. This is specifically so I can fit the cast adapter that is made to mount the flu in. See image attached.

That is what I mean by casting the gallery. I’m not fluent in the terminology, so when I say vent gallery, what I am meaning is the area of the vent opening from the inside of the arch up to where the gallery meets the flu adapter plate.

The area of a cross section of the 200mm diameter flu is 314cm2, and compared to the gallery opening which is 495cm2. The vent gallery opening at the planned size if 57% larger than the cross section of the flu. Is it to big? If I imagine by your analogy of the head of a vacuum cleaner (brilliant way to describe it by the way, can image with the draw of the flu if it is the correct size it would be like the vacuum), at the present width I could possibly recuse the depth of the vent opening to 8.2cm. At this depth the vent opening would be 317cm2. Is this a good idea? While the opening would be 387x82mm, it would open up to meet the flu adapter plate where it would be 25.5x25.5cm, 650cm2. Double that at the vent opening. A good space for smoke to gather before going up the 200mm flu?
If I did reduce the depth of the vent opening to 82mm, it means I could reduce the depth of the vent landing by 46mm.

I attached 2 images of potential arch layouts to incorporate the above. In one, the idea was to slice regular bricks to 40 thick, and to mortar those on the front of the firebrick arch. Then I read your comment, so in the second image changed the front of the outer arch (in from of the vent opening) to incorporate regular clay bricks. I will buy some 100 year old bricks here in the next days. That would allow me to have my decorative arch in brick. The other alternative I had been considering is to tile around the arch. I see you did that was well.

[david s]

1. As the oven floor is always cooler near the oven mouth, insulating under the entry bricks will also help reduce this temperature difference and heat being drawn from the floor to the supporting slab. Some builders include a small gap between the floor bricks of the oven and the floor of the entry to reduce the difference and the heat sink effect.

2/3. I’m not really sure what you plan with the flue gallery when you say you plan to cast it, yet the drawing shows brick construction. The area that gathers the smoke should be at least double the area of the cross sectional area of the 6” pipe. Ie wide but shallow like the head of a vacuum cleaner. This allows you to make the gallery quite shallow to make the oven easy to work.
Regarding using standard bricks on the front I can’t see that being a problem as there will be no flame impingement there.

[daidensacha]

Points of confusion, needing clarification
As I come closer to starting my project, there are some points that are clear to me that I need to clarify. Any advice on the following points would be really appreciated.

1. Insulation under vent landing/ Heat break size/location

Vent area insulation

From Forno Bravo plans.
If you will be building a landing in front of your oven opening, it is not necessary to pour vermiculite concrete all the way to the front of the hearth. Rather, you can end the form where your oven vent will begin, and only pour vermiculite directly under the oven chamber and vent area.
Fill the form under the landing in front of the oven with standard concrete

What I’m not clear on here is where the oven vent begins. The components of the opening as I know them, are the inner arch, the vent landing, then the outer arch. I will have an oven landing in front of the oven, which will be plain concrete.

Q. Up to which point is it best to insulate under the vent area?
Q. Would it be up to and including the outer arch?
Q. Should this insulation be only under the vent area, or also under the arch walls?

Heat Break
I also want to integrate a heat break, although I am unsure where the better place to put it is. I read some put it between the inner arch and the vent gallery, and others put it between the vent gallery and the outer arch. David also has explained to me the difference between heat loss/transference through conduction, and radiation, and the benefit the heat break has with reducing to a degree heat transference through conduction, but not radiation.
By David’s logic, it seems practical to have the heat break between the vent gallery and the outer arch, to prevent the heat transference by conduction to the outer arch, and prevent possible cracking from expansion to the face that is exposed to the outside conditions.

I was planning on 10mm heat break filled with vermicrete insulation. Inner arch, vent area and outer arch come to 460mm (18.11”). This is without adding a decorative arch in front. How do others do this if you want to add a decorative arch in front? Can you replace the outer arch firebricks with normal bricks? I have used recycled bricks a lot on difference projects, and they do break down over time. If I were to use them as the front arch instead of firebrick, i’m concerned what the heat will do to them over time.
Q. Is it better to plan for and add a separate decorative arch in front of the vent area?

