Need urgent advice - dual opening, dual chimney oven - see footprint

[nordic]

We are starting a project this week for a custom built oven that is designed to have 2 individual openings/ Chimneys - see attached image showing planned footprint for the planned layout. I have built an oven before but with a single opening and a clean out hatch at the back but this oven is larger and more complex so before casting everything in concrete ( the footprint of the supporting slab only ) it would be good with some opinions about whether this design will work as planned:

* Dual opening, meaning access from two sides to the oven
* Oven is 3/4 of a circle as it will have to be built around a structural concrete corner on site
* Inside net diameter inside the dome will be 175 cm or 68 inches measured across, with 1/4 of the circle missing.
* It should be possible to place 2 pcs up to 16" pizza's inside one of the cooking areas and up to 3 pcs 10" pizza's in the second cooking area - these can be seen on the attached picture as round brown circles laid out on the floor at the location they would be inside the finished oven
* The plan is to have one large central fire inside the oven that will provide heat for the dome and out to both sides. The area for this fire would be 65cm/ 24-25" in width and around 75cm/ 30" in length and can be seen on the picture as a built up area with firebrick in the middle of the oven
* In the ideal situation there will be installed a clean out tray in in the lineup with the area where the wood is burning for easier clean out. This hatch will be fully closed when the oven is in use and only minimum heat loss through this area. This is the third opening in the oven as seen on the picture, placed in between the 2 regular openings used for cooking.
* The plan is for the 2 cooking entrances to be without a hatch so that the openings are providing easy access and direct view to the flames inside. It will however be built in such a way that 2 hatches ( similar to the one seen placed at the clean out hatch ) can be installed later in case we encounter problems heating up the oven to a high enough temperature due to heat loss through 2 unsealed hatches - this is planned like this so that the oven can be saved without breaking it down and rebuilding it if the design fails.
* The oven should ideally be able to bake a pizza in 3-8 minutes depending on how high a temperature we heat it to, if super hot and busy we will have 2 staff, one at each opening, if less busy we may just cook on one side with one staff
* There will be 2 chimneys on the oven ( placed just behind the 2 openings ) with an approximate inner pipe diameter of 150mm/ 6" - these 2 chimneys will merge into one above the oven and lead out of the building in a larger diameter pipe after the merge. On the picture these chimneys are placed standing on the arches, they will be located right after the arch in the final built oven so roughly 150mm/ 6" further back than now.

I am looking for specific advice as to whether you believe, ideally based on a fair bit of experience, that the design will work, be able to reach high cooking temperatures and whether the 2 chimney, 2 opening hatches will work in terms of air flow, heat loss etc.

The layout shown is done on the floor in the area where the oven will be but all this will be raised up onto a slab poured 30" higher, need to make final decisions about the size, shape and locations for hatches and clean outs over the next few days.

Advice appreciated as crucial decisions will have to be made this week. Please be constructive - will follow up as project will progress - got 6 weeks to targeted first use and hope to post pictures of the oven in use by mid May.

[SableSprings]

Welcome to the forum Nordic! In looking at your picture and reading your overview, I have a couple of initial concerns and thoughts on this...

1) The strength of a dome is based on a complete dome. I'm not convinced that this partial dome will be structurally sound. You will have some significant forces pressing outward where there is no buttressing.

2) You need to plan on at least 10 cm of insulation between the bricks and the concrete pillar. Concrete starts degrading at below pizza temps. Studies have shown that at pizza temps, concrete will degrade. Heating at the very least will cause cracking which will expose the internal metal rebar to the air and result in rust & metal fatigue. You'll need to adjust your dry layout to accommodate mortar seams and spacing for that insulation (especially against that pillar!) In addition, you need to plan for some sort of external covering for the insulated cooking chamber(s). I don't think you want to tie to that pillar, so you need space not only for an outside render but space to apply it from...

3) You want the cooking floor to be heated to pizza temps, but with this design I don't see the cooking floor coming up to temp in a reasonable amount of time. The heat will flow primarily along the top of the dome and out the front opening(s). Floor heating will be very slow as will recovery time after cooking a pizza. Also, those far corners are going to heat much slower than the front part of the oven(s), so you'll probably see some real stresses applied by uneven expansion/contraction...especially to a partial dome structure.

