36" build in coastal VA

[nlinva]

Thanks, David! I think I'm slowly beginning to wrap my head around this. Fortunately, I don't have to decide what my solution is going to be right away, so I'm going to ponder it some more, but I am now leaning toward having more of an insulation break between the front of the vent arch and the eventual front of the oven, which includes the decorative arch.

Also fortunately, I have the luxury of not being pressed for space immediately in front of the oven, so inserting some insulation is not going to cause space constraints. It's more a question of the structural stability of a largely free-standing decorative arch.

[david s]

It’s the total amount of thermal mass in your oven and gallery that is directly proportional to the amount of fuel required to reach a given temperature and exponentially, the higher the temperature, the greater the heat loss.
Also the more insulation you have the slower the heat loss. Some say you can never have too much insulation, but there comes a point where added insulation adds virtually nothing (law of diminishing returns) and cost plays a factor here.
Although the gallery will always be cooler than the dome, it too should be insulated on the outside.

As an example to explain some of this, I just fired my kiln today and the attached firing schedule shows temperature increase with gas pressure changes. It took 6.5 hrs to reach 1120C consuming 13kgs gas.

[nlinva]

Thanks for the feedback, david s ! I was thinking of the issue primarily in terms of heat loss (hence the thoughts about insulation), but it is useful to also think of it in terms of a heat sink (not that those two things are unrelated of course). Not sure yet whether that should change my preferences, but definitely an important additional angle to keep in mind!

Thanks also for the suggestion for the modified homebrew mix to use!

The decorative arch is set back by the 2" reveal, and it is only a bit over 2" deep. I tried it out with different implements, and there is no real way for this to make the gallery feel deeper unless there is some tool with a broad handle that is right near the vent arch-decorative arch transition. So on that front I'm not really worried.

I take your point about the different approach doing castable, allowing a much lighter (and thus less of a heat sink) chimney transition. That being said, would it not be a proportional issue? In other words, is your gallery/chimney lighter _as a proportion_ of the dome weight than mine is? The greater total weight will obviously matter for time to heat up, but if the proportional difference is about the same, the heat sink issue should also be about the same, right?

[david s]

As you have around 25 bricks plus a fair amount of mortar to join them all, you have a lot of thermal mass there in the gallery. Separating it from the dome does reduce some heat transfer by conduction, but it won't see temperatures and the resulting thermal expansion which approaches that of the dome. The evidence of this is that the black soot doesn't burn away in the gallery.
I'm really not sure whether mortar or high temp silicone would be the better solution to join your outer decorative arch, because my approach detailed below is quite different. If choosing mortar I think you'd be better using homebrew, but cut the clay content back because it promotes shrinkage cracking. I now always use 3:1:1:0.5 for this reason.
Adding a decorative arch will further increase the depth of your gallery, making the oven more difficult to work.

I have approached this problem from the other direction. My gallery is very shallow (13cm/5"), set back a little over the oven mouth and supporting a 5" flue made from fibre reinforced dense castable but quite thin resulting in only 10 Kg in weight. Because the light weight does not present a heat sink problem it is mortared directly to the dome and my expansion joint/conduction break sits between the outer decorative arch which is then integrated into the outer rendered shell. Adding this to the gallery depth as well as mortar joints results in a depth of 19cm/7.5" which gives me great access and the facility to use short handled (50cm) tools.

[nlinva]

I've now cut & ground the bottom and middle layers of the tapered chimney throat/transition. That was a lot of angle grinding to do, especially for the layer that sits on the arch, but I'm pretty satisfied with how it turned out. Once the pieces have been mortared together, I'm going to do some light touch-up grinding to make sure all the joints & transitions between bricks are smooth. I want to do as much of that as possible _before_ placing the pieces on the vent arch, because I don't really want to have to smooth anything out once it's installed.



Meanwhile, I have some questions about insulating this part of the build and about the possible need of an expansion joint. I would love any input from those with more experience than I!

My plan is to have a decorative arch of normal bricks (i.e. not firebricks), with about a 2" reveal. The bricks will be on their side, adding about 2.5 inches to the depth of the entry. I'm going to mortar them to the front of the vent arch (for stability) and rest them on the landing.