2. Vent Gallery Construction

I am working on formwork to cast my vent area out of castable concrete directly on the brick arch and vent. It will be 50mm thick, and wrapped in heat blanket before bricking up around it leaving an air gap as well. My reasoning is to prevent heat transference to the outer wall of the chimney.

The planned inside of the vent gallery was to be be 373mm wide x 228mm deep x 235 high (from inner brick arch to top of vent). I have flipped back and forward between half brick, and full brick, uncertain if the half brick width will be enough to create sufficient draw.
Q. Can I reduce the depth of the vent opening by half a brick to 114mm and will it be big enough to draw smoke up and out the 200mm flu??

3. Recommended Flu size
I read that for 40” oven, mine is 1020mm close enough to 40”, the recommended flu size is 200mm. It will be 835mm from the inner arch, to the top of my oven chimney. The oven will be in the corner of an outdoor kitchen covered with a terrace roof. I planned to put a double wall flu on top of the chimney and through the roof within will extend the height.
Q. What would the recommended height be to creat a sufficient draw?

4. Gallery Landing Depth
I have little experience with creating a chimney, and the relationship between the oven opening size, vent gallery size, and flu size (width and height). Any input on my plan, or the correct sizes and dimensions so it functions well would be appreciated.

Image below: My arch plan with the heat break between the vent gallery and the front arch. Also with a 373 x 114mm vent gallery opening. I’m struggling to understand how to reduce the depth of the gallery landing as in this image, it is 460mm including the decorative arch.

Options in Table columns
Half bricks: If I use half bricks with a 10mm heat break between the vent landing/gallery and the separate decorative arch.
Option 1: Reduce half bricks either side of vent opening to 80mm, with a vent opening of 114mm, and heat break of 10mm between vent gallery and decorative arch.
Option 2: Reduce half bricks either side of the vent opening, and the decorative arch to 80mm, vents opening 114mm, heat break 10mm between vent gallery and decorative arch.
Option 3: Per option 2, but removing the decorative arch and placing heat gap between inner arch and the vent landing/ gallery.
Option 4: Per option 2, but increasing outer arch to half bricks(114mm) and and placing heat gap between inner arch and the vent landing/ gallery.

	Half bricks	Option 1	Option 2	Option3	Option 4
Inner arch	114 mm	80 mm	80 mm	80 mm	114 mm
Vent Opening	114x373 mm	114x373 mm	114x373 mm	114x373 mm	114x373 mm
Front of arch	114 mm	80 mm	80 mm	80mm	114 mm
Heat break	10mm	10 mm	10 mm	10 mm	10 mm
Separate Decorative Arch	114 mm	114 mm	80 mm	None	None
Outer vent landing	352 mm	318 mm	284 mm	204 mm	238 mm
Total vent landing	466 mm	398 mm	364 mm	284 mm	352 mm

** The options listed in this table based on the assumption that 114mm x 373mm vent opening will create sufficient draw.
** Outer vent landing is from the outside of the inner arch to the end of the vent landing where it meets the oven landing in front of the oven.
** Total vent landing is from the inside of the inner arch to the end of the vent landing where it meets the oven landing in front of the oven.

[daidensacha]

Apart from the water resistant calcium silicate board which I’ve never seen or used, the standard cal sil, 5:1 vermicrete will suck up any water into itself just as much as blanket will. The good news is that when any of them are dry they just go back to their original condition. The one that I’ve found that does not is mill board. It turns to mush and is very weak when it’s dried.

Two pallets with all my stuff for the oven was delivered Friday and is now sitting in the garage waiting for me to put it together. It’s good to see and feel the stuff, because until now my only sense of how it is has been from images and reading about it.

The calcium silicate board is quite solid, and not what I was expecting. Much harder than I anticipated, and I see why you say it wont have issues holding the oven weight. I don’t really want to give it a water test, although that might happen when I have a left over offcut. I guess as I have enough I’ll just go ahead and cut it 75-100mm larger than the outside of the dome. Just means I’ll need to take care not to damage it or get it wet when doing the bricks.