Freestanding Flue Gallery Design and Attachment

[WaterDog]

I’m working out details of my build and am leaning towards a cast gallery that I can mold independently of the dome. The design allows the gallery to straddle the oven mouth which also allows for a gap between the gallery and the top of the mouth to help moisture escape from the insulating layers like david s does while minimizing overall depth. I've worked out how to accomplish this using foam molds. I’m not necessarily doing this for the insulating quality of the gap but more for the venting and keeping expansion forces isolated.

That said, the gap I have is currently 3/8” or 9mm. Is this enough to accomplish the venting I’m after? My insulation fabric layer will be 2”.

Lastly, what is the best way to secure the gallery. I can certainly add a mortar bead outside along its 9” depth but I'm reluctant to mortar a portion to the dome/mouth area over concerns for expansion differential cracking.

I could run wires between the dome to secure it but prefer a mechanical bond. Would high temp silicone work? I'm thinking of only sealing the bottom 4” where the gallery runs up from the floor along the dome mouth.

Appreciate any insight.

[WaterDog]

Here's an illustration of what I’m planning. I’ve worked out the shapes and will achieve the required precision through the use of molds so I’m not concerned about how to do it. Primarily asking for opinion on the size of the gap and its ability to vent the insulation layer and if securing the gallery via mortar and high temp silicone at the places indicated makes sense.
Thanks.

[david s]

That looks pretty sound to me.
A few points,
1.Allow the perlcrete plenty of time to dry before building over it. Or use cal sil as it’s already dry. Cast in some drain holes in the supporting slab to help expel underfloor moisture.
2.If using blanket as a gap filler between gallery and outer decorative arch, make sure it’s the type that’s exonerated as a carcinogen, especially if there’s any that will be exposed. Or seal off the blanket with some 6:1 vermicrete.
3. Not sure why you are replacing blanket with high temp silicone lower down, but I think it’s doubtful it will stand up to temps seen at those points.
4. where your outer shell joins the stainless flue pipe is a point of stress. I’ve seen lots of ovens with cracks here and it then provides a point of entry for water. If you go higher it will provide more support and shed water better, because of a reduced angle. A gap should be left so it can be filled with high temp sealant which works ok in that location provided you don’t allow flames going up the pipe.

[WaterDog]

Thanks, David. Much of what my approach to this build is in large part thanks to your advice on the many threads I've read. Here is a side view which should provide a more complete picture. My main concern was with securing the gallery since it will essentially be freestanding. Hence the mortar/high temp silicone question. I may be overthinking this somewhat since once the fiber insulation, perlcrete insulation and stucco are done the gallery will be captive. I guess I wanted to see what others thought of the idea.

So, do you think the 3/8" gap to vent moisture from the insulating layers is large enough?

As far as point #4, based on this view, do you still see this as a potential point of failure for the SS flue? The surrounding thickness in the chimney support area will be 1" inch front and back (to minimize overall gallery depth), and 2" left and right. A little unsure of using homebrew at the 1" thickness and if it's not advised I could switch to a refractory formulation (Harbison. HeatStop) for this piece.

All good on your other points. The countertop will have (5) 1/2" weep holes under the 4" perlcrete base. I am not in a rush to complete this project and I like to plan well ahead before jumping in.

[david s]

I think that the 3/8” gap is quite sufficient to act as pressure relief in the same way that a small hole in a saucepan lid acts, but maybe not so good as a vent for drying.
Because moisture travels away from the heat source I believe drain holes through the supporting slab that locate beneath the dome insulation ( in addition to those under the floor insulation) are a better location for venting.
The expanding stainless pipe issue caused by the pipe heating up before the outer shell that surrounds it, is exacerbated by the steel’s much higher conductivity. There is a lot of power in an expanding steel pipe. If a small gap of about 3-5mm, filled with high temp silicone is used the stress on the outer shell is minimized, cracking prevented and water entry prevented.

[WaterDog]

Hmmm,very interesting. Currently, I have the ceramic fiber layer ending at the Perlcrete layer. Wouldn’t moisture entering at the blanket/perlcrete layer continue through the perlcrete and exit through the weep holes underneath?

Or should I have the ceramic fiber blanket ending at the countertop and add a couple more weep holes? A bit concerned with adding holes if I don’t need to..

[david s]

All insulation (except, I presume, the water resistant calcium silicate board) will act like a sponge. Preventing water entry in the first place is probably more important than designing an efficient method of expelling moisture, but a combination of both is important if an efficient working oven is aimed for. A roof over the oven, or an enclosure rather than an igloo style is probably a better solution, but both require considerably more labour and expense.

[WaterDog]

Appreciate your insights. Understood.

Onward!