4) You mention that during use, the firing hatch will be fully closed. You need the fire to burn hot and efficiently. I believe by having the center hatch closed the fire will not have enough oxygen flow to burn well (or at all). The beauty of the Pompeii oven design is that air is brought in low, feeds the fire, and then rises & returns back over the flames. This return over the top of the fire creates a secondary burn, basically a very complete burn of the wood off-gassing.

5) I've seen a large oven done on the East Coast (USA) that was a half-barrel (vault) design with two doors. The vault gave the entire oven the structural strength it needed and each door had an installed chimney above it. That way either side (or both) could be fired up as needed. They could use the system to produce pizza or bread or both based on simply how they fired and opened/closed each side of the cooking chamber.

6) Just to be clear, your chimneys will be placed outside the chamber arches so the each oven can be separated from the chimney exit.

7) I'm sure you've read about this, but for what you are attempting, the more insulation under the floor and over the oven you have...the more efficient and successful your oven will be maximizing heat storage.

I hope some of these concerns/ideas help you in this build. I realize that a lot of these points seem very negative, but that is not my intent...just my experience. I really hope you'll continue to keep us informed on your decisions and build progress.

[nordic]

Hi Mike, - thanks for the advice and your quick reply, really appreciated. This is a project in progress and I will try to work all improvements possible into the final design as this evolves. Just to clear up a few things: The 2 main entrances will be open during cooking so airflow in should be just fine. The 2 chimneys will be placed low on the dome height just behind the 2 entrances.

The oven will have a 4" thick insulated floor above a 4" concrete slab ( 2 inch fireboard and 2 inch firebrick ). The brick thickness of the dome will be 4 1/2" with 2 inch fire mat insulation, 2-3" of perlite plastered insulation above that.

If we encounter a problem reaching high enough temperatures in the oven, the 2 entrances are designed in such a way that retrofitting 2 glass hatches will be easy - these can then be left open or closed to control the balance between air in and heat loss out.

The structural concrete corner behind the oven will also be insulated with 2" fireboard and 2" firebrick ( total 4" ) in order to protect the concrete so I hope this rules out any chances of damage to the concrete The concrete wall is 8" thick and solid.

When continuing to work on the design today my own concerns arising was focusing on the possible airflow inside the oven and how the 2 chimneys would possibly effect that, I am trying to see if we can work in an adjustable port in the bottom of the chimney that will allow for setting the opening size. I forgot to mention in the first posting that the 2 chimneys are planned to merge into one larger vertical chimney out of the building, just working out the details for this over the next few days.

As for the heating of the oven I am expecting a period of up to 2 hours to heat the oven to a good working temperature. Due to the fact that the oven is designed to bake 2-5 pizzas at the same time we are happy with a baking time of 3-6 minutes which should allow for a slightly lower operating temperature, a more crispy crust and time to stay calm when having several pizzas in the oven at the same time. In my experience it is however more stressful working with an oven with low range/ slow baking temperatures than with an oven that is properly heated so I am hoping for being able to achieve a 3-6 minute baking period.

Not sure if any of this information changes some of your feedback Mike, I will try to see tomorrow how we can possibly reinforce the oven structurally to help absorb stresses due to the fact that 1/4 of the complete dome is being replaced by the corner wall.

Will post an update as this progresses, hopefully the oven will end up working within the design parameters with the help of some feedback.

Thanks!

[SableSprings]

I too, am struggling with how the airflow is going to work. From what I understand, you intend for incoming air to flow from one (or both) working openings to the central firebox. I think cooler air will be flowing across the cooking floor on route to the fire and heated gases flowing out along the dome towards the front chimney(s). It seems that this system would not heat that floor very well and with both doors open you would get a very turbulent fire with no clear direction so to speak. I am glad to read that you are not attempting to reach +700F temps on the cooking floor. Most WFO in restaurants I've visited stay in the 500-600 range to allow remaining "calm" working the pizzas and are happy with the slightly longer cook time.

Just to be clear, you mention insulating the corner concrete pillar with 2" fireboard and 2" firebrick. I'm assuming you are clear that the firebrick is not an insulator. Did you mean that outside the oven chamber bricks would be a 2" fireboard and an exterior shell of firebrick? I'm a little confused on that point since you note covering the oven brick with fire mat insulation and an added perlite layer. Thanks for any clarification, hopefully I just am not understanding all the details clearly at this point.

