Heat break. To do or not to do

[Toomulla]

I accept some of what is said, however if a 6 mm gap is effective in stopping heat we may as well just do away with the several inches of insulation and have a 6 mm air gap between the brisk and some waterproof cover. I have yet to see a working example proves the concept

[Gulf]

That is a pretty extreme example. Sort of like comparing apples and oranges. The surface area of a dome is many times the very small area that comprises where the face of an inner arch contacts the outer arch. No one has said that the gap "is effective in stopping" heat transfer. But, my tests show that it does slow it down. Granted, there will never be a proof of concept until someone builds twin ovens, one with heat breaks and one without. I seriously doubt that happening anytime soon.

My advice for "To do or not to do" is to evaluate the main purposes for the oven and the availability and price of the fuel. If the purpose for the oven is for mostly pizza, don't include it. If the purpose for the oven is for extended bakes IE bread, roasts etc.you may want to consider heat breaks. If the oven will be used for extended bakes and wood is at a premium, you definitely want to consider including them imo.

just sayin'

[fnbroken]

I for one intend to include a heat break. Better off having and not needing, than needing and not having. I want to do slow roasts

That said, could someone recommend a suitable product - and maybe even if there anyone in the US willing to ship me some thermal silicone to cover the rope i will stuff in the gap?

[deejayoh]

I for one intend to include a heat break. Better off having and not needing, than needing and not having. I want to do slow roasts

That said, could someone recommend a suitable product - and maybe even if there anyone in the US willing to ship me some thermal silicone to cover the rope i will stuff in the gap?

I used Kaowool Moldable Caulk. Has been in there a few years, and performed solidly. Looks like site below does international orders?

https://www.sheffield-pottery.com/Hi...wool-p/tcm.htm

[deejayoh]

I accept some of what is said, however if a 6 mm gap is effective in stopping heat we may as well just do away with the several inches of insulation and have a 6 mm air gap between the brisk and some waterproof cover. I have yet to see a working example proves the concept

I don't see the logic in that argument. Air is a better insulator than brick; Ceramic Fiber is a better insulator than air. I think we all agree on that? So a 4 inch CF heat gap would probably perform better than a 6mm heat gap, but that doesn't mean the 6mm heat gap does nothing. Personally, I see dramatic drops between my inner arch and the other side of the heat gap. Others have said the same

[JRPizza]

.... I have yet to see a working example proves the concept

I have an idea - those with and without a heat break could post some temperatures, possibly morning after a firing, of the inner arch and the bricks immediately adjacent to the arch, such as sides, top and bottom of tho opening (12, 3 or 9, 6 o'clock), to see what kind of gradient there is across that interface. Since the vent area is not insulated (usually) and open to ambient air I would expect there to always be a temperature differential regardless of configuration, but if the break has any benefit it should show up in the data. It might also be useful to give size of gap and what's filling it for those with a heat break.
I probably won't fire the oven again till this weekend, but will post my temps if others think this is a good idea.

[david s]

I have an idea - those with and without a heat break could post some temperatures, possibly morning after a firing, of the inner arch and the bricks immediately adjacent to the arch, such as sides, top and bottom of tho opening (12, 3 or 9, 6 o'clock), to see what kind of gradient there is across that interface. Since the vent area is not insulated (usually) and open to ambient air I would expect there to always be a temperature differential regardless of configuration, but if the break has any benefit it should show up in the data. It might also be useful to give size of gap and what's filling it for those with a heat break.
I probably won't fire the oven again till this weekend, but will post my temps if others think this is a good idea.

It’s worth a try, but I think there are too many variables to make comparisons.
1 oven size
2 how well insulated the oven is, both over and under
3 is flue gallery insulated, how much
4 insulated or non insulated door and how well sealed
5 ambient temperature

in my build, because I have made my cast flue extremely light it does not act as a strong heat sink. It is also well insulated. I have my heat break/expansion joint between the flue gallery and the outer decorative arch. This way the dome and flue gallery are both free to expand within the outer shell and decorative arch, being disconnected from them therefore not creating expansion stresses.

[JRPizza]

True, but I was taught if you torture data enough it will tell you anything you want to know
Seriously, I agree that there are a bunch of variables, but if we plot something like temperature at arch vs difference on each side for the two configurations we might see a trend. Trends are probably the most we would be able to discern, but people are always saying "show me the data" so some might be better than none.

[Toomulla]

Let me say I have no issue with the installation of a heat break for those convinced that it has a value. As the Devil's Advocate I am raising that I have seen no empirical evidence of its value,oven builders can satisfy themselves as to its value, my view is that the bricks abutting the vent area seldom reach a saturated state at the very high pizza temperatures and the temperature gradient is already dropping across those bricks, the major heat loss in my oven is the insulated door.

