Heat break?

[drseward]

I am planning out a build of a Pompeii oven. In reviewing the FB PDF file on the Pompeii oven (a wonderful resource, BTW) as well as the many great posts on the forum, a question comes to mind. There is a lot of attention paid to adequately insulating the oven for efficient performance (reaching pizza temperatures rapidly with minimal fuel as well as good retained heat for other baking).

In considering the design, it seems that there is weak link (in terms of thermal performance) where the mass of the entry meets the mass of the oven dome and floor. In the situation where the fire is out and an insulated door is in place to retain heat for cooking, wouldn?t this continuity between the mass of the oven itself and the entry arch and floor be the source of significant heat drain?

It seems that some kind of heat break between the oven and the entry would significantly improve performance without adding any significant expense. Is there a reason, structurally or otherwise, why a heat break is not used here?

Any thoughts would be appreciated.

Mark

[Frances]

Re: Heat break?

It has been discussed before (I forget where right now) and the verdict was that it wasn't really worth the bother - the heat loss not being big enough to weigh up the fiddling around constructing a heat break. I believe Ken had some neat ideas on how it could be done.

Stucturally of course its better to have the entry firmly meshed in with the oven dome and any kind of heat break will weaken that link. But I'm also quite sure that it is doable, if you really fancy the extra work...

[Ken524]

Re: Heat break?

Yeah, I tinkered with some ideas. I think I posted a few drawings on the official "Heat Break" thread; but I ended up sticking to the plan and not worrying about it.

I agree with Francis. You will compromise structure and it's probably splitting hairs anyway.

If you follow the plans, you will be amazed at how well the oven performs.

[christo]

Re: Heat break?

my entry way is made of bricks on their thin side so their conduction path is a bit less than bricks on thick side, but I'm very happy with the performance.

One thing I wonder about the floor - it accepts heat from the dome and other radiated sources - but as each brick does not fully contact the other - ash being the insulator does it make a significant difference on heat transfer?

My design required my entry to contact the dome to support the opening. I'm looking forward to seeing your descision and to see how performance is if you go to the isolated dome.

Christo

[wlively]

Re: Heat break?

I did build a "heat brake" if you will into my entryway floor. My initial goal was not so much efficiency but to keep the temp down on the floor of the arch and landing. You would never know by looking that it is there. At the seem of the entryway floor I cut the side of the joining bricks (first ones outside) at 45deg angle. From on top you see no difference, but a side view would see that the bricks are only touching at the very top edge. I filled the empty space with vermiculite concrete.

Probably not a significant amount of "braking", but it was the only thing I could think of that would be invisible. Can't say what difference it makes, as I have never felt anyone else's oven, but I can measure a difference between the two points of the floor. Has been too long since I was all geeky with measurements, I seem to remember being in the 100+ degree range.

If anyone is interested I can measure next time I fire it up.

[drseward]

Re: Heat break?

Thanks for the input everyone.

Wade, I find what you did to separate oven floor from the entry floor intriguing. If it occurs to you the next time you fire it up, I?d be interested in hearing what the temperature differential is.

Christo, I had the same question about how much heat transfer there is to the bricks of the entry floor considering the less than complete contact from brick to brick and the ash filling the small voids. Perhaps someone else could measure the differential to compare Wade?s results.

Frances and Ken, tell me what you think the compromise in structural integrity would be to isolate the entry from the oven. I am considering a single brick wide arch attached to the oven dome and a detached arched entry/chimney. The separation between the arch on the dome and the arch on the entry would obviously be sealed. Does that make sense?

Mark

[christo]

Re: Heat break?

Please measure next time you fire up.

I'll measure mine without a break at 4 inches inside the oven and 4 inches inside the entry and report also.

Christo

[DrakeRemoray]

Re: Heat break?

Even if you gain a few degrees by building a heat break, the biggest problem I have when baking is getting the oven to cool down fast enough. Retaining heat is not an issue in a properly insulated oven, even with the potential leakage you mention.

See this thread for temps over time on the standard build.
http://www.fornobravo.com/forum/f30/...cord-6502.html

Also, always welcome another coloardo builder! Where in Colorado are you?

Drake

[drseward]

Re: Heat break?

Hi Drake,

I'm just north of Monument, CO.

It sounds like it's been a few years since your build--did you source your firebricks locally? Do you recall where you got them?

Mark

[DrakeRemoray]

Re: Heat break?

You bet!!

I got my firebrick and the decorative stone surrounding my oven from Robinson Brick (303) 783-3000.

I got the casting stuff for my cast vent and insulating blanket and board (which I used to make my door but would work under the oven) from a refractory supplier called United Western Denver (303) 388-1224

I got the loose perlite from American Clayworks 303-534-4044.

The hardest stuff to find was the dry heatstop mortar. You can try Hillen Corporation - Colorado Demolition and Building Materials up on the north side of town. But I would confirm it is the dry mix, not the airset/premixed...

Good luck! Let me know if you have more questions!

[drseward]

Re: Heat break?

Wow Drake. That's a lot of help--thanks.

Mark

[wlively]

Re: Heat break?

Since you guys asked for it, I finally fired up my oven again.

Here is a picture of my landing with temp measurements on each side of the break. Inside oven floor temp 550+ and TC floor reading of 360 or so. Interestingly, taking various floor measurements there seems to be a 20-30deg avg drop across each brick as you read hot side to cool side.

[Lars]

Re: Heat break?

This forum is great!!

I love how you are all discussing the merits of the heat break at the opening. But, it kind of reminds me of when I went cross country bicycling.

I had read all sorts of things about touring bikes, how to reduce the weight, the skinny tires, etc. I loaded down my bike with my clothes, sleeping bag, 1 man tent, map, water bottle, and set off for Chicago.

On the first expansion joint I hit, on an interstate bridge across the river into Iowa, I popped my 'ultra light weight' racing tire inner tube in 2 places!

That day, I went and got a plain old heavy innertube, and a big fat plain old bicycle tire, and never worried about shaving off an ounce here and there ever again. In fact, loading down the bike made it kind of fun to ride -- a little more momentum up and down hills.

Yes, I realize this has NOTHING to do with pompeii ovens, or even pizza, but as all of you know, the perfection we strive for must always be tempered by the reality of the task... and I wanted to become a 'laborer'.

Lars.

[Les]

Re: Heat break?

That day, I went and got a plain old heavy innertube, and a big fat plain old bicycle tire, and never worried about shaving off an ounce here and there ever again

Lars - you nailed it. I had road bikes, then mountain bikes. We recently bought a couple of beach cruisers - not looking back...

[Rodneyf]

Re: Heat break?

This is great! I am at the stage right now where I will start my entry arch and was going to put a 2mm piece of cardboard between the entry and vent which I would remove after.
My theory was that an air gap, be it ever so small would still act as a heat break and I think my entry arch will be strong enough to be self supporting without being keyed into the vent arch.
Am I going down the wrong path?