First try with bricks and morter

[comuscomp]

If a problem occurs then a rebuild may be in order but for now it's too far on to stop and modify.

First drying fire seemed to go OK.



so did the second.



on the third day I got the arch temp to 320C and the walls to 270C and kept it there for a couple of hours. I just couldn't resist the temptation and it seemed a waste of heat not to, so first pizza fell into the oven.



And 3 minutes later it was ready.



seemed like a good time for celebration.



then play time with a sourdough garlic baguette



then a rosemary and sea-salt bun.



back to the bricks and the back structure wall built and cavity filled with vermiculite.



First layer of vermicrete 10:1 and covered with stainless steel plasterers lathe ready for the second coat.

[comuscomp]

Left it to dry for 4 days, gave it a second vermicrete coat and then a thick coat of waterproof render.



Just got to make some doors for the bottom and front arch.



tried using a trivet over the wood embers, the chicken tasted great.

[oblertone]

Nicely finished, I didn't think you'd left enough room for insulation but credit to the finished item. Are you planning an inner door as well as one to keep the cats out ?

[comuscomp]

It's a multi function affair, Oven for pizza, bread etc. Bar-B-Q using a trivet, griddle plate and drip tray, and a smoker in the front.

I'm making an inner door to close off the oven, a vented inner door to allow smoke out with the chimney capped, and double thick outer doors to keep the smoke in and the cats out.

the wood store doors are half made, rain stop play at the moment.

I found that the chimney needs extending so I'm looking for a 100mm flue pipe about a metre long to improve the smoke draw, think the chimney is a bit too narrow. Most of the smoke goes up the flue but some still billows out the front.

It could probably do with a bit more insulation, the side wall on one side get warm to the touch but that can be another project if it doesn't fall down. Just got to paint it now and maybe make a roof for it, and a bigger wood store, and a prep area, and a seating area and lights and........

[downunderdave]

You don't say what size your oven is, but for round ovens they require5" diam pipe for up to 24" internal diam and 6" up to 36" diam and 8" over 36" diam. Work out your floor area to compare to the dome. The height also increases the draw, but the diam is more important.

[comuscomp]

The actual inner oven dimensions are 58cm (22.84") deep x 65cm (25.6") wide and 44cm (17.32") high.
Which I believe is .377 sq metres (584.7 sq in) or 16.58 cubic metres (7.03 cubic ft).

A 24 inch diam internal area (pi * rad squared) is 452 sq inches for a 5"(125mm) pipe,
A 36 inch diam is 1017 sq inches for a 6"(150mm) pipe.
So it seems like I have to widen the chimney to 6 inches(150mm)from the 110mm I have.

The inner oven door is 23cm high x 40cm wide.

any ideas what the optimum height of a chimney should be?

[downunderdave]

May 19, 2017 14:55:32 GMT comuscomp said:

The actual inner oven dimensions are 58cm (22.84")  deep x 65cm (25.6") wide and 44cm (17.32") high.
Which I believe is .377 sq metres (584.7 sq in) or 16.58 cubic metres (7.03 cubic ft).

A 24 inch diam internal area (pi * rad squared) is 452 sq inches for a 5"(125mm) pipe,
A 36 inch diam is 1017 sq inches for a 6"(150mm) pipe.
So it seems like I have to widen the chimney to 6 inches(150mm)from the 110mm I have.

The inner oven door is 23cm high x 40cm wide.

any ideas what the optimum height of a chimney should be?

I think your calculations are way out. As your oven is a bit taller than a half barrel then 3.142 x 0.325 x 0.325 x 0.58 divide by 2 = 0.096 but add on the thin slice below the half cylinder 0.58 x0.65 x 0.115 = 0.043 
gives you a total volume of 0.096 + 0.043 = 0.139 m3 (4.9 ft3)

yes you will will need a 6" diam pipe for your oven. A taller pipe will give you more draw but it won't really matter, just make it tall enough to get the smoke away from you.

