How long to preheat a Pompeii pizza oven?

[Nabukodonosor]

Hey guys, I hope you're all doing fine. I have a question. I am about to build my first Pompeii pizza oven. I was just wondering...how long do I need to preheat it before I can insert a pizza in it? I've read somewhere from a guy that owns one of these that it takes 4-5 hours to preheat it. Also, how much wood do I need? I'm just doing some calculations.

[Neil.B]

I'm new to this but believe it depends on the size of your oven, thickness of your bricks (thermal mass) and insulation below cooking floor and around oven.
I think it takes about 2 hours for a 36"-42" oven with 4.5" walls... but could be wrong.

I'm building a 32" oven with 4" walls and 4.5" insulation, so i think it will heat up quicker.

[MarkJerling]

I've found that, while I can heat the oven in 2 hours, it heats more effectively if I heat it for 4 hours with a smaller fire than at 2 hours with a large fire.

[david s]

That is because the firebricks are not highly conductive. Being slow to heat they’re also slow to cool making heat retention a useful characteristic.

[MarkJerling]

That is because the firebricks are not highly conductive. Being slow to heat they’re also slow to cool making heat retention a useful characteristic.

Very true. Here's a link to my most recent temperature profile:

[danhem]

I've found that, while I can heat the oven in 2 hours, it heats more effectively if I heat it for 4 hours with a smaller fire than at 2 hours with a large fire.

When you say ‘smaller fire’, how long would you say it takes for the dome to clear?

working on fire management at the moment and whilst I can get to pizza temp in a couple of hours, I can feel that the oven isn’t fully saturated and that i’d struggle to throw out any more than 6 pizzas. My dome clears after 90 mins.

I’m not so worried about heat sink - I see zero spike in temperature through the hearth floor and dome.

I guess my real concern is the oven floor. After a strong fire I’ll let the embers sit on the floor for 20 mins or so. After clearing the floor for pizza space, the floor temp drops from 850f to 650f pretty quickly. In an ideal world I’d like to maintain the floor at 750f for a sustained period.

I have 2.5” ceramic fiber board under the floor bricks and as said above, it doesn’t register an external heat sink until the following morning (where I see the temp spike by around 30f).

in this case I can only conclude that I haven’t saturated the floor with enough heat. I have been concerned about damp insulation but I’m around 20 full blown fires in now so assume that any damp has been driven out.

[MarkJerling]

When you say ‘smaller fire’, how long would you say it takes for the dome to clear?

working on fire management at the moment and whilst I can get to pizza temp in a couple of hours, I can feel that the oven isn’t fully saturated and that i’d struggle to throw out any more than 6 pizzas. My dome clears after 90 mins.

I’m not so worried about heat sink - I see zero spike in temperature through the hearth floor and dome.

I guess my real concern is the oven floor. After a strong fire I’ll let the embers sit on the floor for 20 mins or so. After clearing the floor for pizza space, the floor temp drops from 850f to 650f pretty quickly. In an ideal world I’d like to maintain the floor at 750f for a sustained period.

I have 2.5” ceramic fiber board under the floor bricks and as said above, it doesn’t register an external heat sink until the following morning (where I see the temp spike by around 30f).

in this case I can only conclude that I haven’t saturated the floor with enough heat. I have been concerned about damp insulation but I’m around 20 full blown fires in now so assume that any damp has been driven out.

I'd say the dome clears in about 2 hours, but my oven needs more time to fully saturate the dome. Once that happens, the dome holds the heat well. I find that my floor cools too quickly for my liking and I suspect that's because I used 45mm (1.75") thick fire bricks for the floor. I could pull them out (they're set on sand) and add another layer underneath but I think I'll just keep it as it is for now so as to get a better idea about the temperature profile.

If I try to heat the oven with a big fire over a short period, then the dome gets to about 950F and the floor gets to 800F but drops off pretty quickly. In one hour, the dome is at 900F and the floor at 730F. That's fine for my use, but after that, the dome drops to 700F after 2 hours and the floor to 580F, which is fine for the dome, but a bit cool for the floor in my view.