[nordic]

As for the airflow my logic would say that a larger fire requires more air to burn/ combust, the fire needs to be in proportion to the oven mass/ volume to be heated. A larger oven will require more heat and air to heat up but the final amount can be reduced by an energy efficient design. Obviously having 2 openings leaves one additional point of escape for heat and may slow down heating up the oven and limit the maximum achievable temperature in the dome. Hope not having to install the hinged door in the openings but will do if we have to.

As for the internal airflow, yes, this would most likely be sucked in and travel above the floor to the central fire pit, but I guess this is the case in any oven and that the amount of air needed is directly proportional to the amount of wood you burn/ amount of heat you are trying to generate so not sure if this will necessarily effect the ovens performance. I am more worried about the potential heat loss out through the 2 openings.

The insulation towards the solid concrete wall should be OK and according to code here but would be able to increase the thickness of the fireboard if needed. But assume the firebricks have significant different temperatures on the side facing the oven and the side facing away so the exposure to the fireboard would be greatly reduced anyway. Not sure.

The build up of the dome would be 115mm / 4 3/4" firebrick, 2 " firemat and then 2-3" perculite plaster, possibly followed by a 1-2" final plastering that will be painted.

Fully aware that deviating from traditional and well proven designs comes at a risk so have to apply good judgement here. Will try to see what further parameters can be improved over the next few days.

A totally different concern is the internal dome height, the inside net diameter of the oven is 175 cm/ 68" - I am planning a dome with the initial 8" of the outer walls being vertical after which the dome will gradually rise up to a highest point of 80 cm/ 30-31" above the floor, a proportion of around 45% dome height to the internal diameter, does this sound reasonable? If the dome was to be completely spherical I guess this should be 34-35".

Know this is lots of details to digest but as the design parameters has to be locked down within the next few days any advice is appreciated. The supporting slab will be poured early next week and after that I will have a few weeks to finalize last details during the actual building of the oven but as parts takes time to source and import the planning needs to be a step ahead of the execution :-)

[Gulf]

On this forum was a build called Pompeii Oven with two openings. I seem to remember it being more well documented with pics than it appears today. However, like many of the builds that have deviated from the Pompeii design, there is not much feed back on how well it performed. Piet Pompeii was the builder and sadly his profile has disappeared also.

[SableSprings]

... A totally different concern is the internal dome height, the inside net diameter of the oven is 175 cm/ 68" - I am planning a dome with the initial 8" of the outer walls being vertical after which the dome will gradually rise up to a highest point of 80 cm/ 30-31" above the floor, a proportion of around 45% dome height to the internal diameter, does this sound reasonable? If the dome was to be completely spherical I guess this should be 34-35"...

I've not seen this perimeter to height ratio used in design nearly as much as the door opening height to internal dome height. From your data of a inner dome height of 80 cm (30"-31"), your door opening would need to be 50 cm (~19") high to meet the normal target ratio of 63% ... are you incorporating this into your plan? The 63% is certainly not absolute, it simply is a "target number" that helps make sure that the flow of air in and heat out is fairly smooth. Lots of ovens are above or below that 63% magic number and work just fine...just making sure you are thinking about it in your plans. That draw (breathing) of the oven certainly defines how well the oven performs and heats.

Dome designs often vary based on purpose. James did a good overview of the differences between the Naples (low dome) vs Tuscan dome (higher dome) and it might be worth your time to look at that thread (link below)

https://community.fornobravo.com/for...signs#post1426

Since you are building a fairly unique design, hopefully you will keep posting as you build and share what you learn (as Joe noted above, the previous build with two openings didn't continue posting with much performance info but did seem to be a success--albeit with different goals than your build).

[david s]

A different solution comes to mind that you might consider and it is to have two smaller ovens rather than a single larger one. In Vicodin Aquense on the Amalfi coast in Italy there is a pizzeria known as “the university of pizza” They have three pizza ovens and sell their pizza by the metre.
The advantages of multiple ovens are
1. You can tailor oven fuel consumption to volume of traffic.Eg on quiet nights you only operate one oven.
2. Allows each pizza cook to control his/her individual oven.
3. Sticking with tried and true design eliminates the possibility of design failure.

