NZ build

[JSeward]

Hi All I have finally stopped lurking and decided to join up. What a great bunch you are! I have built my base and about to start the hearth and dome, though not really working to a formal set of plans I am getting nervous about the dome. My base is 1.4m give or take, the inner diameter of the dome was hoping to be 1.0m, do I have enough room for the landing and chimney ? It is looking a little shorter than most pics I have seen. Width of opening I was hoping to be 50cm and about 30cm high. Dome inner height of 50cm. Any help much appreciated

[david s]

With the measurements you've provided it sounds like a squeeze. You don't say if you are building the oven in brick, casting yourself or using a modular kit, but if it's brick then a half brick will be 112 mm which only leaves you 88 mm each side for insulation and rendered outer shell or siding if building a dog kennel style. If casting or buying a modular kit the walls are usually thinner so that might be a better way to go, or just make the oven smaller. You don't say what the other dimension of your supporting slab is, but most designs need extra room to accomodate the depth of the entry, so this also needs consideration.

[JSeward]

Thanks David, base is 1.4m square, building out of brick, bricks available are 75x115x230. What diameter would you recommend for this size of base ?

I am guessing 100mm of vermicrete on the outside is the norm, so using the above 970mm would seem like it fits. My main worry was the minimum length of the entry required to allow the oven to draw properly, I think this is really what will dictate the diameter of the dome.

Cheers

[david s]

Ceramic fibre blanket is twice as good an insulator as vermicrete so you can make your insulation 50mm thick and save some more space. Your bigger problem is the depth of the entry.. Lay some bricks out on your slab to see if you can work it out. Casting an entry is a way of making it much shallower than a brick one. Mine is only 130mm deep and then another 65mm for the decorative arch in front of it. You can also set the entry slightly inside the circumference of the dome to save more space.

[JRPizza]

JSeward, do you have any experience using CAD programs? Laying bricks and using templates are great tools, but using CAD can help you do some quick resizing and moving things around. Two things that I think will help you shorten up your landing is to go with a smaller flue and move the inner arch as far into the oven as possible. There are recommendations for flue size here on the forum, but for your size oven the recommendation would be for an 8" round if you went with round. If you shrunk your oven to around 36" (.915m) the recommendation is for a 6 round that would let you have a smaller vent area. Moving the inner arch in towards the center of the oven lets you start the vent "earlier" which could shorten up the landing length too. Lastly, you could add a cantilever to the front of your hearth to gain some room - several builders have done that.
I recommend reading several of the posts on sizing a vent landing and consider using some CAD program or hand layouts to get a quick feel for what is possible.

[JSeward]

Thanks gents, I have sketch-up to hand, but what am I actually trying to achieve ? Sounds like I can make a small entry work ? Is it the building of it that is difficult or is it the fact that a short landing causes venting/draw issues ? I am confused

[david s]

So long as there is plenty of draw from the flue provided by a combination of pipe diameter and height, the entry below the pipe can actually be really quite shallow. Think of a vacuum cleaner head that is wider than the pipe but far shallower. So long as the entry funnels nicely to the pipe and the draw is adequate there are no smoke issues. This is quite difficult to achieve if making the thing in brick because it's hard to transition from a rectangle to a circle. Casting allows this far more easily as pics show.

[JSeward]

Ahh I see, great analogy thank you.

[JSeward]

So, I got back to the oven after a short hiatus. So the dome , do I build it on the hearth or outside, are there any pros and cons ? I am thinking 25mm vermicrete, 25mm ceramic board then 25mm floor tiles. 50mm vermicrete under the dome walls. Do I mortar the first soldier row down? Thanks for any help

[JSeward]

Hi All, I am trying to keep costs down and make things as easy as feasible with my build . Is it feasible to cast my own cooking floor i.e. rather than use bricks ?? there must be a reason more people don't do this ???

[UtahBeehiver]

Contact David S. He is our resident cast concrete expert.

[JSeward]

Many thanks

[david s]

Hi All, I am trying to keep costs down and make things as easy as feasible with my build . Is it feasible to cast my own cooking floor i.e. rather than use bricks ?? there must be a reason more people don't do this ???

Some manufacturers do, but the problem with calcium aluminate castables is that any large castings are subject to cracking because of the uneven heating. This is why many builders lay the floor bricks loose to allow for differing temperatures and expansion. The larger the floor bricks are the more likely they ar to crack. A cast floor made with Portland cement won't last too well but if made with a more refractory cement (calcium aluminate) it can take the heat. Ideally these castings should be fired slowly to at least 1000C when it will be properly sintered. In practice no one does that because you need a large kiln and plenty of time and money spent on kiln fuel to take the material up to temperature slowly and progressively. If done this way the material is essentially the same as a firebrick. Unfired it only relies on the chemical bond of the cement. A cast floor does work, but needs to be done in a few sections unless you don't mind some floor cracks.
25 mm for the floor is way too thin IMO, it won't hold enough heat. You really need 50 mm. The usual under floor insulation is 100 mm of 5:1 vermicrete or 50 mm of cal sil board. You can use both but your plan looks a bit too thin to me.

[JSeward]

Thanks David, yeah, doing some more reading I realised the 25mm was too thin. I have 50mm of calsil board and was planning on 50mm of floor now.

So I need to fire the floor ?..., I cant just pour it like normal concrete, what would happen e.g. cracking or spalling ? cracking is fine, the gaps between brick are, after all cracks, but as long as the performance isn't to badly affected I am fine with that.

The problem is it hard to get 50mm fire brick brick here in NZ. Certec have them but they aren't standard, and getting 500mm square 50mm thick tiles is $70 per tile, otherwise, 75mm is a bit too thick for a floor no ? or at least too high for my current plans....

Thanks
John

[david s]

So I need to fire the floor ?..., I cant just pour it like normal concrete, what would happen e.g. cracking or spalling ? cracking is fine, the gaps between brick are, after all cracks, but as long as the performance isn't to badly affected I am fine with that

Ok. In that case get a proprietary dense castable refractory that uses calcium aluminate as the cement component in the mix. The floor takes a real beating and the home brew, while OK as a mortar for the dome is not suitable as a castable to use as the oven floor going to pizza temperatures. If you can, although not absolutely necessary, add some stainless steel needles (melt extract fibres) at the rate of at least 2% of the weight of the dry mix.Both castable and needles should be available from refractory suppliers.