Modular ovens and thermal mass

[ikhan42]

Hello,

I am looking at maybe buying a modular wfo rather then building one from scratch. I have a question about these modular ovens or is it 2 questions LOL.

Okay from what I have seen of these prefab modular ovens they are relatively thin and from my understanding of thermal mass that is the reason these ovens have such quick heat up times. First question is. is this assumptions correct ?

If the answer is yes then read on to my next question otherwise ignore the ramblings of a senile man.

from what I understand of the vaulted ovens is that they retain heat for days due to the thickness of the walls which makes sence to me. So here is my question.

What is stopping me from buying a modular ovenkit and increasing the therman mass by adding refractory cement to the outside before the insulation layer to increase the thickness of the oven and also hopefully the thermal mass. Kind of like what they do on top of the scotts oven after the bricks are layed they add I thnink from memory 4 inches of cladding to increase its thermal mass. Is there a structural reason why this couldnt be done on the igloos?

Hope you guys understand what I am asking. LOL not to sure if I asked it right.

thanks
ikhan42

[dmun]

Re: Modular ovens and thermal mass

Extra thermal mass means more heat up time and more fuel. It also means that the oven stays hot longer, and you can bake repeat batches of bread. It makes sense if bulk baking is your goal, particularly if you're firing the oven every day, in which case the extra time and fuel is not such an issue.

So the question you need to ask yourself is: am I, or do I aspire to be a commercial baker? Do I have access to huge quantities of firewood? Will I not be disappointed if it's hard to bring my oven up to pizza temperatures? If so, then thermal mass is the way to go.

People do add thermal mass to the outside of modular ovens. Usually it's more like half an inch, rather than four inches, but there's no reason that it couldn't work, if that's what you wanted.

[Frances]

Re: Modular ovens and thermal mass

I think it'd make good sense to add to the thickness of a modular oven so's the walls would be as thick as the brick built ovens are. Wouldn't it?

You'd end up using a bit more fire wood but with the option of baking two or three loads of bread - it would also retain heat for three days, as long as you add plenty of insulation.

[nissanneill]

Re: Modular ovens and thermal mass

ikhan42
dmun has it in a nutshell as many of these premanufactured ovens are designed to heat up in less time but cannot absorb nor hold the heatbin the thinner walls. Yes you will use less wood and heat it to pizza temps in under 45 minutes and cook many pizzas in it provided you keep a reasonable fire ging during the cooking period. Shut it up to maximise the heat absorbtion for a bread bake, rake out the coles, sweep or mop the floor and have a single bread bake, a second one unlikely.
Again, it is your choice, add extra mass for heat retention and use more wood (not significantly more, but more) and add possibly a half hour to get up to temperatures required.
My 40" Pompeii heats up in around an hour and uses a wheelbarrow load of wood which is about 25kg of redgum. The dome reaches 500+˚C and the hearth around 430˚C. Keep the fire going and cook 20 to 30 pizzas when I allow it to cool to around 350˚C (dome) for a bread bake with the door on, a second one if needed (but never needed as I can get more bread, rolls and buns in on a single bake), then the oven cools a little for a roast (1 1/2 to 2 hours) then the apple pie, and then I shut it up and it is still 200˚C next morning.
You choose, but get onto James and see if/when he anticipates the FB modular ovens being available here in Australia.
Incidentally, where abouts are you located down under?

Neill

[ikhan42]

Re: Modular ovens and thermal mass

Thanks for the info guys I am not looking at doing commercial bread baking but I do like to do slow roasts ie cook beef slowly for 3-4 hours. I was leaning towards the scotts oven as it seems easier tand quicker to build. I will do pizzas in it but am looking for more a oven that I can bake in, do pizzas in, slowly cook biryani sealed with flour, and more importantly do tandoori chicken and naan breads in. We hold pretty big parties some times 60+ people so am looking at something that will stay hot longer. The modulars dont in my opinion fit this bill without re doing the fires.

I just dont have time to build the igloo anymore out of fire bricks as I am a perfectionst and the top 1/2 of the oven looks like something I will never be happy with.

Speaking of the igloo I will ask this silly question as it just came as a light bulb in my head.

Why cant the pompeii oven be built up to saye the 3rd or 4th course when things arent all that scary and the a top be mad for the oven out of cast refactory cement? Is this not possible cause of the keystone ???

The reason I ask this question is most of the modular ovens seem to come with the a cast hat that sits of the igloo type ovens.

