2024 Neapolitan oven build

[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]

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]

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]

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]

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?

[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]

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.

[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]

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]

Casting the vent gallery

I painted the ply formwork and casting plaster/ vermiculite mould with a release agent. Centered the mould in formwork with a 50mm space between the mould and formwork.

I used dense castable refractory concrete for the vent form.

Following images. Sanded the casting plaster/ vermiculite mould so it was smooth. Important was the flowing form from 373x128mm vent opening to the 200mm diameter opening for the flue.

Painting the mould with release agent worked well, allowing easy removal of the formwork and the mould from the concrete. The mould also absorbed water from the concrete, which softened the mould making it really easy to scrape out.

It’s designed to have 50mm thick walls, so it’s weighty, but I can lift it and there is excess concrete in places that I can remove once it has dried.

The last image shows how the top is designed to fit the flue adapter plate that the flue will sit in.



I’ve got to give credit to david s for the inspiration for this vent. His generosity in sharing his expertise with vermiculite and casting the vent form was really helpful and made this possible. In the beginning it took me a little to get my head around, figuring out how I could adapt his method to my needs. The final outcome is perfect for my planned oven.

[UtahBeehiver]

Nice casting work.

[Tobi]

Hi, I follow your post, because I saw that you really plan accurately. I'm building a pizzaoven right now and did a lot of research. What I was also interested you told about is the iron band around the first standing? brick round. How will you fix it? Because I think it would go in the Hight of the ovenmouth, so you can't just put it completly around. One oven builder I know told me he would put a door band "Ofendichtband" between the bricks and iron band because of different movement while heating the oven. I also wrote you an personal message, because I'm also from Bavaria and more or less in the same state of the oven. Maybe we can share some Infos. Thanks for your great planning. I nearly finished my construction under the oven. Images attached. Best regards tobi

[Tobi]

[daidensacha]

Tobi Hi Tobi, I‘ve been pretty preoccupied, we are building a new terrace in which I will build my oven. As you might know, with the weather here in Bavaria its a lot.of preparation to have the terrace ready for paving.
Page three of posts in this thread, David helped me by bringing my awareness to the fact it would be good to think about reinforcing the Neapolitan dome. He provided sketches, but if you want to look at a good example with images of how the bands are attached to the front of the oven, look in Napolitian style 106cm build inside a hobbit house. Basically upright angles on the side of the arches, with threaded rods across the top of the arch attaching the angles. The bands are then attached to the angles with brackets and threaded rod that can be tightened. I bought stainless steel pieces cut to size and had them welded at a friends fabrication workshop. I bought the stainless steel band online, and will attach the ends to the brackets with studs. It is 14mm stainless rod.

I‘m still a way off but hope to start the foundation of my base next week. My terrace will have a roof, so once that is up I am free to build regardless of the weather.