Thermal break between dome and chimney box

[mrchipster]

Re: Thermal break between dome and chimney box

Stainless steel at the entry on the floor level is a good idea to prevent abrasion and many commercial ovens have this, but stainless is way more conductive than brick so is somewhat self defeating if the objective is to reduce heat conduction.

A thin piece of stainless in contact with the brick floor will not transfer much heat because the contact point will be minimal less than 1/8 inch X length of steel. and I plan to hold it back with ash gap, which will further reduce the thermal transfer.

Chip

[david s]

Re: Thermal break between dome and chimney box

Stainless steel at the entry on the floor level is a good idea to prevent abrasion and many commercial ovens have this, but stainless is way more conductive than brick so is somewhat self defeating if the objective is to reduce heat conduction.

[mrchipster]

Re: Thermal break between dome and chimney box

You may be right. My initial thoughts are that a hemispherical entryway will stand on its own weight, simply butted up against the oven inner arch. A workaround for the L-shaped tab might be the near-45degree cut brick with a shallow angle cut filled with high-temp silicone. Another option might be a stainless steel insert, an option for me because of a brother who is highly skilled at fabricating all kinds of steel.

I guess great minds think alike, yes? I have two designs in mind for the stainless steel entryway floor, again to be fabricated by my bro. I like the idea of sealing the door with ceramic rope. Here is my idea of incorporating it into the (FB designed) door instead of remaining exposed to elements.
John

I made a modification to the sketch in a couple of ways.

1) I believe the L shape is very strong just not a sharp corner at the 45.

2) hi temp silicone might be a good idea I do not know what it is heat rated for.

3) Take out the first metal flange from the door removing the U and leave the outer edge an L shape for the rope to sit in.

I like design 1 for the floor as it keeps the entry devoid of things that will catch the oven tools.

Chip

[GianniFocaccia]

Re: Thermal break between dome and chimney box

an L shaped brick like you propose will induce significant stress at the corner

You may be right. My initial thoughts are that a hemispherical entryway will stand on its own weight, simply butted up against the oven inner arch. A workaround for the L-shaped tab might be the near-45degree cut brick with a shallow angle cut filled with high-temp silicone. Another option might be a stainless steel insert, an option for me because of a brother who is highly skilled at fabricating all kinds of steel.

I guess great minds think alike, yes? I have two designs in mind for the stainless steel entryway floor, again to be fabricated by my bro. I like the idea of sealing the door with ceramic rope. Here is my idea of incorporating it into the (FB designed) door instead of remaining exposed to elements.
John

[mrchipster]

Re: Thermal break between dome and chimney box

Any thoughts?

One other important thought. an L shaped brick like you propose will induce significant stress at the corner I highly recommend a smoother transition for your corner. Cutting a near 45 degree angle and then smoothing the transition with a grinder might be the way to go.

Chip

[david s]

Re: Thermal break between dome and chimney box

This is where I placed mine. I filled the bottom with 7:1 vermicrete which has some elasticity, did the same at the top, but didn"t worry about filling the sides. Because of the design of my floor, I couldn't put the gap at the inner end of the entry.

[mrchipster]

Re: Thermal break between dome and chimney box

The question is, how big a gap is too much? And, where does the thermal brake do the most good?

Not having a great deal of experience in thermal transfer I do not know but I believe you are on the right track and since your oven is built and you have experimented I will strongly consider your experience. Many thanks for your insights.

Chip

[Lburou]

Re: Thermal break between dome and chimney box

You said you would have a 1/2 inch air gap between the inner arch and the flue but when I look at your photos it looks like it might be about 1/2 inches already, what type of gap do you have now?


Chip

Its about 3/16ths of an inch. What I want to communicate is that I wanted more of a temperature difference between the dome and the inner arch. I don't think my design hurt the performance of the oven....But, a slightly larger gap could make a larger temperature difference.

The sixteenth inch gap between the granite and firebrick on the entry floor gives about half as much temperature difference as the difference between the inner arch and the dome, ergo, the larger the gap, the slower the heat loss. The question is, how big a gap is too much? And, where does the thermal brake do the most good?

[mrchipster]

Re: Thermal break between dome and chimney box

. My other thought was to leave a true (1/2" gap) and fill it with say, a 1/2-1" strip of 1/8" -thick stainless steel on the inside and insulation (or not) behind the stainless.

My plan right now is to incorporate a stainless steel entryway floor so there will be no gap between the entryway and oven floor.

Any thoughts?

The idea of stainless at the entry is a great idea from more than one perspective. The entry floor can be easily cleaned and the underlying area could be entirely insulation no need for conducting brick mass there.

The stainless could be bent down at the edges and provide a nice clean edge that would not catch tools and it would provide a turned down wrap over the insulation below.

I have been toying with the idea of a metal bumper strip at the edge of the inner arch to prevent what others have experienced and that is excessive chipping of the edges of the inner arch.

If a piece of stainless angle was placed at the corner of the inner arch and wrapped around the inner arch it would create a nice strong corner. It could be built to fit loose enough to not cause any thermal expansion issues.

The problem would be fabrication of this very unique shape. I am not sure if you could heat angle with a torch and get it to bend into such a radical curve.

