Scratch build thread - 36-inch inner diameter dome oven

[mnagy]

And then this would be the female end:

[mnagy]

The main door of the oven would just be placed in the tunnel. I would think air could get out that way before there was any pressure built up no?

[david s]

I'm a big advocate of vents for the underfloor insulation as well as for the dome insulation. After lots of thought I don't believe that a vent at the top of the dome is the ideal location for the dome vent apart from it being a bit unsightly. I have a system where the insulation that surrounds the flue pipe can exit between two terracotta collars, but this requires some specialised fabrication. A far simpler system that won't be seen is to drill some holes in the base of the flue pipe where moisture can escape.(see attached pic)

I also now have 4 vents in the supporting slab that line up with the dome insulation, as well as one in the centre for the under floor insulation. The reasoning for the placement here is that as the top of the dome on the inside gets hottest the quickest because of heat rising by convection, The dome insulation will dry out fastest in that location. As moisture accumulates in the lower parts of the dome insulation and will want to travel away from the heat source and down by gravity. Additionally, any steam pressure higher up will force moisture down, particularly as it hits the cooler outer rendered shell and re-condenses. Moisture will pass readily through blanket or lean verm/perlcrete.
Imagine if you had a saucepan of boiling water and it had a drain in the base of the saucepan. If you blocked the hole in the lid the steam pressure would very quickly empty the saucepan.

I remember when firing a new kiln for the first time and the recommendation was to fire it very slowly, empty apart from shelves and props up to service temperature. I was very surprised to notice water dripping from one front corner at 400C, creating a pool of about 100ml. I thought that this would be well above the point of any moisture not having already been vaporised.

Any vent or drain holes should be covered with mesh.

[mnagy]

Right now I feel good about the downward drainage through the floor. A cap vent on top wouldn’t look bad (especially if I glue a tile to it), but I don’t fully understand the benefit yet. I’m thinking the concern is water/pressure building up between the dome brick and the rendering layer?

[david s]

Steam pressure can be enough to crack the outer shell, but the main issue is to efficiently remove any moisture that can accumulate in the insulation layers. Because water conducts heat, wet or just moist insulation will not work well and heat loss will be high, as well as considerable amount of energy is required to vaporise it. Vents allow its exit efficiently.

[mnagy]

What about something like this. I would put it on the back of the dome, about 20 degrees up from the bottom. It open up the fiber blanket layer to the air.

[david s]

Yes, that could work ok, nut because of the process I explained, the lower down on the dome that it is the better. You would need to ensure that it didn't provide an entry for water.

[mnagy]

Perfect. I think it should be good in terms of keeping water out. The vent has angled grates that should do the trick. I’ll use 3 of them: at 12-o’clock (back of the dome), 4:00, and 8:00.

I suppose in a windy rainstorm some water could get in, but I bought a custom cover for it which will generally be on when not in use.

Thanks again for yet another helpful catch!

[mnagy]

What do you guys think about these vents? Some of the guys on TikTok are saying that allowing the insulation to be exposed to the air will cause dew to form on the inside of the shell, causing lots of water to accumulate. What are your thoughts on this? https://www.tiktok.com/t/ZT8rVYkGb/

[mnagy]

Finished the oven!!!

https://www.tiktok.com/t/ZT8rtW9N3/

This was such a rewarding project. I feel connected to history/humanity with how little this process has changed in 2000 years.

Thank you to everyone on this forum (david, beehiver, and others). Some serious ideas, SAVES, and general guidance that was shared for free just because these guys love ovens.

Will enjoy this thing for years to come.

thanks!

[david s]

What do you guys think about these vents? Some of the guys on TikTok are saying that allowing the insulation to be exposed to the air will cause dew to form on the inside of the shell, causing lots of water to accumulate. What are your thoughts on this? https://www.tiktok.com/t/ZT8rVYkGb/

I disagree as dew forms on the hotter side. eg dew forms on the inside of your car windscreen, particularly on a cold morning and you have the car heater on. As the sun rises and if the temperature of the outside air gets hotter than the inside of the car the water condenses on the outside of the windscreen. Any minimal early morning dew forming on the area of the vents will be quickly evaporated as the air temperature rises above the temperature of the insulation. The more important and much greater volume of moisture in the oven will be affectively removed with the vents located, preferably under the floor, (where they won’t collect dew), or low in the dome.

[mnagy]

Dew forms on surfaces that are colder than the outside air. Like on a cold glass of water sitting on your grandma’s nice coffee table. The concern isn’t condensation forming around the vent. It’s condensation inside the dome insulation layers. Where the fiber blanket (exposed to moist outside air) is warmer than the inner surfaces of the dome and rendering.

[UtahBeehiver]

These ovens are a labor of love. Now you get to explore the nuances of cooking in WFOs. If you are interest, look up thread from Karangi Dude, he takes cooking in WFOs' to a whole other level.