Question 1: I can mortar them to the landing, or I can use high temp silicone (same stuff as the heat break/expansion joint at the inner arch). Which is better?

The reason for the silicone would be that it would allow a little bit of movement relative to the outer landing, in case the vent arch expands due to heat and pushes against the decorative arch. However, there would be a lot of weight on that silicone, and I'm not sure how well that would work. Also, do I use the homebrew mortar formula I used for the dome, or should this be regular mortar (say 6:1:1, i.e. type N)?

Question 2: As currently designed, the chimney transition is the full depth of the vent arch. This means this part will also butt up against decorative arch. My guess is that this part will get hotter than the vent arch itself. If it makes a difference, I could mortar the decorative arch only to the 2" overlap with the vent arch here, and stuff the remainder of the joint gap with compressed insulation blanket and/or the high temp silicone. Is that worth doing?

Question 3: Relatedly, I can also easily cut off 1" off the front of the chimney transition section. I could stuff the resulting gap with 1" of blanket or, if I want some extra rigidity, with 1" of the CalSil board. On the other hand, this makes the chimney wall at the front much thinner (would be just 1.5" thick or so for about 2" of its height, before it tapers further inward). Is this worth doing?

At the moment, I'm leaning towards mortaring all around (arch to landing, and arch to chimney transition), for more structural strength. But if people feel that either the added insulation or some joint flexibility are important to have, I can readily be convinced :-).

[nlinva]

I've ground the the top layer of the chimney transition so that the top part fits around the anchor plate, while the bottom part widens down to the next layer.

The middle layer is half-height bricks horizontally, both to make sure there are no joints that line up between successive layers and to provide a little more height for the gradual tapering from the initial arch opening up to the 6" pipe diameter.

The bottom layer will be curved to fit the arch.

The top and bottom layers will be mortared together in halves, so they're easier to place. The middle layer will be put in place in individual pieces, which will make it possible to correct for any inadvertent non-level placing of the bottom

[nlinva]

With the arch drying, I've turned to the chimney transition. It needs to be higher than some of the other set-ups I've seen because the Shasta anchor plate sticks out a ways below the mounting plate. This will help prevent any lateral movement of the chimney, so I'm less worried about cutting it down smaller than the initial 14"x17" size, which removed the holes for mounting screws (tapcons).

The first picture shows the part that extends below the plate. The top level of the chimney transition will narrow to closely fit around this part, with any gaps sealed using high-temp silicone (the same stuff I am using in the expansion break between the dome and the vent arch).

I already cut down the plate to be less deep front-to-back, but I'm also going to cut the sides to make it roughly square. This will allow for a smaller footprint of the top transition layer, and thus less weight on the arch.

[nlinva]

In the end, the vent arch was an easier build than the inner arch, both because I now have more experience and because I dry-fitted it beforehand to measure what the cuts at the top needed to be like. It did take more mortar than I had anticipated, so I had to mix an extra batch, but other than that it went pretty smoothly.

I did forget to put plastic wrap over the arch form, which david s had suggested. Hoping that the ram board covering the form won't stick too badly to the arch. At least I do have the half inch pieces that raise the form, so hopefully that'll make the removal a bit easier.

On the first picture, you can see that the landing sticks out in front of the arch. This is where the decorative arch will go (see previous post).

[nlinva]

With the dome closed, I spent the past 2 weeks thinking about how best to do the vent arch, calculating how deep it needs to be, how big the chimney hole should be, how much brick I want to have on either side, etc.

The piece of slate I cut for the vent landing is 14" deep. In front of it will be a piece of slate that is the full width of the hearth slab; together they are sized to reach the front of the slab. This meant that I did not want to cut the vent landing shorter, even though once I decided to cut the chimney anchor plate, I was able to make the arch less deep.

Fortunately, I have been planning to add a decorative arch to the front, so instead of putting that in front of the vent landing, I'm now going to put it right up to its front edge, making the actual vent arch a little more than 2" less deep than in the original design. This does mean I'll have to cut 2 small pieces of slate equal to the size of the reveal of the decorative arch, but that should be straightforward.

