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Old 09-11-2006, 02:38 PM
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Question Thermal break between dome and chimney box

I've poked around the site and did not find much on this subject. Or I looked in all the wrong places....

It seems like a pretty big heat sink is attached at the front (or back) of many ovens in the form of an un-insulated chimney.

Has any one used extra insulfrax or other insulation to thermally isolate the chimney similar to the floating hearth concept? Does the general consensus think it will make a reasonable difference in heat retention?

Thanks

Christo
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Old 09-11-2006, 07:50 PM
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Default chimney

Most people use triple wall engineered flues for their ovens, which have a LOT of insulation between the inside and the outside. They are designed to withstand a chimney fire of highly flamable creosote without setting the house on fire (you see a lot of them installed in a wood frame riser).

Even masonry chimneys are supposed to have a half inch of free air space for insulation between the refractory flue tile and the four inches of masonry surrounding it.

So yes, the chimneys are supposed to be insulated.
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Old 09-12-2006, 03:53 AM
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Default Thermal Break

I'm not doing very well communicating my question.

I'm asking if it makes sense to break the oven in half and provide separation between the oven half and the pizza entry tunnel (piehole) that supports the chimneys on most ovens.

Sort of an addendum to the island hearth discussion - breaking more of the oven off and isolating it.

In most ovens I've seen here, the pie hole and chimney support extend about 16 inches from the opening in the dome. That seems like a lot of masonry (even if you exclude a triple wall chimney duct) that could suck heat from the dome. There is also extra flooring bricks that could pull heat from the floor.

I was considering leaving a 1 inch gap at top, bottom and sides around the dome opening and filling it with castable refractory insulation after I build the chimney section.

I guess the real question here - do oven entrys feel warm to the touch, does the floor of the oven entry pull significant heat from the oven floor?

As always, Thanks!!!

Christo
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Old 09-12-2006, 07:56 AM
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Default

The oven entry feels warm, but per our fireplace discussion, remember that there is radiant heat from the oven glowing through the entry. Perhaps an engineer could answer the question whether the heat conducting from the oven into the masonry of the landing area and chimney vent areas is significant compared with radiant heat loss and convection (cold air entering through the opening). From the practical standpoint, I doubt you have much to gain here from the pizza making standpoint. The real question would be whether this improves retained heat baking.
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Old 08-19-2011, 09:07 AM
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Default Re: Thermal break between dome and chimney box

I am designing a thermal break between my dome and my chimney/entrance.

I have attached a few photos of where I am at this time but looking for advice and input before I start cutting brick.

The first photo shows a 45 degree chamfer below the floor leaving a 1 inch vertical surface. I will be filling the void with FB Board in the chamfered area and the entrance brick will have a similar but opposite angle cutout. I will be leaving about a 1/8 inch gap in the floor to fill with ash.

My real question comes regarding how to handle the thermal break on the arch. I believe I have three options.

1) Looking at the remaining photos you can see that I have drawn 2 arcs on the face of the inner arch. The lighter inner arc represents the location of the interior the brick for the outer arc; this leaves a 1 inch landing for the sealing face of the inner insulating door.

The darker outer line represents where I would cut a relief chamfer into the inner arch similar to what I have done to the floor in photo one.

The brick at the base of this arch is a minimum of 5 inches deep going into the oven on the inside of the arch and gets up to 6 inches at the outside edge. At the top of the oven the bricks are 7 inches minimum deep and become 8 inches deep at the thickest point. So I believe that the internal arch is strong enough to be able to deal with the chamfer.

you'll notice in the photos that I have a tape measure pointing to the lower part of the arch and considering not chamfering this area because I have gone to the trouble of buttressing this area with additional bricks. The thermal loss in this area albeit potentially significant may be offset by the weakening of the inner arch. I would still maintain the 1/8 inch gap in this area between the inner arch and outer arch.

This chamfered area would then be filled with FB insulation.

2) The second option would be to not modify the inner arch. And have a similar design associated with the outer arch (chimney area) putting the chamfer on the outer arch brick.

The problem I perceive by putting the chamfer on the outer arch is that it may weaken the area underneath the chimney stack. I will be using a double wall (no internal insulation) stainless steel stack so the weight is not significant.

3) the third option would be to put a small chamfer going back about a half-inch into both the inner arch and the outer arch transitioning to a flat parallel to the existing face.

This third option would be a little more difficult to create but it is an option.

One other thought, I'm not sure if FB insulation is the best choice for this break, other options are insulating firebrick which would be very difficult to cut, FB board which may not be so difficult to cut but still difficult, or Perlcreate, or vermicreate. clearly the FB insulation or the vermicreate Perlcreate options would be the easiest to install.

In any case I will be putting a sheet of cardboard as a spacer between the bricks that will burn out after a few firings.

Thanks for your thoughts and advice in advance.

Chip
Attached Thumbnails
Thermal break between dome and chimney box-th_floor_11.jpg   Thermal break between dome and chimney box-th_arch_1.jpg   Thermal break between dome and chimney box-th_arch_2.jpg   Thermal break between dome and chimney box-th_arch_3.jpg   Thermal break between dome and chimney box-th_arch_4.jpg  

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Old 08-19-2011, 11:34 AM
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Default Re: Thermal break between dome and chimney box

With regard to what material to use for the insulation in your arch thermal break, why not cut a strip of ceramic fiber blanket and stuff it in the gap, then seal it in with mortar?

Love the addition, but will it be worth it in terms of heat retention? What do you plan on baking with all that insulation?
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Old 08-19-2011, 11:46 AM
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Default Re: Thermal break between dome and chimney box

Quote:
Originally Posted by calipizzanapoletana View Post
With regard to what material to use for the insulation in your arch thermal break, why not cut a strip of ceramic fiber blanket and stuff it in the gap, then seal it in with mortar?

Love the addition, but will it be worth it in terms of heat retention? What do you plan on baking with all that insulation?
I would like to use my oven all year round and with temps in the -20F range in the winter I will be trying to retain all the heat I can the wintertime.

I feel that optimizing loss will be a significant issue here in winter.

Chip
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Old 08-19-2011, 11:48 AM
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Default 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.
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Old 08-19-2011, 12:57 PM
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Default 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
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Old 08-19-2011, 06:19 PM
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Default Re: Thermal break between dome and chimney box

Quote:
Originally Posted by GianniFocaccia View Post

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.
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