With a longer but somewhat smaller fire, I get the dome to around 1050F and the floor to 960F in just under 4 hours. I've not yet tested to see what would happen if a build a bigger fire for 4 hours, or built a fire for longer. An hour after raking the coals and clearing the floor the dome drops to 910F and the floor to 730F, which is almost identical to the shorter heating period, but after that, at the 2 hour mark, the dome is still 890F and the floor 650F. So, on that basis, it seems unlikely that I'll be able to keep the floor hotter, unless I add more thermal mass, but time and practice will tell!

But, at least the insulation seems to be working because at the 3 hour mark, the dome is still 825F and the floor 625F. After 4 hours, the dome is at 716F and the floor at 618F. Because my oven's floor is 42" diameter, I can cook 3 large pizzas easily at one time, but I find it's most practical to only do two at a time, adding one to the rear after rotating the other and moving it to the side. I have not yet tried to do more than 12 pizzas directly after each other so it's hard to say how much heat is lost each time I add a fresh pizza. Also, I've found that adding a small log to the coals from time to time helps.

[danhem]

Ok thanks for the info, very interesting.

I think like you, my issue is in heating up the floor sufficient to saturate. I’m pretty happy with how the oven retains heat for 48 hours after a pizza cook - with the door in overnight I can bake bread the following morning, roast meat during the afternoon and cook ribs the following day after that.

my floor bricks are 2.5’’, during a fire zero heat comes through the hearth - however the following morning heat through the hearth spikes/drains by around 30f and this has been constant for some time now. I find it strange that during the entirety of a fire the underfloor records zero heat drain, yet overnight (woth oven door tightly fitted) heat begins to drain through the bricks, ceramic fiber board and concrete.

id be really quite pleased with my oven’s performance if I could figure the floor issue out. I’m hoping your longer and slower fire method is the key to a better floor performance.

[MarkJerling]

Ok thanks for the info, very interesting.

I think like you, my issue is in heating up the floor sufficient to saturate. I’m pretty happy with how the oven retains heat for 48 hours after a pizza cook - with the door in overnight I can bake bread the following morning, roast meat during the afternoon and cook ribs the following day after that.

my floor bricks are 2.5’’, during a fire zero heat comes through the hearth - however the following morning heat through the hearth spikes/drains by around 30f and this has been constant for some time now. I find it strange that during the entirety of a fire the underfloor records zero heat drain, yet overnight (woth oven door tightly fitted) heat begins to drain through the bricks, ceramic fiber board and concrete.

id be really quite pleased with my oven’s performance if I could figure the floor issue out. I’m hoping your longer and slower fire method is the key to a better floor performance.

I'm finding heat retention, with the door fitted, is very good. I fired it up on Friday and it took until Tuesday midday for the temperature to drop back to ambient. I've also checked my floor slab and that does not heat up at all, so I'm confident that my underfloor insulation is working well. I'll try a longer fire next time and see what happens. I wish I had thought of putting a heat sensor under the floor!

Here's the revised heat profile with the door fitted:

[mefornaio]

Ok thanks for the info, very interesting.

I think like you, my issue is in heating up the floor sufficient to saturate. I’m pretty happy with how the oven retains heat for 48 hours after a pizza cook - with the door in overnight I can bake bread the following morning, roast meat during the afternoon and cook ribs the following day after that.

my floor bricks are 2.5’’, during a fire zero heat comes through the hearth - however the following morning heat through the hearth spikes/drains by around 30f and this has been constant for some time now. I find it strange that during the entirety of a fire the underfloor records zero heat drain, yet overnight (woth oven door tightly fitted) heat begins to drain through the bricks, ceramic fiber board and concrete.

id be really quite pleased with my oven’s performance if I could figure the floor issue out. I’m hoping your longer and slower fire method is the key to a better floor performance.

