Flu width, height and joint to oven dome

[Mark1986]

Hi!

I have been building my pizza oven for a while now.

In my first design I made an eror by casting the flue directley into the homrbrew dome of my pizza oven, which made it crack open.

So I removed the flue from the oven, and will now be adding a new one on top of the dome.

Now, I didn't really think out how I was going to make a joint between the metal flue and the dome.

Here is some of the work I did yesterday:



The flue is about 5" on a 30" inner size pizza oven.

My questions

• Is a 5" wide flue big enough for my pizza oven?
• Is 40" length for the flue to long for my oven? I am thinking about buying a shorter one. (someone said it looked quite long)
• How can I go about making a joint to the oven itself?

My plan until now

1. Wrap the flue in one layer insulation blanket
2. Use heat resistant kit to connect it to the dome
3. Use homebrew to make a little upward funnel/vulcano shape against the flue and insulation blanket. Going for quite a tight fit having to compress the insulation blanked and just making it a bit heigher than the insulation blanket.
4. Sealing the homebrew funnel/vulcano shape to the flue with a thick layer of heat resistant kit

Hope someone can give me some feedback on my plans

Best,


Mark

[rsandler]

1. Usually a 6" flue is recommended for a 36" oven; you've got a smaller oven, so probably fine? A 5" flue has about 69% as much cross-section as a 6", but your dome has about 58% as much volume as a 36" oven. Not sure if those are exactly the relevant quantities, but the height of your flue will also help.
2. No such thing as too tall from an airflow perspective (within reason); if it were really super tall you might have trouble with instability/wind, but I don't think you're in that territory, particularly if you can mount it securely.
3. Not sure I have great answers here--Trying to tightly pack with mortar probably won't work, as the metal will expand differently from the mortar, cracking it. It looks like you have a double walled pipe, which already has insulation inside, so doesn't need to be wrapped in CF blanket. The manufacturer surely sells a base plate that fits, and could be attached to your dome either with tapcons or with high-temp caulk, but I'm not sure you can fit it on the mating area you have available, even if you cut bits off the base plate. You could try casting a platform a bit higher, so that you have a flat area to place a base plate?

[Mark1986]

1. Usually a 6" flue is recommended for a 36" oven; you've got a smaller oven, so probably fine? A 5" flue has about 69% as much cross-section as a 6", but your dome has about 58% as much volume as a 36" oven. Not sure if those are exactly the relevant quantities, but the height of your flue will also help.
2. No such thing as too tall from an airflow perspective (within reason); if it were really super tall you might have trouble with instability/wind, but I don't think you're in that territory, particularly if you can mount it securely.
3. Not sure I have great answers here--Trying to tightly pack with mortar probably won't work, as the metal will expand differently from the mortar, cracking it. It looks like you have a double walled pipe, which already has insulation inside, so doesn't need to be wrapped in CF blanket. The manufacturer surely sells a base plate that fits, and could be attached to your dome either with tapcons or with high-temp caulk, but I'm not sure you can fit it on the mating area you have available, even if you cut bits off the base plate. You could try casting a platform a bit higher, so that you have a flat area to place a base plate?

Thanks for the advice!

About point 2: You think this looks ok aesthetically?
About point 3: No its a single wall pipe. That's why I am trying something like I am describing. What would you recommend in that case?

I also have this bigger pipe lying around, maybe I could construct something with that?

[david s]

The draw of a flue is primarily controlled by the internal diameter, but can also be increased by extending its height. I think you will probably be ok with the 5" with it being tall. The other important factor is to create a smooth flow by funnelling the gallery (see first pic) which you obviously can't do now.
If you sand/rasp the inside of the inner casting, so the pipe is not a really tight fit, it will provide enough room for the pipe to expand without stressing the inner dome casting. I don't like anchor plates because they require drilling into the casting which is asking for problems. I prefer using the method shown in my pic.
If you go a fair bit higher with the insulation around the pipe, the outer shell will support the flue pipe higher up. A 10:1 vermicrete above the blanket is easily shaped and weak enough not to cause problems with the flue expansion.
When applying the outer render do not render against the pipe or its expansion will crack the outer shell. Wrap some cardboard around the pipe at that point. Then wrap some plastic around that so the cardboard can be easily removed. Render up to the plastic covered cardboard.Once the render has set hard the cardboard and plastic can be easily removed and the gap filled with high temperature silicone. The gap needs to be around 5mm. This method works well, I've done heaps.
The pics should tell the story.

[Mark1986]

david s Thanks for the great explanation!

About the funneling gallery
I was already working on something like that with a rasp:


Maybe I can expand this gallery further? I didn't want to move more towards the back chamber, as it will connect the flue gallery/opening directly. That is not a good idea right? Maybe onlye expand it sideways?

About the vermicrete flue support
Ok I will work on that! A cardboard thickness, ok great. I was going for a yoga mat rapped around. But that might be excessive.

Btw can I apply homebrew render over it afterwards? I want to seal and paint it.

[david s]

A 4:1:1 sand cement hydrated lime makes a good render.. I use random fibre reinforcing in mine. AR glass fibres as used by the concrete countertop guys are really good.and far easier than stuffing around with chickenwire over a compound curve.

[Mark1986]

A 4:1:1 sand cement hydrated lime makes a good render.. I use random fibre reinforcing in mine. AR glass fibres as used by the concrete countertop guys are really good.and far easier than stuffing around with chickenwire over a compound curve.

Do you mean this as a layer over the vermicrete? Or as a substitute for it?

I got all the right stuff in house, glass fibres and such

[david s]

This is a mix to go over the vermicrete layer. The high proportion of lime imparts great workability as well as flexibility and some crack self healing properties.
Don’t use standard glass fibres, you need to use Alkaline Resistant AR ones which are zirconium coated to prevent their corrosion.