Need help with oven planning

[pocahontius]

Hello,

my name is Jens and I’m looking forward to build a first Pompeii Oven this summer. I read the plans version 2.0, watched a lot of Youtube videos and checked this forum, but now I have a few questions, where I could not find definitive answers for. I want to draw plans and calculate the size of the foundation, so I need to know what materials I use.

1. Hearth: Is it better to use a ceramic isolating board or isolating concrete?
2. Hearth: Is it better to construct the island hearth for additional thermal mass or will this get to massive? There will be occasions once a year, where I will make pizza or cook for about 50-60 people. But most of the time, I use it for 4-6 people eating a pizza/making some food or for baking some bread for me and a two neighbors.
3. Dome: Should I apply an addition 2“ of high heat mortar around the firebricks for additional thermal mass (see question 2)?
4. Oven Dome Insulation: I want to use ceramic insulating blanket as this seems to be the more modern option. The outside of the oven will be finished with small tiles. How do I construct this? If I put a 3“ layer of the blanket on the dome, this will be soft, so the tiles will fall off. Do I have to build a vermiculite layer on top of the blanket first in this case? How thick is enough to give the outside enough stability?
5. Weather: What product / color / material is best to protect everything against water? Do I have to put a waterproof layer of color directly on the vermiculite? I didn’t get the idea out of the english manual, sorry…

Greetings and thanks for your help!

Jens

[pocahontius]

Can anyone give me a hint? That would be really great!

Thanks a lot
Jens

[david s]

1. As a general rule calcium silicate board is a better insulator than a 5:1 vermicrete, but if you double its thickness you get pretty much the same result. The board is way more expensive, but is already dry and saves some labour.
2. 3” thick firebricks is plenty of thermal mass. Remember that added thermal mass, just as increasing the size of the oven will increase both fuel consumption and time taken to reach working temperature. If you want to cook frequently for only 4-6 people then a smaller oven may suit you better.
3. Same applies
4.Tiles are best glued and grouted against a hard cement outer shell. Rendering a cement shell against the springy and lumpy blanket is difficult but can be done. A layer of around 1.5” of 10:1 vermicrete provides a good substrate to render against as well as covering the lumpy blanket surface and restoring a true hemispherical form. It also increases gome insulation.Over which a hard cement shell around 12-15 mm thick is plenty strong enough.
5. Waterproofing the outer dome traps water in as well as keeping it out, so is not always necessary, depending on weather conditions, both rainfall and humidity. Tiles and grout should do a pretty good job of keeping out the water. A roof over an unwaterproofed oven dome is probably the best but most expensive option.

[pocahontius]

Thanks a lot for your help! Then I'm sure that I will discard the additional thermal mass. Can you give me a hint how long it will take to bring the ovens to temperature?

If I build the 36" vs. the 42" with only the firebricks, blanket, vermiculate layer and tiles on top and without the thermal mass?

I have absolutely no idea how the differences between the two will "feel". But I would be happy with the larger space inside.

Jens

[UtahBeehiver]

It takes roughly a couple hours for my 42" ovens to get heat saturated. As you mentioned in you original post, the occasionally you will do large parties, with the right planning, you can accomplish this with a 36". I rarely use the full 42" space in my oven and for 4-6 people, a 42" is way overkill for for small groups, takes more fuel to fire, longer to heat, etc. You are not limited to 36" or 42" you can make the ID any size you want, some builders have done 39" ovens for example.

[pocahontius]

So which size would you recommend as sweeetspot for me?

[UtahBeehiver]

Only you can make that decision.

[pocahontius]

That's sure :-)

Thanks!

[pocahontius]

Hello Gulf,

I now switched to this old thread from me.

now I understand what Russell meant with the wire mesh on top of the dome. Did you made this to fix the nut in the vermicrete?

The vent pictured in this message is the same size as the other one I showed you (4cm diameter), but has a removable (screwable) cap. I could shorten the pipe, welder something to the sides of the pipe that the thing sits more stable in the vermicrete (or if I weld some sheet metal pieces to that, I could screw it to the keystone of the dome (with some distance of course, that that steam from the blanket can get into it. Or, as David S said, drill a lot of holes from the side into it.

