Chimney Design

[Mr.Pizza3]

Like many of you I am too staring and stalling and planning and going over the multitude of options of how exactly I want to build my chimney

I have thought about the center chimney over the dome with a squirrel tail and I do like it

but I am also looking to build a nice masonry chimney over the front using a clay flue and surrounded by a nice outer brick shell

my main issue comes in with thermal breaks

I have the room to add a break between the walls of my outer arch and inner arch and the issue comes in to when I build my brick base for the clay flues to sit on

it seems that due to the size of my oven I will be requiring at least an 8” chimney so my base for that will be tieing into the outer front dome in order to make a base large enough to accommodate a proper sized chimney

with that being said how are you incorporating a thermal break here?

it seams structurally there is no real way to do it and my fire brick that starts the flue will end up sitting on the dome unless I would also insulate the beginning of the flue ? would that then remove the need for a thermal break there? If I tie into my dome is it even worth adding breaks on the sides ?

I figured at minimum i will have a break in my floor that’s the easy one

but any input would be great



my thought now is as follows

build my outer arch

build my masonry base for the chimney
insulate

render

then build a decorative arch around this and then around my clay flue so the decorative arch up top would be separated from the dome and the sides would have vermiculite inbetween the decorative and outer arch and I’d be done

[david s]

Firstly, be aware that a brick chimney might be adding too much weight on an arch that is not buttressed. The problem is compounded because of thermal expansion which can challenge structural integrity.
Regarding a “thermal break”, which is a term I dislike because a small gap of around 1/4-1/2” would be considered ridicuously inadequate if used as either underfloor or dome insulation. Separating conductive material will prevent heat travel by conduction, but a small gap will do little to prevent travel by radiation. A gap’s primary function is to act as an expansion joint so the hot and expanding inner oven parts can move freely inside the cooler outer shell and decorative arch without stressing them. I have seen many ovens and read many reports of cracking caused by the lack of attention to this design detail. To slow heat travel from the dome to a heavy heat sink gallery is a good idea for efficiency to save fuel and improve performance. My approach was to reduce the mass of the gallery (12 kg), insulate it and place the expansion joint between the gallery and the outer decorative arch. But this is only one approach.

[Mr.Pizza3]

DavidS

alright well maybe I’m understanding things wrong but my train of thought has been that the arch you build into the dome is considered the “inner arch” and after the dome is complete the next arch you build that also includes the flue opening gallery etc would be considered the “outer arch” and those are the two people add the expansion gap in as well as one in the floor

then the decorative arch would be the one that goes on the outside of your outer arch if you chose to do that sort of like the attached picture

or are you considering the entire fire brick tunnel as the inner arch and saying I should add an expansion joint between my fire brick and decorative out arch ?

my plant was to basically build my front similar to the attached photo and then have my 8x12 flue go up about 2-3 feet and have the outer chimney “shell” go up 1.5 - 2.5 feet having the flue extend slightly beyond it and then capped

[Mr.Pizza3]

DavidS My main question is the top of the flue is going to rest partially on the dome like seen here is that going to be an issue expansion wise?

I will still plan to add the gap between my arch wall and inner arch wall but there obviously won’t be one where the base of the chimney is set

Just Curious if this will have any effect?

[david s]

The inner arch which forms the oven mouth is pretty well buttressed by the dome that surrounds it, but the outer arch is generally not buttressed. The decorative arch (if fitted) that sits outside the outer gallery arch is integrated to the outer dome shell. Because there is insulation between the outer shell and the inner dome, there won’t be a great difference in temperature to cause you problems there, but bear in mind that ther will be considerable weight on it if you build a brick chimney on top of it. You could simply make the outer shell a bit thicker there to support it.

[Mr.Pizza3]

DavidS
So to be clear your saying the base of my chimney sitting partially on my dome is going to be okay and to continue with the plan of the expansion gaps between the walls of the outer arch and on the floor

secondly my outer decorative arch which will carry majority of the weight of the chimney should see some form of additional wall on the side or some form of buttressing to help support the chimney

[Mr.Pizza3]

DavidS

and when you say a thicker shell your talking about a thicker render on my dome so it does not crack and press down my insulation where the chimney will be ?

[david s]

I was envisioning something like this with the gallery also surrounded by insulation