Gap Between Vent and Dome

[dottavio]

I finally finished my dome and I am ready to start my vent. I have been doing some research and noticed that some builders leave a gap between the dome and the vent. The idea, as I understand it, is to create a thermal break between the two.

Not sure if that thought is valid or if leaving a gap will compromise the structural integrity of the chimney, which in my case will be a Duravent with an anchor plate.

Once again I turn to the Forno Bravo forum for your thoughts and feedback.

Fire away......pun intended

[david s]

Re: Gap Between Vent and Dome

If you insulate between the the entry and the dome then you are essentially building two separate structures. In my opinion it is better to insulate the entry from the outer arch so the whole inner dome and entry are one and free to move while the outer decorative arch is tied to the outer shell.A compromise is to use some insulating firebrick between the entry and the dome, but these lack strength and won't provide a thermal expansion joint.

[Faz]

Re: Gap Between Vent and Dome

Hi,

On my build the complete entry arch is separated from the dome. There is a half inch gap between the brickwork. I worked some of the external vermicrete into this gap but most of it is an air gap. There is also a layer of vermicrete between the dome floor and the floor of the entry. The first pic below shows the vermicrete thermal break (it is the rough textured material which sits just outside the dome arch). I have tiled over this break so it nice and smooth and cannot be seen on the completed oven (see the second picture).

[dottavio]

Re: Gap Between Vent and Dome

Kaz,

Thanks for the pics.

I sometimes get hung up on the science of this whole project. Looking at what you did there is not a "true" thermal break as the heat can conduct through insulating concrete and the tiles, but then again your design must stop some heat flow

How much cooler is your vent compared to the dome?

Also, to David's point, since you now have two seperate structures did you have any stability issues with your chimney? I was looking to install a 4' length of Duravent pipe and am concerned that the vent not being connected will not give its base enough stability.

Then again, perhaps this is all over thinking on my part as usual....

Doug O

[Faz]

Re: Gap Between Vent and Dome

Yes there will be some heat conducted through the tiles, but as that layer is only approx 15mm thick in total the amount of heat lost through this route is greatly reduced.

I haven't measured the actual temperatures on the floor, but there is a noticable difference when placing my hand across that break area.

[david s]

Re: Gap Between Vent and Dome

I'd imagine that you should be able to hold your hand against the entry even when the oven is SERIOUSLY HOT. Thus you keep the heat only where you want it. I like your solultion with the tiles overlapping the vermicrete.

[dottavio]

Re: Gap Between Vent and Dome

I am cutting my vent bricks as I am typing this, lunch break...

Very interesting Kaz. You've definitely given me some food for thought. I suspect I could even rip the fire bricks to 1/2" and accomplish the same thing as the tile. My floor is inside my oven so I can rework if necessary.

Thanks

Doug O

[mnagy]

I’ve been thinking about this issues for my oven as well. I was thinking about using some (1/2”) ceramic fiber board between the inner and outer arch. I would have it match the size of the outer arch so the oven door fits inside and rests directly against the inner arch brick. I was also thinking that the oven-side of the door would be made of the same ceramic fiber board.

I realize that this takes away all of the structural benefit of the outer arch, but I’m hoping that my inner arch will be enough. My inner arch height is 57% of the dome hight, so the force coming into that arch on top will be at 45 degrees, so half the force is perpendicular to the arch (not great).

[Giovanni Rossi]

I wouldn't use ceramic fiber board (or any type of insulation for that matter) in an exposed state. It won't stand up to abrasion and you don't want the loose fibers in your cooking space, or anywhere in the environment. david s is an advocate of old school wood doors. I'm going to go that direction.

[Gulf]

I placed a cardboard spacer over the face of a friend's oven. After the entry was completed, I cut the cardboard leaving just enough room for a pull tab. Lessons learned: Wax both sides with paraffin. We got about 95% of the spacer out. The rest we left to eventually burn out.

[NCMan]

I am a firm believer in heat breaks/isolation joints. I use ceramic fiber tape to separate the oven floor and any "adjoining" structures. Complete oven isolation. Works like a charm. I just leave it slightly recessed and caulk over it w/high heat sealant

[Macrinehart]

Love this thread and I've been thinking about this question for my oven. In my case the floor is under the dome and there is no thermal break for the floor. I was planing a thermal break for the outer arch but not sure what I'll do about the floor, which is built. I could take the bricks out on the front, cut them and put them back with a thermal break like NCMan.

[Giovanni Rossi]

Hey Mac,

I'm right with NCMan on thermal breaks. Not having them was in the negative column when I was deciding on buying a pre-cut kit. However, I haven't noticed any issues so far and I'm still getting days of retained heat cooking. My dome also sits on the floor. I suppose some heat is conducted from the cooking space to the landing; but, it is nice to have a slightly cooler area to pull a pizza to if the bottom is browning too quickly.

[Macrinehart]

Correction - Giovanni Rossi - just to clarify, does your floor have a thermal break on the floor between your oven and landing?

Thanks!

Mac

[Giovanni Rossi]

Sorry Mac, I've been experiencing the wonders of Iceland for the past 2 wks and haven't been following the Forum.

No, I do not have a thermal break in my floor. Once the floor tiles were in place I could have easily cut a gap before laying the entryway and arches. But, the kit plan interlocks the entry arch and vent arch bricks. Since I didn't want to mess with that, it seemed silly to have a thermal break in the floor and not between the dome and the entryway. My comment "I suppose some heat is conducted from the cooking space to the landing; but, it is nice to have a slightly cooler area to pull a pizza to if the bottom is browning too quickly." was trying to put a positive spin on losing some floor heat to the outer landing.