Anyways hope I am not scaring anyone to much with my hair brain ideas

BTW nissanneill I am from sydney live in a suburb near Baulkham Hills

Thanks again
ikhan42

ikhan42
dmun has it in a nutshell as many of these premanufactured ovens are designed to heat up in less time but cannot absorb nor hold the heatbin the thinner walls. Yes you will use less wood and heat it to pizza temps in under 45 minutes and cook many pizzas in it provided you keep a reasonable fire ging during the cooking period. Shut it up to maximise the heat absorbtion for a bread bake, rake out the coles, sweep or mop the floor and have a single bread bake, a second one unlikely.
Again, it is your choice, add extra mass for heat retention and use more wood (not significantly more, but more) and add possibly a half hour to get up to temperatures required.
My 40" Pompeii heats up in around an hour and uses a wheelbarrow load of wood which is about 25kg of redgum. The dome reaches 500+˚C and the hearth around 430˚C. Keep the fire going and cook 20 to 30 pizzas when I allow it to cool to around 350˚C (dome) for a bread bake with the door on, a second one if needed (but never needed as I can get more bread, rolls and buns in on a single bake), then the oven cools a little for a roast (1 1/2 to 2 hours) then the apple pie, and then I shut it up and it is still 200˚C next morning.
You choose, but get onto James and see if/when he anticipates the FB modular ovens being available here in Australia.
Incidentally, where abouts are you located down under?

Neill

[ikhan42]

Re: Modular ovens and thermal mass

Ok here is my question in reverse, it appears that if I add more mass to the outside of the pompeii oven it starts to take on the characteristics of the scotts oven ie doesnt heat us as hot as the traditional pompeii but will retain heat for longer ie idel for breads.

So now I will ask what if the scotts oven is made with less thermal mass ie not so much refractory cement poured over the oven after construction then would it not have the faster heating times of the igloo ovens ? Reach the same temperatures ? Be good for pizzas?

Lust pondering sounds like thats all i do these days is ponder and contemplate
Thanks anyways
ikhan42

[nissanneill]

Re: Modular ovens and thermal mass

ikhan42

if I add more mass to the outside of the Pompeii oven it starts to take on the characteristics of the Scott's oven ie. doesn't heat us as hot as the traditional Pompeii but will retain heat for longer ie ideal for breads.

Assuming you are using the 4 1/2" brick thickness for your dome walls, why would you want to add extra thickness and thermal mass?
Most cast refractory domes are only around 2" thick, that is why they heat up quicker and use much less wood to achieve the heats required for your pizzas, OR breads, OR merely baking.
If you only want to bake, then you won't have a fire in there, a door will be on and closed to hold in the heat and there will be less heat stored in the much thinner dome.
An oven made by a brick width (4 1/2") will need more wood, and a longer fire time to provide the heat for the bricks to store. You must also allow the material time to absorb the heat and to transfer it into and through the brick. Remember that the bricks will absorb the heat muck more slowly than say a metal which have high thermal transmission. For example, get a 6" piece of metal, any metal but lets use aluminium, hold it at one end and put the other end into a flame. See how long it takes before you drop it as it gets too hot to handle. Now, get a piece of your dome brick, similar size and length and repeat the exercise. I'll bet that it takes a lot longer before it gets too hot to handle.
So your oven works the same way, a fierce fire to provide the huge amount of heat, and time to let it 'soak' ie to penetrate the bricks as much as possible. The deeper the heat in the brick, the more heat is stord within the dome and consequently the longer you can use this stored heat for whatever you wish BUT at the temperature you need.
How well your oven is insulated and when you close the very well insulated door, will govern how hot and how long you can retain that heat to bake.

So now I will ask what if the scotts oven is made with less thermal mass ie not so much refractory cement poured over the oven after construction then would it not have the faster heating times of the igloo ovens ? Reach the same temperatures ? Be good for pizzas?

Remember that the Scott oven is made with the 4 1/2" brick thickness and if you want to bake numerous bread bakes, you can add extra thermal mass. The more the thermal mass, the longer and hotter and more wood you will use. Yes, with a single brick arch thickness, you will achieve pizza temperatures quicker but these ovens were designed not to get so hot but more for baking breads and several bakes.
Hope this helps.

Neill

[dmun]

Re: Modular ovens and thermal mass

Remember that the Scott oven is made with the 4 1/2" brick thickness

And a lot more thermal mass above and below...

if the scotts oven is made with less thermal mass ie not so much refractory cement poured over the oven after construction then would it not have the faster heating times of the igloo ovens ?

The barrel vault oven is less inherently strong than the hemispherical oven. There's a lot of lateral stress on those side walls, and being rectangular, all the forces are exerted sideways. That's partly what all that concrete is doing over the brick layer: buttressing.

James, the founder of the FB site, built two barrel vault ovens, and his frustration with them led him to the research that developed the Pompeii oven.

Can we ask why you want extra mass? What is it you are planning to bake?

[james]

Re: Modular ovens and thermal mass

I think Dmun hits it on the head. I always think of Notre Dame catherdral when I look at the Scott oven. The lateral (outward) thrust of the barrel vault dome pushes the oven walls outward -- which is why Notre Dame has those huge flying buttresses. A structural necessity hidden as a design feature. The Scott oven is encased with 4" of concrete in part to hold everything together. I built my first oven with Alan (he was a great guy by the way), and we even embedded rebar in the hearth and the cladding concrete to hold the oven in.

