Pdx 42" update

[Macrinehart]

Heavy packaging. The weight of the chimney pipe and cowl combined is 27.2 lbs.

[david s]

Yes chimney cap (cowl). Presumably they supplied one to fit a double wall flue pipe.
my 150mm (6”) x 900 mm ( 36”) weighs 2kg. I dont know what diameter of pipes you have, but even for your extra length 18.5kg sounds heavy. Maybe the packaging is heavy.

[Macrinehart]

But you will need a cowl that has an attachment flange to accept a double pipe.

The product I got was 304 SS double wall, inner and outer walks are .48mm thick with ceramic fiber blanket pre-installed. The 48" section weighs 40.8 lbs (18.5kg), shipping weight, I haven't unboxed it yet. I do plan to build a pergola later. On option is to add a roof support so the smoke chamber is not carrying the full weight of the chimney.

Regarding the cowl, you're referring to the chimney cap? Or something at the base to support the chimney? I have a chimney cap.

[david s]

I’m surprised that you think it’s heavy. Maybe yours is a lot thicker than the flue pipes I get. Mine are 0.5mm thick. I usually do single walled flue pipes because stainless pipes are not cheap and even the base of the pipe is far enough away from the oven operator (or little kids) for it to be a danger. If the flue pipe penetrates a roof or is going through a pergola it is required to have a double flue to reduce the possibility of timber catching fire from excessive heat. I’ve done plenty of double flues and it’s not that much harder. But you will need a cowl that has an attachment flange to accept a double pipe. I also stuff some ceramic fibre blanket between the pipes, at the top and bottom so that the inner pipe is centred inside the outer one.

[Macrinehart]

Today a section of my chimney arrived. I purchased a 4 foot section, double-walled SS pipe, which I understand is recommended size for the oven size I'm building. However, I was surprised at the weight, and realize now that this chimney will be pretty heavy when fully constructed. Given that, I'm having doubts that smoke chamber I built will be strong enough to support it.

My plan was to seat the chimney through the opening at the top of the flue gallary, and to fashion a collar around the outside that will rest on top of the smoke chamber. I was then going to use a vermiculite mortar to build up additional support around the base of the chimney, perhaps until about 1-foot is secured. Does this seem like a reasonable approach?

david s - my approach follows the drawing you posted, with exception that your drawing appears to be based on a single wall pipe, cutting tabs on the bottom. My plan is to fabricate a collar or tabs from an extra piece of SS Sheet metal I have and screw those into the outer walk of the pipe with sheet metal screws.

Considering the weight I will give the castable material another week to dry before I start. Seems like 2 weeks of dry time should do. Thoughts?

[Macrinehart]

Here's an update - today I used the shop vac to remove mold sand down to the bottom of the sand form, and then created a couple of shallow side channels along the shoulders of the form (no more that 3" on either side).

I can tell this is reducing pressure - for instance some small cracks were formed in the sand around the top opening. I think this is having some desired effect, allowing some shrinkage while also maintaining support.

I think I will gradually continue to remove material over the next few days.

[Macrinehart]

I think after a couple days I will use my shop vac to clean out a hollow in the center of the sand mold and a coulple of channels to either side. Perhaps that will continue to provide some support while allowing for compression as the material shrinks.

[Macrinehart]

Thanks david s. I recall reading ingredients your post on another thread about the homebrew ratio and based on that I reduced the clay ratio by half (3:1:1:1 down to 3:.5:1:1). I guess that puts me at about 9%. I'm not extremely precise on the mixing so it could be a bit higher or lower. I had to mix a couple batchs and ran out of sand with just a hair less that 3 parts on the final batch.

Yesterday I was working in relatively warm temps - about 80F, but today we're forecasting a high of 68F with rain, with overcast but dry weather and highs from mid 60s to mid 70s and lows in the mid 40s to mid 50s for the balance of the week. pretty moderate.

I'll be sure to dampen those towels daily.

[david s]

Great job. Because of the very high clay proportion in home-brew I think it is better to halve the amount because clay will impart shrinkage. It is generally recommended not to exceed 7% if added to a standard mortar. This is because of the shrinkage problem. If removed from the mould too early you do risk damaging the cast. If you wait too long the shrinkage becomes more of a problem. The setting and curing is highly temperature dependent so it's difficult to advise when to demoed. For my climate it's about two days, you'll have to be the judge in your situation. You may be able to very carefully remove most of the sand mould from the top to allow free shrinkage of the cast. However, having easily done it a repeat go is simple and cheap and you'll certainly do a better job the second time anyway. Make sure you damp cure it for at least a minimum of a week before proceeding. Good luck.

[Macrinehart]

After reading several posts on castable ovens and the smoke chamber I decided to change directions and use sand to create a mold for the smoke chamber. One of the benefits of changing my mind after starting down a path is I have extra unused materials that find there way into usefulness. In this case a piece of stainless steel that was left over from a prior abandoned effort found usefulness as almost perfectly blocking the underside of the opening for my chimney.