You don't mention your current firing method but if you are not seeing any hearth absorption something is wrong. A 200° drop in how much time? Your hearth bricks should be saturated at the end of the fire up or at least 75%

Raking the coals over the hearth is an excellent practice to equalize the floor and achieve a deep charge. Your might need to extend the soak or perhaps increase the footprint of the fire rather than its volume, if that makes sense. What I mean by foot print is have as much floor real estate in contact with embers during your firing process and not just a fire say in the middle or off to the side.

As I charge my mobile oven 36" at the half way point my entire hearth is covered and remains that way till near the end. The inherent problem with operating one of these oven on a part time basis is the multitude of variables. Prior to the pandemic we were in operation for 4 days a week and even at this interval for over 5 years now the oven has its own ways.

Moisture in the board is EXTREMELY difficult to drive out. I would suggest doing a series of fires over about a weeks time and installing the door after each one. Before I put trim on my deck we had some water infiltration and I swear I could feel its presents even at 4 days a week for a good 2 weeks.

A longer slower burn to a point always will give you a deeper and more even charge to the oven overall. There is a balance unique to YOUR individual oven you just need to find by usage.

[danhem]

I think I need to burn a fire longer. I have been basically setting a stack alight and adding logs every 10/15 mins until the carbon clears. I think my longest fire has been 3 hours, which so far has delivered a heat profile suitable for 4-6 Pizzas. Sealed over night with a door from there the oven does retain heat quite well.

we have 20 friends coming next week so I’ll plan for a longer fire and hope that the floor soaks up more of the heat.

if building the oven was a learning curve l, the actual fire and heat management is also something that needs time to master.

Im not 100% happy with the wood I’m using for fuel. Seems that quality hardwood available in the West isn’t available here. All the pizza restaurants use a variety of Pine. It’s a hard wood with no sap residue. My source of wood is dry but it burns through very quickly so I’m not sure how much heat it is actually giving off, but at I guess I need to work with Whats available here in Thailand.

[MarkJerling]

I think I need to burn a fire longer. I have been basically setting a stack alight and adding logs every 10/15 mins until the carbon clears. I think my longest fire has been 3 hours, which so far has delivered a heat profile suitable for 4-6 Pizzas. Sealed over night with a door from there the oven does retain heat quite well.

we have 20 friends coming next week so I’ll plan for a longer fire and hope that the floor soaks up more of the heat.

if building the oven was a learning curve l, the actual fire and heat management is also something that needs time to master.

Im not 100% happy with the wood I’m using for fuel. Seems that quality hardwood available in the West isn’t available here. All the pizza restaurants use a variety of Pine. It’s a hard wood with no sap residue. My source of wood is dry but it burns through very quickly so I’m not sure how much heat it is actually giving off, but at I guess I need to work with Whats available here in Thailand.

Can you get mai khaen? I understand that should work well.

[mefornaio]

You already said it.

"if building the oven was a learning curve l, the actual fire and heat management is also something that needs time to master."

I can assure you it's not the wood. As a general rule we use oak and citrus or Almond and oak or citrus for our mobile. For a period of about 6 monts last year I was running exclusively the crap from the grocery store (some kind of nasty pine mix) Sure I had issues with it but honestly once you get to know your oven through repeated and frequent usage you'll get the hang of it. There really is no substitute for this method.

Just keep at it it will come.

[danhem]

Can you get mai khaen? I understand that should work well.

The wife seems to think that it’s an expensive wood. Seems that there may be a wood called mai khaen hua that would be more affordable for burning. I’ll ask my supplier if she can get some.

thanks for the tip

[danhem]

You already said it.

"if building the oven was a learning curve l, the actual fire and heat management is also something that needs time to master."

I can assure you it's not the wood. As a general rule we use oak and citrus or Almond and oak or citrus for our mobile. For a period of about 6 monts last year I was running exclusively the crap from the grocery store (some kind of nasty pine mix) Sure I had issues with it but honestly once you get to know your oven through repeated and frequent usage you'll get the hang of it. There really is no substitute for this method.

Just keep at it it will come.

Appreciate the motivational speech. This weekend will be a real test of the oven. A long slow fire is planned so I’m pretty confident that the oven will perform.

cheers.