Pdx 42" update

[Macrinehart]

I gave the new tier of my flue galley a week to cure, keeping it damp throughout. Today I cleaned out the sand and took out the angle grinder to level the top and enlarged the opening to allow for about a 1/8th to 1/4th-inch gap around the exterior of my single to double-wall chimney coupler. The concrete is still damp, but it's looking good! While I am still a little concerned about the seam, my sense is that once the work is complete, even if I get some cracks the chimney should still have a good draw. When I'm finished the chimney will be roof supported so I'm not to concerned about load bearing properties of the flue galley.

At this point, I believe I need to give the concrete another week before starting to fire the oven. While I wait I'm going to try and solve for the oven door.

[Macrinehart]

So today I made the first attempt to repair my flue gallery. This time I purchased a single-wall to double-wall pipe coupler. This coupler seals the interior of the two walls and it has a 3" deep and 8" diameter flang on the base which I felt would make an excellent coupler to the flue gallery.

to start I re-filled my gallery with wet sand. Then I centered this coupler on top and filled it with wet sand. Next I cleaned and dampened the already dried concrete-mix I previously molded in place. Using a concrete bonding adhesive, I made a slury of sand, concrete and adhesive per directions on the bottle and spread that around the top and exterior sides of my existing concrete-mix. Then I mixed up more of the casting home brew and used that to build up the flue gallery around the pipe coupler.

My experience at this point is that the wet concrete wanted to slowly slump. After messing around for a while trying to mitigate this without much success, I finally decided to wrap the damp concrete in plastic cling wrap with the hope that a little pressure from the outside could hold everything in place while it slowly dries. Finally, once everything was satisfactory I pulled the coupler out.

My hope is first that I get a reasonable bond between new and existing concrete. I presume if that works then once the material is set up I'll have to do a little work to get the coupler to fit. If it doesn't work, then I'll have to knock this gallery down and redo it.

[Giovanni Rossi]

Sorry, late to the party on this one. You could get an 8" chimney bottom plate, trim it to the height you need and anchor it to the top of the flue gallery. Then connect that to your transition collar with an 8" male-male union.

If you're firm on not wanting to drill into the flue gallery, you could have another go at smoothing out the enlarged hole and use a 9"-8" flue reducer and the union. But, those reducers are pricey.

[david s]

I don't think I can help you apart from explaining how I do it. My ovens are way smaller and with a double flue setup with inner 5'" and outer 6” pipes. Rather than installing the double flue pipes as one unit, I install the inner pipe as normal first, then slide the outer one over the inner one from the top, then tucking some blanket between inner and outer pipes at the top. Then securing the outer pipe with a custom made bracket to the roof supports and a Dektite seal between outer pipe and roofing.

[Macrinehart]

Hi fellow oven builders!

Once again, today, I find myself questions whether to move forward or backward and am seeking advice.

I've encountered some issues with the smoke chamber. I built this chamber with a slightly over 10 inch opening in top to accommodate my double walled pipe. Unfortunately, the pipe I purchased has an o.d. of 10.5". So after some thought, I decided, no problem, I will cut down the smoke chamber with my angle grinder to get the right sized opening. This was not a perfect solution.

While the new opening is adequate, due to unevenness of the material used to build the chamber, I now have some rather significant gaps between the outer wall of the chimney and the top of the chamber. I think the largest is about 1inch.

Perhaps I could mix another batch of cast material and patch this, but I'm not sure about that; I'm concerned that it won't bond well and lead to cracks. I don't need the patch material to support weight, but it will not have any structural support below, so frankly I'm just not sure it will work or even last that long if it does work initially.

Another complication is that the double wall chimney pipe is not sealed at the bottom. To seal the pipe, I have to purchase an adapter that converts from 8" double wall to 8" single wall, and this adapter has a tongue that will extend down into the smoke chamber by about 3". I could just cut off the tongue portion, but the manufacturer advises not to as it could impact the seal and allow creosote into interior of the double wall pipe.

So, if I go backward, then the plan is to demo the smoke chamber and rebuild it with a slightly over 8" rough opening on top. Then I attach the adapter to the pipe and it can sit on top with just the tongue extending through the opening. What I don't like about this is the lost time.

Will the first approach work? Are there other options?

[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.