So now I have a vent arch that is approx. 12" deep (about 11.75 actual arch, plus 0.25" expansion break). I put gasket rope on the top/outsides of the inner arch, with all the pieces that butt up against the inner arch L-shaped, overlapping with the inner arch about 1.25" (enough to cover the gasket rope).

I wanted to have at least 2" of brick on all sides of the brick part of the chimney, and I wanted a relatively smooth taper to the actual chimney, which is 6" inner diameter. With a 12" deep arch, that means the front-to-back dimension of the opening is about 8". I dry-fit the arch to figure out which brick was the first to allow a taper that would make for a vertical opening. It was the 6th brick in the arch (7th overall, because there is 1 brick below where the actual arch starts). Tapering these bricks made for a left-to-right opening of about 10". The corners are rounded, but the initial opening is close to 80 square inches.

Each of the bricks at the top of the arch starts 2" wide and finishes 2.5" wide. That means the front-to-back opening is down to about 7" at the top of the arch. Left-to-right is still about 10", because I didn't want to make the bottom of those bricks too thin, so that part of the opening is basically vertical.

The picture shows the opening as it currently looks.

[nlinva]

RandyJ that makes sense. I do hope it is more losing interest in posting than losing interest in finishing the project. I had a moment of sadness thinking about possible never-to-be-finished ovens out there....

You have been incredibly efficient and fast with your oven-building; I'm quite jealous! And I love how the lighting over your oven has turned out. I might try to do something a little similar once I get to that point. First up, though, is to finish cutting the bricks for the vent landing, build it, and then place the chimney, so I know where to cut through the roof :-).

[RandyJ]

This is a difficult question to answer. Many people lose interest in posting about their ovens if they are not fully vested in the first place. Most i have followed have finished. Some rake a few years to do so. I am almost done with my 2nd oven. It took me about 3 to 4 months to complete the first oven and if I take the middle of this one out where I was working 60 hrs a week and stressed out I am about the same again. Granted I am a bit outside of the normal with regard to how fast the oven gets built. This time I started at beginning of May and was fully cured by end of June and cooking pizza on July 2. That included pouring the base slab and building everything to get a finished oven then I just finished my roof structure and am tending to some final details. Trying to get everything done before it shows here.

[nlinva]

Random musing......

Has anyone from FB or one of the forum moderators ever tried to get a sense of what percentage of people starting an oven build actually finish it? It's such a big investment in terms of planning, acquiring materials, etc., that you'd expect it to be high. But on the other hand, it's much more work than most people (me included!!) realize, not just ahead of time, but even as you're going along.

From a glance at the forum, it certainly looks as though many threads are not completed, but of course that does not necessarily mean those ovens were not completed!

[nlinva]

I do have a friend with a drill press, so that's a good suggestion for redrilling holes. One added advantage of doing that is that I can more easily ensure they are exactly in the middle of the bricks I'm drilling holes in.

david s, I looked at your proposed solutions to the problem, and I quite agree with the downsides of the anchor plates you list (cost, , size, risk of splitting the brick/casting that you're screwing it onto). It never even occurred to me that it might be possible just to do without, but at this point I'm going to stick with and just make it smaller.

This is one of those cases where more experienced people have a better sense of what is essential and what is not. Never having set up my own chimney system before, I just assumed that an anchor plate would be essential. Maybe next time...

[david s]

I can probably cut the anchor plate down, for instance to 11 x 17" or so (I don't think the width is an issue, but the depth is). However, this would mean cutting off the holes for the mounting screws, leaving only the caulk to hold the plate onto the brick. I can probably try to drill some new holes, but not sure how cleanly I can do that.

The larger the anchor plate, the more stability to the chimney against lateral pressures, so in principle I like it. However, as I mentioned earlier, my chimney goes through a roof, and will get held in place there. That means the anchor plate will comparatively few sideways stresses (I suspect), compared to otherwise unsupported chimneys. So I think I'm OK with cutting down the plate, but as always, there is a chance I'm overlooking something important and I'm happy to defer to those who are more experienced.

Here are some solutions to this problem.

https://community.fornobravo.com/for...ery#post446435

Russell has a solution that may suit you better

https://community.fornobravo.com/for...ate#post381172

[david s]

Don’t cut it with a jigsaw, use an angle grinder with a cut off blade, easy.