What I do not understand is the size. You said it is too large. These pieces are 1,5" in diameter. Is this a problem with the venting? Or did you say this because it would not fit through the wire mesh? I thought that a slightly larger hole would help to let the steam escape easier. These pieces are absolutely weatherproof. If I use another method to fix it, it seems ok to me or do I get something wrong here?

In the link you send in the other thread, there is one nut in the wire mesh and I think this will be layed on top of the blanked in the vermicrete right? But what is that nut in the keystone of the dome? Is it only for the large paddle helping you to create a perfect circle? Or has that something to do with this vent system?

Another question that just appeared to me: What is this heat barrier in the floor and entry arches for? Is it only, that the oven does not cool down so fast? I have no use for days of days of retained heat. More important seems to me, that the floor has no edge, where the pizza peel would grab to and the possibility of using the entry with lower temperatures, but I cannot judge this, as I haven't used a wood fired oven yet. But perhaps there is something to it, that you would recommend it to me, as it seems to be easily build.

Greetings
Jens

[Gulf]

Hi Jens,

Sorry for the late reply. I'm going to include a couple of links. One is to another thread where I discussed the oven vent bushing that I isnstalled. The other is to a Google Photos Album where I have added some select photos of the the Oven Vent. In the album I have a drawing of a cross section of my oven. At the apex of the dome the drawing depicts a hole through the vermicrete. That hole is filled with scrap ceramic fiber blanket. I wanted the the hole to allow steam, pressure, or moisture to have access to the vent. The hole is filled with scrap ceramic fiber insulation. The insulation does not restrict pressure but, does insulate the hottest part of the dome. I included that drawing also to show how the ceramic fiber layer separates the firebrick dome from the vermicrete layer.

My first thoughts of your vent cap was that it was too large. After looking at it more and reading your desciptions, I really like it. It is very low profile. I think that it will work just fine. I agree that you will need to cut the pipe or get another short all threaded nipple. The pipe does not need to go below the render any further than absolutely necessary. The pipe should not be anchored into the vermicrete. The vermicete has no strength. The short nipple should be anchored into the stucco/render. You mentioned welding but, the electrical locknuts come in 1.5" sizes also. You may want to incude a couple of thin washers. I'm not sure from the description exactly what size ventilation holes are in your vent. But, if they are large enough for your local ant poputaion, I would cut and fit a wire screen to fit inside of the nipple.

I hope that this helps.

[pocahontius]

Hello Gulf,

ok, great. I'm now sure I got it right, but my English is often not so good to express it clearly. The only thing I wasn't sure about was the strength of the vermicrete.

Do you have any tipps about my other question: What is this heat barrier in the floor and entry arches for? Is it only, that the oven does not cool down so fast? I have no use for days of days of retained heat. More important seems to me, that the floor has no edge, where the pizza peel would grab to and the possibility of using the entry with lower temperatures, but I cannot judge this, as I haven't used a wood fired oven yet. But perhaps there is something to it, that you would recommend it to me, as it seems to be easily build.

Greetings Jens

[Gulf]

What is this heat barrier in the floor and entry arches for? Is it only, that the oven does not cool down so fast? I have no use for days of days of retained heat.

You answered your own question correctly but, I would add that they are also thought of as expansion joints. They are not necessary for a pizza oven. I do mostly retained heat cooking and have a large masonry flue gallery and flue that would have drained a lot of heat from the oven.

[pocahontius]

I can not judge this, as I haven't used a wood fired oven before. I thought of using it as a tool for everything. Mostly pizza and bread, but I want to have to possibility to use a tuscan grill and cook in it. Most of the time, I think I will make pizzas with friends, and we want to use the retained heat to bake a few breads the next day with my neighbors. But this will be about 5-10 breads max. My flute is a from a German supplier, but is similar to the duravent type from the fornobravo manual. It is surrounded with isolation. From my readings I heard that an oven like this easily retains the heat for 1-2 days when using a door and no heat break. Can someone confirm this? Then I would not use a heat barrier.