From there I sculped the sand to roughly match the shape of a Styrofoam model I'd built for the just abandoned form. And then the anchor plate I purchased became a guide for sizing and positioning the base of the chimney. Considering comments on shrinkage I used the anchor plate to provide rough opening size and then added about an 1/8" - 1/4" to the radius.

Finally, I mixed up some home brew using a ration of 3:.5:1:1 of sand, clay, lime, and concrete. It was pretty warm today, so throughout the process I was misting my work area keep things damp. Built up the concrete around my mold all in one shot. It seems to have gone pretty well! Everything is covered with damp towels now.

Based on my reading, I will be letting this cure for a couple of days, and then removing the supports under the sand to allow for shrinkage during drying. It's cooling off to the mid-60s for the next few days, should be pretty good weather for slow curing.

[Giovanni Rossi]

Welcome back! I was wondering what happened to you. Glad it was just a technical glitch.

I'm far from a pro, but your situation is similar to what I did.

So, for #1 and #2:

I ran out of time to finish as winter approached and I didn't want all that water in the refractory when the temps hit single digits. So, I covered the dome with some loose layers of blanket and did a few days of small fires to dry things out a bit before putting it to bed for the winter.

You're clearly past that point now and your dome is well cured.

Once spring rolled around, I fit the blanket on properly and did the standard routine of drying fires before adding the pericrete insulation layers. I had such a wet spring that I redid the drying fire routine once I added the pericrete. Then I brushed on the waterproof render.

I ran into some terminology confusion during my build as "render" is used in many locales to refer to what we call stucco in the U.S. If your stucco layer is non-waterproofing, insulating or not, you're going to want to have that on and then do your slow drying routine. If your stucco is waterproofing, the fellows on the forum would say to dry first then apply.

Looking forward to seeing you cross the finish line.

[Macrinehart]

Hello Fellow Oven builders - long time no chat. I have actually been locked out of my account for months, that password recovery email is not working for me. I decided to try again and luckily was able to guess my password with 5 attempts, so Mac is back!

I am also not finished with my oven. Here is where things stand:

1. The dome is finished
2. The opening arches are completed
3. I have wrapped the dome and arches with insulation and covered the entire structure with stucco wire lath.
4. The landscape crew build a beautiful patio around the oven

Items to do:

1. Smoke Chamber construction
2. Curing
3. Stucco
4. Chimney
5. Food prep/service area and permanent cover.

I couple of questions for the pros:

1. Should I do the oven curing after adding the smoke box and before applying stucco?
2. Any special considerations in curing if the build is taking years instead of weeks/months?
3. My smoke chamber is going to be cast. What are the recommended ratios for a cast smoke chamber?

Attaching some recent photos. Show, the new patio, a model of the smoke chamber, and the the cast I am building.

[Macrinehart]

mongota - Thanks you for the response. I'm going to try an idea out, and if it fails then I will follow your advice and move backward to move forward.

My idea is to purchase use 3 pieces of Stainless sheet metal to isolate the chimney landing from the chimney itself. My notion is that 2 pieces will be attached to the inner and outer portions of the chimney landing, keeping them isolated from each other while providing a level, smooth base for the chimney. Then I'll place one rectangular piece of SS above without anchoring it, and finally I will anchor the chimney to the upper SS sheet, using the stair-step brick method to build the smoke chamber and taper to the anchor plate.

I do not think I will need any gasket material. The idea is that the plates will provide less friction for movement, while the brick weight of the smoke chamber will provide a stable foundation for the chimney structure. I'll insulate and stucco around the smoke chamber base for added stability and to keep it from slipping out of alignment with the opening.

My two concerns with this approach are that repeated heating and cooling could cause a missalignment between the chimney and the opening in the arches. I think experience will tell, and if it doesn't work then I can deconstruct this part and try something different.

[mongota]

I'm in a bit of a conundrum offering ideas as to what I did, as my landing arch is a bit different than yours. I made it so the entire flat surface of the landing arch, where the chimney anchor plate would be installed, was independent from the dome, versus yours where part of your flat is your dome arch and part is your landing arch.

Yes, you can bevel or shape the bricks for smother flow. I rounded off mine, it was quite easy and fast with a diamond blade on an engle grinder.

Post 79 and 80 in my thread show photos of the rounding of the bricks, as well as notching them to accept the cermic fiber rope gasket. Not sure if these will help, but...https://community.fornobravo.com/for...570#post393570

I do see your sticking point though where the 5/8" gap is located. Perhaps think about laying gasket material on the flat at the top of the dome arch? Then cutting a rabbet into the bricks that tyou add to the top of the vent arch so the rabbet will overlap and cover the gasket, it'll air seal, but not be mortared.

These are the tough parts of the build. Try a few differnt ideas and I'm sure one will lead you on your way.

I hesitate to write this because we all want to always keep moving forward, but: if you feel stuck, you can shift into reverse for a bit. Consider knocking a few bricks down and rebuilding something that better suits your end design. It's okay to back up if that's what you need to do to get moving forward.

[david s]

Google "Forno Bravo cast flue gallery" . You'll get lots of hits. It's a better way to search than using the Forno Bravo search function.