[JRPizza]

No problem. I was hoping that if you didn't have a break you could contribute some data from one of your firings. There are really only three possible outcomes - breaks help, don't help, or there is so much noise in the data a conclusion can't be drawn.
I too have a metal door with a metal face, and agree that if you look at percentages of heat lost the door might be the biggest contributor. I can comfortably put my knuckles on my inner vent arch the morning after, but they get uncomfortable if I put them against the door! I plan on adding a wooden face to my door one of these days which will probably help with heat retention.

[david s]

No problem. I was hoping that if you didn't have a break you could contribute some data from one of your firings. There are really only three possible outcomes - breaks help, don't help, or there is so much noise in the data a conclusion can't be drawn.
I too have a metal door with a metal face, and agree that if you look at percentages of heat lost the door might be the biggest contributor. I can comfortably put my knuckles on my inner vent arch the morning after, but they get uncomfortable if I put them against the door! I plan on adding a wooden face to my door one of these days which will probably help with heat retention.

Just be careful if planning on a wooden face for your door. I like the look of a wooden door and it's traditional, but be aware that it is also vulnerable. I have a 1" insulation panel attached to mine which is fine for roasting and baking for a few hours. What you can't do with my oven and door design however, is to place the door at pizza temperature and leave it until morning in an effort to retain the heat for the next day. Not only is the wood vulnerable from heat slowly penetrating the insulation panel, but the heat from the floor can also be enough to burn the door from the bottom. Customers of my mobile hire oven are sometimes unable to follow instructions. Note the direction painted on the surviving part of the wooden panel.

[deejayoh]

I too have a metal door with a metal face, and agree that if you look at percentages of heat lost the door might be the biggest contributor.

Totally agree with this. And I think it's not so much the door, but the seal around it. Best thing I have done for heat retention was put some big green egg lid gasket around the lip of my door. The before/after on that was about 50 degrees improvement in heat retention a day for the first 2 days.

[deejayoh]

True, but I was taught if you torture data enough it will tell you anything you want to know
Seriously, I agree that there are a bunch of variables, but if we plot something like temperature at arch vs difference on each side for the two configurations we might see a trend. Trends are probably the most we would be able to discern, but people are always saying "show me the data" so some might be better than none.

I think a valid test of effectiveness would be to take the same measurements you suggest while the oven is firing. If the heat break is working, you should see a bigger temp drop for ovens that have it vs. ovens that don't for all the readings except the 12 o'clock reading. (There the oven exhaust will overwhelm the heat break)

That should account for most of the factors that David correctly points out would affect readings taken later.

[david s]

Fired up my oven to cook a couple of chickens this afternoon. I took some temp measurements and have recorded the results. I fired the oven for 1.5 hrs after which the oven interior was white. I then left the oven with the door off for another hour (no flame). The following readings were taken, in C. The numbers in brackets were from the same locations but taken 30 mins after, still with no door in place. Crown of dome interior 420 (390), base of dome 390 (345), Floor, oven side of flue gallery 439 (282), entry, flue gallery side of expansion gap 210 (187) outer decorative arch next to expansion gap 180 (154) top of outer decorative arch 103 (98), bottom of outer decorative arch 133 (105) Make of these numbers what you will, but as I've previously said the primary function of having a gap is to act as an expansion joint rather than as an insulating space to retain heat.

After a further 30 mins i placed the chickens in the oven (3 pm and a very hot day) and cooked them for two hrs. The oven was a bit hotter than I would normally have it, but was running out of time so I covered the birds with alfoil for the first hour. They were perfect. We had a really hot afternoon, note the temperature gauge in the shade. The oven is also standing in the hot sun so the outer decorative arch was a lot hotter because of this than usual.
Happy Christmas all.

Dave
Dav

[JRPizza]

David, I am also cooking some birds and took the opportunity to make some measurements. I fired the oven for about 2.5 hours (counting spreading the coals) before I closed the door to starve the coals and then clean out the oven. Replaced the door and took some readings after 4 hours (total time). Top of the dome was 540F, and the center of the floor was 520F.
With the door off to measure, I read 280 hot side, 196 cold side (12 o'clock), 206, 149 (3 o'clock), 323, 226 (6 o'clock), and 179, 142 (9 o'clock). Not sure what these mean yet, and there is quite a temperature gradient along the vent arch even over a few inches. If we get some more data I'll plot it up. I can't remember if you said the oven you measured had a heat break or not?
Got some quartered potato's in a cast iron pan going right now and getting ready to put in the chickens