pizza oven heat retention

[taiwanpizzaman]

Success!

Thanks James and all you other guys. After reading about firing the pizza ovens and noticing that ALL the walls should be white-hot I was like So this weekend I made an "Agressive" fire. I stacked wood at the back of the oven, the two sides and then the middle-front. The middle fire was lit first and after an hour the two side fires. After two hours the back still hadn't lit up and it looked like oxygen starvation, so I parted the fires and ended up with a blazing inferno

We baked ten pizzas and five loaves of bread (without the door!). When I put the door in the oven stayed hotter than 200 degrees centigrade for more than an hour! If i knew it was going to stay hot for so long I would have planned a couple of roasts as well! Cheers

Colin

[taiwanpizzaman]

Re: pizza oven heat retention

Thanks guys, I think it might be the fires. I'll experiment with bigger fires and see how it goes. Thanks to all of you out there for your input, it's a great forum and I appreciate all the help. PS: Next time I'm building a round oven!

[CanuckJim]

Re: pizza oven heat retention

Colin,

Mine is also a barrel vault design, built deliberately with high mass for multiple bakes. My experience is that ovens with this design require firing that is not necessarily hotter, but longer in duration. This gives the mass that you do have time to heat up. If you don't fire longer, the mass will wick heat away from the cooking surface. As an example, if I want to bake bread at 550 F, the mass below the floor should be in the 400 F range. Otherwise, the temp drops off too quickly.

Jim

[james]

Re: pizza oven heat retention

Hey Colin,

I have moved this thread to Heat Management. Seems like a good fit.

If I were to summarize (in inches):

The floor is 33"d x 25"w
The dome is 17.6"h
The dome insulation is 4.5"
There is 4" sand under the cooking floor
The oven opening is 11" high
The floor is a single firebrick (2.5" or 4.5"?)
The dome is a single red clay (4.5" or 8"?)

Hmmm. I think this oven should cook better than it is. I can't see anything out of whack. Among the things we know:

1. The dome isn't too high.
2. The opening isn't too big.
3. The floor isn't too thick.
4. You can definitely fire and cook with a single layer of brick.

I can only think of four possible things.

1. The baked stone isn't really an insulator, and it is wicking heat.
2. Is there a lot of concrete around the dome that could be wicking heat?
3. There is still a lot of moisture left in the oven (very possible).
4. You aren't building big enough fires (also possible).

If the problem is either 3 or 4, keep burning better fires to see if the oven starts coming up to heat faster and holding heat better.
James

[taiwanpizzaman]

Re: pizza oven heat retention

I use mostly logs that I cut into fairly thin strips. I wouldn't know what wood it is. It is not very soft, so it burns slowly in the beginning, but once firing burns quite nicely. It doesn't burn very long though. If I keep a very big fire going, the pizza oven works very well, we cooked more than thirty pizzas last weekend, but you have to keep piling on the wood to keep it hot enough. I'm looking for a way to make this oven more efficient, perhaps reducing the internal size.

[spareRib]

Re: pizza oven heat retention

Everything I've read suggests that a single layer of red brick should be sufficient. Likewise, with 10cm of concrete/vermiculite, you shouldn't have a mass problem and black bricks should absorb a fair bit of heat. How big of a fire are you building, what materials are you burning and how long does it take to burn down?

[taiwanpizzaman]

Re: pizza oven heat retention

Hi guys

The picture was only to show how the barrel was formed during building and the insulating material on top. After that the front section, chimney and door was built at 63% of the dome height. More insulating material was added to the front and then the entire dome was covered with 10cm concrete. I used that picture because I want to know if that single layer of brick is enough to retain the heat in the pizza oven.

Thanks
Colin

[spareRib]

Re: pizza oven heat retention

Indeed, it looks like the door is the same height/width as the cooking chamber and quite shallow compared to the size of the opening? You might want to compare with the dimensions of Alan Scott's designs.

[RTflorida]

Re: pizza oven heat retention

I'm not too familiar with the barrel vault concept, but, based on the picture - could the entry be too large??? Looks quite large in relation to your internal measurements. This could be the source of drawing too much cool air. I would also think you would need a well sealing, insulated door to have a chance at retaining much heat.

RT

[taiwanpizzaman]

Re: pizza oven heat retention

Hi guys

Thanks for your input. I've used a door with a thermometer in it. The initial temperature with only the bed of coals and ashes goes up to 160 degrees celcius and then quiclkly drops to 140 degrees in about twenty minutes. Over the next two hours it drops to about a 100 degrees which it holds for five hours. After about twelve hours the temperature is between seventy and eighty degrees.

The hearth base consists of 10cm concrete, 10cm building sand and then black firebricks. These firebricks are used for the temple furnaces and are rated to 1000 degrees celcius. They hold the heat extremely well, they remain warm eighteen hours after firing. I've included some pics.

[maver]

Re: pizza oven heat retention

before you add insulation or another layer of firebricks (which would increase the mass of the floor - possibly aggravating the problem) try a draft door to verify whether your concern about drawing too much cold air is an issue. By your dimensions I presume this is a barrel oven as well. Could you be more specific about the insulation under the hearth and what the order of layers in your dome is? If you do add a firebrick layer to the hearth you should also include a layer of insulating board under that. Given the cost of firebrick you might take as much of your existing hearth out, place in a layer of insulating board and then brick back on top. If you have to adjust the height further then use ordinary red birck under the insulation. My 2 cents.

[Xabia Jim]

Re: pizza oven heat retention

You could try your friend's suggestion using a temporary set of firebricks on the hearth....might be worth the try. If that helps, you could make it permanent.

It's a bit of a puzzle though....why would the red bricks not hold the heat? ....using a lot of wood should build up the thermal mass in the masonry.

It can't hurt to put an additonal layer of insulation around your oven. What about a door and chimney cover to keep the heat in longer for bread?

Maybe post a few pictures....

Good luck....Jim

[taiwanpizzaman]

Re: pizza oven heat retention

The oven is insulated with a 12cm coating of baked stone (similar to vermiculite), portland cement and lime. On top of that i have another 10cm of concrete cladding. The initial firing was done with gas and after 26 hours of continuous firing the top of the oven was barely warm. No matter how much I fire it, the top never gets hot and has never cracked, that's why I think it must be the single layer red brick.

[dmun]

Re: pizza oven heat retention

I suspect an insulation problem, rather than a masonry mass deficiency. Tell us how your oven is insulated

[taiwanpizzaman]

Hi guys, great site!

I finished my pizza oven about a month ago. The inner dimensions are 85cm x 65cm. The dome height is 45cm. I built the oven based on info I gathered on the internet. I used firebricks for the base, but used normal redbricks for the dome. The reason was financial, the firebricks here in Taiwan are thirty times more expensive than housebricks, no kidding! The oven works really well, but it needs an enormous amount of wood to keep it going for pizzas and it doesn't retain heat for baking bread.

A friend of mine here builds kilns and he suggested raising the oven floor with another layer of firebricks to reduce the dome height. The oven chimney is in front so it looks like the cool air sucked in at the bottom is cooling the oven too much. I was thinking about using fireclay and mortar to build another inner dome (with difficulty), to increase the oven's heat retention. Any suggestions will be appreciated.

Colin