To Thermal Break or not to Thermal Break? That is the question.

[Greenman]

Re: To Thermal Break or not to Thermal Break? That is the question.

I used S/S outside the inner dome and vermicrete around the rest of the break. A year on it seems to be fine.

The temp either side of the thermal break convinces me that it works. It is very significant and I am using it again for the oven I am building for my daughter.

[stonecutter]

Re: To Thermal Break or not to Thermal Break? That is the question.

Would stuffing the gap with a lean mixture of vermicucrete work?

Sure, but you still should either use metal or rigidizer because its friable.

[Les]

Re: To Thermal Break or not to Thermal Break? That is the question.

Since metal conducts heat WAY more than these bricks, is there a real benefit with filling the divide with metal. These bricks can absorb and retain heat pretty well, in regard to conduction, they are pretty poor. I have no doubt that the break will cause no harm (if constructed properly) but no one has posted empirical evidence that it helps. Any one with the break, please document the delta across the divide, with pictures so we can compare ( without the door on, that's just another variable) If the data supports the benefit, we should get it incorporated into the plans. Here is a test I did last week? https://www.youtube.com/watch?v=03Pwaf8NlOU

[NCMan]

Re: To Thermal Break or not to Thermal Break? That is the question.

I plan to use a heat break in both my floor and between my oven arch and vent arch. I have limited experience w/this, so I only speak my opinion, having lots of experience w/concrete and masonry, just not brick ovens. I would not build one any other way. Besides the benefit of the heat breaks, to me, they also act as a control/expansion joint. They also open up design differences you would not usually have w/out them.

[NCMan]

Re: To Thermal Break or not to Thermal Break? That is the question.

I meant to also add that I am not using any type of metal for a heat break. I have some ceramic fiber strip that I bought and will use that for a heat break on the floor, just outside of my hearth bricks. On the arches, I plan on using rope, as Stonecutter and others mentioned.

[stonecutter]

Re: To Thermal Break or not to Thermal Break? That is the question.

I didn't put a break between the vent landing and the granite.

My feeling is that the break benefits the oven construction in the form of an expansion gasket, primarily. As far as improving heat retention over an oven without one, I would have to be convinced with major differences in performance to buy that. When the oven is going full blast, a break is a non factor IMO. Long term heat retention for baking after no flame or coals are in the oven?... Maybe.

As mentioned by others( and myself) since it's no trouble to add a break, doesn't cost anything and has no downside....why not add one? Even if there was only a tiny performance gain.

[NCMan]

Re: To Thermal Break or not to Thermal Break? That is the question.

I kind of look at it like this....done correctly, it certainly doesn't have negative consequences. It's plusses are it can act as a heat break, an expansion joint and as an isolation joint. Not to mention possible design changes that can be made to save a few bucks.

[SevenAcre]

Re: To Thermal Break or not to Thermal Break? That is the question.

Good stuff guys. Happy fourth! Hopefully some of those ovens out there are firing!
-J

[kanoer54]

Re: To Thermal Break or not to Thermal Break? That is the question.

I'm certainly not an expert on this matter, but i left a 3/8" gap between the inner arch and the vent area, stuffed it with hi-temp rope gasket and covered it with hi-temp caulk to avoid any chance of fibers popping off. At the floor area, I left a 1/4 inch gap and filled it with ashes. After a year of use, I still have no chips from pizza peels, cast iron pots or dutch ovens being slid in and out. Sometime I will need to take the IR and check the temp difference.
jon

[deejayoh]

Re: To Thermal Break or not to Thermal Break? That is the question.

Since metal conducts heat WAY more than these bricks, is there a real benefit with filling the divide with metal. These bricks can absorb and retain heat pretty well, in regard to conduction, they are pretty poor.

That statement is true, except that the metal used isn't solid - it's a thin-walled piece of tubing. So you can't compare the conductivity of a solid piece of firebrick to a SS tube that's a sixteenth of an inch thick. SS is about 10x as heat conductive as firebrick, but you're using about ~1/8" of metal (combined) across the top and bottom of the tube vs. 2.5" of firebrick. So 10x heat conductivity on 5% of the material probably works out to a small improvement.