I know I say this all the time, but unless you are running a commercial bakery, the Scott oven is not the right design.

James

[ikhan42]

Re: Modular ovens and thermal mass

DMun,

I dont want the extra thermal mass hence my question if I do a scotts without the extra mass around it would it not function the same as a Pompeii?

The reason I asked about adding extra thermal mass for the modular oven is because they look way to thin to me and look like they would cool down fairly quickly.

I am just looking at options and an oven that is relatively easy for me to put together. I am scared about building the pompeii as I know I am fairly anal and would probably never finish the pompeii if I dont do it perfect.

The Scotts oven looks like a no brainer thats why I am leaning towards it.

Also any one have any comments on making a modular hat for the Pompeii in a build rather then cutting the wierd odd sized bricks. I am assuming this is imparcticle due to keystone but guess my curious mind needs to know LOL.

ikhan42

And a lot more thermal mass above and below...

The barrel vault oven is less inherently strong than the hemispherical oven. There's a lot of lateral stress on those side walls, and being rectangular, all the forces are exerted sideways. That's partly what all that concrete is doing over the brick layer: buttressing.

James, the founder of the FB site, built two barrel vault ovens, and his frustration with them led him to the research that developed the Pompeii oven.

Can we ask why you want extra mass? What is it you are planning to bake?

[dmun]

Re: Modular ovens and thermal mass

It's an enduring myth that the barrel vault oven is easier to build than a hemispherical dome. They are both big projects that involve a lot of heavy lifting: In retrospect, moving and lifting stuff was the hardest part. It's true that a fully-fitted, no mortar showing pompeii involves a lot of fussy fitting, but they work just as well if there are gaps between bricks.

You need to ask what you want to do with the oven. Even a thin wall modular oven will hold LOTS of heat, enough to bake a turkey for hours, a standard thickness pompeii, even more.

[james]

Re: Modular ovens and thermal mass

The vent and transition from the barrel vault are very complicated and poorly documented with the Scott oven. It is worse than anything you have to do in the Pompeii oven. One of the reasons the barrel vault oven doesn't seem difficult is that there are few drawings or photos of that part of the oven. When I asked Alan about it, he laughed and said that it was an initiative test -- and that everyone did it differently. :-)

If you are looking to build an oven that doesn't require cutting bricks, you could build a barrel vault oven without a vent, and work your way around the smoke.

James

[ikhan42]

Re: Modular ovens and thermal mass

Thanks for the info I would imagine the transition into the vent for either is not really all that well documented and is really something that is done differently on every build for both the barrell vault and pompeii or other home made igloo ovens.

But I am now leaning back to the igloo oven. Back to my other question is it possible to do the pompeii up to say the 4th or 5th chain and to finish the top off with a pre cast refractory cement hat that fits on top. I think there will be strength issues as there isnt the locking keystone brick but will ask the question anyway.

Thanks
ikhan42

The vent and transition from the barrel vault are very complicated and poorly documented with the Scott oven. It is worse than anything you have to do in the Pompeii oven. One of the reasons the barrel vault oven doesn't seem difficult is that there are few drawings or photos of that part of the oven. When I asked Alan about it, he laughed and said that it was an initiative test -- and that everyone did it differently. :-)

If you are looking to build an oven that doesn't require cutting bricks, you could build a barrel vault oven without a vent, and work your way around the smoke.

James

[mr g]

Re: Modular ovens and thermal mass

Hi Ikhan42, not sure if this info helps but I and a few other friends built our entire ovens from castable refractory. I've attached some photos and have plenty more if you're interested.

Note - the sand mould was designed to hold up for 10 domes so a little extra effort went into making the mould but you needn't worry too much if you're only casting one dome. Once we got going we were casting an entire oven in less than two hours.

One oven is already being used to cook 2 minute pizzas and it works beautifully. The oven seems to take about 1.5 - 2 hrs although we haven't fired it enough times to have reliable data an that yet. We haven't got any doors either so I can't help you with how long it holds the heat for but in terms of thermal mass - there is around 420 kgs worth of castable (75mm thickness) plus the floor tiles so I expect to be able to cook anything that I need to for a typical back yard situation. Of course you can add another inch to give you greater heat holding capacity.

Cheers
Mr G

[bernerdog]

Re: Modular ovens and thermal mass

James,all due respect but the transition in both ovens remains the most mystical and least documented step and ,I suspect, likely a bit different in all builds. I have built both and while the forno bravo plans and forum contributions are clearly more helpful, that step remains a challenge according to almost almost all postings. Consider the "teardrop", dropping wall, metal vs. brick, arch or no, and other issues, we may do a disservice to new builders in suggesting that step is as easy as any other. Not so for me and a good deal of feelings one`s way can be expected. John