[nordic]

Based on feedback and some other concerns I had about the layout of the oven we took another look at options today and came up with a small change that will allow for using the oven in a more flexible way.
By moving one of the openings across so that the 2 openings now are symmetrical and 90 degress offset from the corner, it will be possible to fire up the oven in one of two ways, either one large central fire or 2 separate corner fires that burn at the back of the oven. I believe this second layout will make it easier to work the oven and by having both options I am now pretty confident this will work just fine. So the oven will have 2 openings and in between then there will be a sealed third clean out opening with a glass hatch. Inside the oven, just behind the glass hatch, we will position a hole in the elevated slab for cleaning out ashes - this hole will lead into a collection tray mounted under the slab. The cleanout hole will be closed with a flat steel plate hatch when using the oven to prevent coal from dropping through this hole until the hatch is manually opened. The flooring below the cleanout hatch will have a slate cover and unable to burn for safety.

Due to the changed location of the 2 hatches it now works better connecting them by adding a bit to the slab in front and turning this area into a table.

Attaching 2 images showing the 2 different ways the oven can be fired up.

We have now rigged down the initial mock up used for deciding on the parameters and will be working towards pouring the slab middle of next week, hope to have oven ready for first test by late april/ early May.

I am sure more issues will come us during the actual building and will post here for advice. Thanks a lot for the initial input, it has been valuable in taking a second look at some of the solutions initially thought of.

[david s]

Your plan does not indicate the location of the chimneys and why the need for two? A hood over both entries with a single flue would be easier and cheaper.
Two entries will certainly reduce an oven’s efficiency. Why then do you require a third for a clean out when you already have two openings?
you must insulate between the inner bricks and that back concrete wall.

[nordic]

Hi David, - the 2 chimneys are planned located immediately behind the arc seen on the images, roughly 10 cm further back than where they are now. As you say, an alternative could be to build the oven with just one exit chimney ( the 2 chimneys planned will merge into one chimney above the oven and exit the building from there ) but I feel it will be better to have an escape route for smoke and gases at a low point in the dome just above each entrance to reduce chances of having smoke coming into the room. This oven will be inside a building.

As for the third opening - I really prefer now having to clear out all ashes through the cooking openings and across the outside table plate in front here. By making a drawer under the oven and a small internal hatch we can greatly simplify cleaning the oven and reduce the amount of dust coming into the room. The added benefit is that the cleanout hatch will allow for viewing the fire from the main lobby area so even though the main function of the hatch is for cleanout it will add some ambience as well.

I got around 2 weeks to decide whether to remove this hatch as it will be easy to just build the dome across where it should be when we get that far, will take another look at the chimney design early next week and work out how this ties into the main chimney coming above the oven.

[SableSprings]

With the size of this oven, I'm thinking that your 2 - 6" chimney pipes might not be able to do the job properly. This oven size would normally be using an 8" pipe. Be aware that when you start a fire you will definitely want to preheat the chimney(s) to get flow started up & out. I have visited several restaurants with wood fired ovens going and most of them have incorporated a hood with an exhaust fan to ensure no smoke comes into the dining area.

Most of these flue fans are located farther up the stack to reduce/eliminate any noise. Having a powered exhaust system would seem to me to be a wise and worthwhile investment. David's comment about a single hood sweeping above the three openings and collecting smoke/off-gases into a single flue would also certainly be worth considering.

I have an ash slot and ash collection bin for my oven and agree totally in its value. I love being able to pull ash into my bin just before a party and empty it the next day or later...

Both David & I have mentioned the need for insulation along the concrete wall. I know you have acknowledged that it will be done...it's just that the dry brick layout in your pictures continue not to show the insulation/spacing as it will be when completed...makes me a little concerned.

Anyway, looking forward to this build as it is certainly going to incorporate lots of new ideas & solutions.

[SableSprings]

Nordic, how's the plan coming along? There was a recent post regarding restaurants and the use of hoods & powered venting/exhausts that I thought you might find interesting. The original thread is older, but recently there was an addition update that might be helpful for you. Here's the link:

https://community.fornobravo.com/for...438#post403438