That said, I put the SS in more for the appearance & durability than for any perceived benefit. I didn't like the idea of a gap in the bricks, and it seemed like the best choice after researching the conductivity.

[david s]

Re: To Thermal Break or not to Thermal Break? That is the question.

That statement is true, except that the metal used isn't solid - it's a thin-walled piece of tubing. So you can't compare the conductivity of a solid piece of firebrick to a SS tube that's a sixteenth of an inch thick. SS is about 10x as heat conductive as firebrick, but you're using about ~1/8" of metal (combined) across the top and bottom of the tube vs. 2.5" of firebrick. So 10x heat conductivity on 5% of the material probably works out to a small improvement.

That said, I put the SS in more for the appearance & durability than for any perceived benefit. I didn't like the idea of a gap in the bricks, and it seemed like the best choice after researching the conductivity.

The stainless should be as thin as possible/practicable, say 0.55mm and preferably only covering the gap at the top (of the floor)

Using a hollow steel tube at say 50 x 25mm (2"x1") around 1/16" thick would give you a total mass of around 1.75 Kg/m
http://onesteel.com/showsizerange.asp?productID=207
The equivalent solid mass of my dense firebricks would also give you a mass of 1.75 Kg/m.
So there is the same mass in the gap if you fill it with a 1/16" steel tube and it's way more conductive. A steel tube will also not act as an expansion joint.
Not particularly efficient.

[NCMan]

Re: To Thermal Break or not to Thermal Break? That is the question.

I'm certainly not an expert on this matter, but i left a 3/8" gap between the inner arch and the vent area, stuffed it with hi-temp rope gasket and covered it with hi-temp caulk to avoid any chance of fibers popping off. At the floor area, I left a 1/4 inch gap and filled it with ashes. After a year of use, I still have no chips from pizza peels, cast iron pots or dutch ovens being slid in and out. Sometime I will need to take the IR and check the temp difference.
jon

Jon......that's pretty much what I plan to do. On the floor, I am using some ceramic fiber strip, which I will hold down about an 1/8" or so, then cover w/some slurry mix. Should do the trick.

[deejayoh]

Re: To Thermal Break or not to Thermal Break? That is the question.

The stainless should be as thin as possible/practicable, say 0.55mm and preferably only covering the gap at the top.

Using a hollow steel tube at say 50 x 25mm (2"x1") around 1/16" thick would give you a total mass of around 1.75 Kg/m
onesteel
The equivalent solid mass of my dense firebricks would also give you a mass of 1.75 Kg/m.
So there is the same mass in the gap if you fill it with a 1/16" steel tube and it's way more conductive. A steel tube will also not act as an expansion joint.
Not particularly efficient.

the conductivity coefficient I referenced before is based on the same volume of the materials, not the same mass of the material - so the weight isn't really a relevant statistic for comparison in the way you are trying to use it.

Thermal Conductivity of some common Materials and Gases

Thermal conductivity is the quantity of heat transmitted through a unit thickness in a direction normal to a surface of unit area, due to a unit temperature gradient under steady state conditions.

[david s]

Re: To Thermal Break or not to Thermal Break? That is the question.

the conductivity coefficient I referenced before is based on the same volume of the materials, not the same mass of the material - so the weight isn't really a relevant statistic for comparison in the way you are trying to use it.

Thermal Conductivity of some common Materials and Gases

Thermal conductivity is the quantity of heat transmitted through a unit thickness in a direction normal to a surface of unit area, due to a unit temperature gradient under steady state conditions.

As stainless is dense and highly conductive it should be as thin as possible.
Generally the denser the material the poorer insulator it makes. As density is Mass / volume then the mass used is important.

[oasiscdm]

Re: To Thermal Break or not to Thermal Break? That is the question.

I'm with Deejah.

SS looks better, serves a purpose. I did surround mine with Calsil so only have about a 1/2 inch of SS touching my landing where I left a small gap 1/8 so it doesn't touch, the landing granite.

On testing it works very very very well.

when tested with no fire having sealed the oven. When fire is raging radient heat makes the entry Granite hot but still not as hot as the fire bricks just inside the oven.

Cheers guys