[downunderdave]

IThere's seem to be a few barrel ovens of this design on this site. I'm sure it will function fine, but there are some design flaws.
1. It is taller internally than it is wide. This may create problems when cooking pizza in that the vault is a long way from your pizza. You may find the top not cooking quick enough and will have to raise the pizza up to the roof for 10 secs or so to get the top cooked enough. Neapolitan ovens are the opposite in that their dome is lower in relation to the oven diameter.ie less than a hemisphere. The upside is that you have plenty of height in the doorway to take a large roast or turkey.
2. The vault should be laid on bond, or staggered rather than laid as separate arches. This would increase its strength.
3. The end walls should sit under the vault rather than beside it, to reduce the tendency for the expanding vault to push the end walls out.
4. A barrel vault is inherently unstable unless braced or buttressed. More so if the semicircular arch is sitting on walls, in this case it appears two bricks higher than the semicircle.
There is a reason the dome has been the preferred form for the last couple of thousand years and that's because it's structurally superior.
I'm sure this design will work for many years as the design flaws I refer to come from kiln construction where the expansion and contraction is around double what you'd experience in an oven, but still applicable.

[comuscomp]

so now I learn of the problems. I have deconstructed the front arch and made the chimney bigger and rebuilt and rendered.

now so many questions are leaping out at me. I don't know if there are answers or not. this is what happens when someone is let loose with a cement mixer, a load of bricks, a load of aggregates, a project and a prayer.

1. What are the optimal dimension ratios of a vault?
2. Should the depth be deeper than the width?
3. What is an optimum height for an arch ceiling?
4. I have seen somewhere on this forum that the door should be 63% of the height, is that arch height or ceiling height?
5. Would it be practical to lift the fire brick floor, form another 100mm of vermicrete inside the oven and then re-lay the firebricks to increase the height of the floor to ceiling?

Most of the pictures of vault oven arches shown on here weren't laid on bond and that hasn't been mentioned before(good to know if I re-build), I laid the first 3 layers on bond but not the arches. The oven end walls are formed under the arches so that will give a bit more support.

It isn't taller internally(44cm) than wide or deep (58cc x 65cm) but you are right that the tops of the pizzas could colour a bit more by the time the bases are cooked.

thanks for all the input, I'm learning all the time.

[downunderdave]

1. Optimal would be internal height half the width.
2. Depth can be up to double the width.
3. Optimal height is same as the radius (half width) Taller and you'll have trouble cooking the tops of pizza. Lower and you'll have a low door height and a less stable vault (more sideways thrust).
4. Yes 63% of internal ceiling height give or take 4%
5. Yes a better option than tearing down the existing vault IMO, but 4" vermicrete will be difficult to dry with the oven already over it. Look at calcium silicate board which is already dry but way more expensive.

My apologies when I said your internal height was greater than the width. What I meant was that the height was greater than the radius.

The chimney should be outside of the oven door, not in the internal oven chamber. This makes for better circulation, less heat loss and more economical fuel consumption.

[comuscomp]

Thank you that is very useful information.

My chimney is outside the oven chamber, it forms part of the bar-b-q arch/ smoking area at the front.

as for the cal sil boards, it would be cheaper to buy another layer of fire bricks to attain the height and finance, who must be obeyed, says no.

It works, so we will use it as it stands and improvise where necessary, if it becomes unassailable then I have a plea to finance.

[downunderdave]

May 26, 2017 18:04:21 GMT comuscomp said:

Thank you that is very useful information.

My chimney is outside the oven chamber, it forms part of the bar-b-q arch/ smoking area at the front.

as for the cal sil boards, it would be cheaper to buy another layer of fire bricks to attain the height and finance, who must be obeyed, says no.

It works, so we will use it as it stands and improvise where necessary, if it becomes unassailable then I have a plea to finance.

A second layer of firebrick is another option, but you need to use insulating firebrick which are very lightweight and easy to cut with an old wood saw. If you use dense firebrick your floor will be very thick and take ages to heat up. The floor and arch should be balanced in terms of thermal mass. You may be able to find an old pottery kiln for free and use the insulating bricks from it. In the meantime as you say, just use the oven as it is and you'll be able to assess its performance.

[comuscomp]

Now with a widened chimney and some front doors.