Chimney to Opening Proportions/Ratio?

[DBD]

Hi everyone, first post here after lurking the forums. Hope I can have some insight here to my question:

I am making my own diy oven, similar to those you see molded by a yoga ball. That brings my internal oven diameter to only 28 inches. I'm not finding any recommended proportions/measurements for such a small size so my question is: for my oven size, can you recommend for me the appropriate chimney diameter and appropriate opening size that would be best for thermals in my build?


Here's my current setup so far.

https://drive.google.com/open?id=1ma...ACz9xSuPG7zFDy


Thanks for reading.

PS. The reason I am going with this small size is because I simply can't find any other mold that would render a larger diameter right now. So my plan is to make this small oven all good and well (and some learning) and use it in the future to mold an even bigger one. Thoughts?

[david s]

I suggest you keep the ball for yoga, cut up the ply formwork for future oven fuel and start researching this site for cast ovens.
To answer your question re oven mouth the height of the mouth should be approx 63% of the chamber’s internal height. Make the width however wide you feel comfortable working with. The larger the mouth the greater the heat loss. The smaller the mouth the more difficult the access.

[Gulf]

Here is a quick search of Forno Bravo of some other builders that did or were thinking about building using this infamous method which has spread like wildfire over youtube, pinterest, and social media sites. This search only used the term "gym ball". You could use also "yoga" or "medicine ball" to define your search. But, the overwelming response from people with experiece is negative about this method..

[UtahBeehiver]

Concur with David and Gulf

[DBD]

Thanks Gulf, David, and UtahBeehiver,

I'm switching to some more research on this site as recommended, super glad I posted here. I will be posting progress of my build throughout the months when I figure things out!

Thanks so much again.

[L555BAT]

Hi

I’m new to the forum but have been using it to gather information as I’m just starting my own oven build. I too am planning on building to approximately the same size 700mm (28”) internal diameter. I’m building to that size as it’s all I have space for. I have no intention on using a yoga ball to form it.

I am hopefully going to pin down my dimensions today if I can get a break from my work, I will try to detail them here later and see what people think.

I don’t mean to hijack this thread but thought it was very interesting.

[L555BAT]

These are the dimensions I intend to work to, anyone feel free to comment. I have acquired all the materials to make it happen and look to start the dome next week.

Dimensions MM. Inch

Internal diameter. 700. 28
Internal height. 430. 17
Opening width. 400. 16
Opening height. 270. 10.5
Entrance projection 280. 11
Oven floor. 50. 2
Dome thickness. 75. 3
Insulation blanket. 25. 1
Vermiculite concrete 75. 3
Base insulation. 100. 4
Chimney diameter. 125. 5
Chimney height. 500. 19.5

[david s]

You need a 150 mm diam flue IMO.

[L555BAT]

Thanks for the info David, I wasn’t quite sure which of the two sizes I needed from my research, but then I acquired a 5 inch piece of twin wall flu pipe for free so that is why it’s being used.

if that’s the only comment, I’m quite pleased! Best get on with it as the good weather here in the UK will be short lived!

[UtahBeehiver]

David does small ovens commercially in Australia so I would seriously consider his suggestion on chimney diameter. Too small and you will get more smoke out the front. The double wall pipe also needs to be wood fuel rated not natural gas fuel. What is base insulation? Dome Vcrete ratio 8-10 to 1, under dome if vcrete 5:1

[L555BAT]

Thanks beehiver, good points! The twin wall is wood fire rated I’ve now confirmed, but I hadn’t considered it previously! For my base insulation I’m using 100mm (4”) thick aerated building blocks, as I had them surplus from an extension I built. Thanks for clarifying the mix ratios for my vermiculite, that’s what I had read so will work to that.

I’ll see if I can easily acquire a 6” flu pipe. Is there any need to use twin wall or am I just as well using single?

thanks

[danhem]

Thanks Gulf, David, and UtahBeehiver,

I'm switching to some more research on this site as recommended, super glad I posted here. I will be posting progress of my build throughout the months when I figure things out!

Thanks so much again.

I think most would suggest the use of an IT (indispensable tool) to create the dome brick layout. It's an great tool that may take some setting up but works wonders once its ready and will make sure each brick is laid according to your dimensions, in your case the inside of each brick would be set perpendicular to the oven center and also measure 14" from the same oven center point.

Take a look at this link:

https://community.fornobravo.com/for...-on-a-fb-theme