See drawing for below

The other possibility is not to use angle but a larger sheet or a number of smaller sheets that just sit between the inner arch and the entry arch. The material does not really need to wrap around the corner into the entry of the oven it just needs to protect the corner. Cutting the stainless to match the arch would be quite easy. And multiple pieces could be cut to lock together to prevent shifting. Additionally attaching a piece of insulation gasket rope to the stainless should be a piece-o-cake and further secure the stainless should they be multiple pieces.


Chip

[GianniFocaccia]

Re: Thermal break between dome and chimney box

The 1/4 inch gap seems quite large at the floor but I think the sides could tolerate this type of air gap

True to form, I didn't fully explain the configuration of my gap. I plan to maintain a continuous brick surface between the arch and entryway via the entryway brick that butts up against the inner arch (see illustration) The contact area in this graphic has been set at 1" and the gap here is 1/2". This is my idea of a thermal break. I could make the contact area less than an inch but am concerned that over time thermal cycling may make the portion of contacting brick brittle and subsequently break. My other thought was to leave a true (1/2" gap) and fill it with say, a 1/2-1" strip of 1/8" -thick stainless steel on the inside and insulation (or not) behind the stainless.

My plan right now is to incorporate a stainless steel entryway floor so there will be no gap between the entryway and oven floor.

Any thoughts?

[mrchipster]

Re: Thermal break between dome and chimney box

I didn't find much on this subject either. But did manage to put heat brakes in three different places on our oven entry. You can see the brakes and read my comments citing actual temperatures here.

If I could go back, I'd make the air space between the dome and the inner arch a bit wider.

You said you would have a 1/2 inch air gap between the inner arch and the flue but when I look at your photos it looks like it might be about 1/2 inches already, what type of gap do you have now?


Chip

[Lburou]

Re: Thermal break between dome and chimney box

I didn't find much on this subject either. But did manage to put heat brakes in three different places on our oven entry. You can see the brakes and read my comments citing actual temperatures here.

If I could go back, I'd make the air space between the dome and the inner arch a bit wider.

[mrchipster]

Re: Thermal break between dome and chimney box

Second, since heat transfer rate is partially a function of the difference of the two entities swapping heat, over time, a hotter entryway will wick less heat from the oven than a colder entryway will. Safety issues aside, I believe a hot, well-insulated entryway heated primarily by exhaust gasses will benefit from an outer door and more thermal mass provided a heat break exists.

John

John,

In order to reduce the thermal loss via the flue I am considering using a chimney cap damper - one of the products is called a Chim-A-Lator it is a fairly simple device that caps the chimney and seals it to reduce down drafts and flue core cooling. This would add to the thermal retention of the entry.

I am still thinking over my options on the thermal break. The 1/4 inch gap seems quite large at the floor but I think the sides could tolerate this type of air gap. As long as they were backed up with tightly packed insulation, insulation brick or board. I would not want any flames to get out via this large a gap.

I still think that I will not compromise the integrity of my inner arch by doing the changes in options 1 or 3 because the bricks are so integrated into the structure of the dome and are significantly oversize compared to the thickness of the dome.... but I am still thinking this over.

Thanks for your input.

Chip.

[GianniFocaccia]

Re: Thermal break between dome and chimney box

Chip,
Kudos for taking a proactive approach to what David accurately describes as a great idea to isolate a heavy uninsulated entry (giant, exposed heat sink) from the main oven.

I have two thoughts on this:

First, a thermal, or heat break (not brake) is a good idea, IMO. Not for baking pies, but for extended (multiple-day) cooking using residual heat. I will be going with a heat break closer to your second option, which I think is easiest and best since it doesn't compromise your inner-arch and a hemispherical entryway is inherently stronger than a straight-sided entryway. My original plan was a 1" gap filled with Insblock 19 between inner arch/entryway, but tscarborough's recommendation of a simple 1/4" air gap has merit. I have also considered filling the gap with ceramic blanket or ceramic rope since there won't be any exposure to the inside of the entryway and food in any way. At this point I am still undecided on which direction to go.

Second, since heat transfer rate is partially a function of the difference of the two entities swapping heat, over time, a hotter entryway will wick less heat from the oven than a colder entryway will. Safety issues aside, I believe a hot, well-insulated entryway heated primarily by exhaust gasses will benefit from an outer door and more thermal mass provided a heat break exists.

There are a few builders here who have reported (no cook/insulated door) residual heat losses of -100F per day which I think is fantastic. This is the primary reason (along with the $350/cord cost of oak locally) I'm going with a thermal break. I am interested to see which direction you choose.

John

[david s]

Re: Thermal break between dome and chimney box

Folk are always banging on about insulation being so important, yet attach a heavy uninsulated entry to the front of the oven and another heavy uninsulated decorative arch to the front of that. I believe it is a great idea to isolate these from the main oven to reduce heat conduction as well as allowing the oven to expand reducing pressure and resulting cracking problems around the entry and decorative arch. I have been advocating this for years. My approach was to make the entry as light as possible (8 Kgs cast refractory) and isolate the decorative arch with an 8mm gap filled with vermicrete.
My outer arch, after two hours firing, is still only warm to the touch so something is working ok.There is some discussion about all this on a thread somewhere.Keep searching.