Pdx 42" update

[Macrinehart]

It would be advisable to drill some drain holes through the concrete to assist removal of moisture from under the cooking floor.

I used copper pipe to set 8 holes in the concrete, they are covered by about a half inch. I'll drill a pilot hole up from the bottom and down from the top to finish.

[david s]

It depends a bit on what type of insulating material you place on top of the concrete. Calcium silicate board has a bit of give in it (you can dent it easily with your fingernails) and vermicrete cast over the concrete slab won’t care how rough the concrete is. It would be advisable to drill some drain holes through the concrete to assist removal of moisture from under the cooking floor. Use a small diameter drill first (1/4“) in case you hit some rebar, then enlarge them to 1/2”.

Regarding the flue, you need an 8” diam glue pipe for a 41” internal diam oven. With a front flue design the oven is a cross draft system and requires more pull from the flue than an updraft system as in a conventional fireplace. This results in smoke pouring out the front, particularly at start up if the draw isn’t strong enough. In addition some decent funnelling to the base of the flue pipe helps gather the smoke smoothly before rising and exiting out the flue pipe. A square or rectangular flue section is not quite as efficient as a round one so some additional cross sectional area is required if departing from a round pipe. Don’t attempt to save money by going galvanised rather than stainless because you’ll have to replace it down the track which will end up costing you more in the long run.

[Macrinehart]

Today I decided to take the form off. It's been 12 days since I poured the hearth.

I think it looks pretty good. The top is a little rough aftwr removing the loose material and I think it could use some patch work.

Sides are better except around the attempted bulldoze, which was done too late in the curing process. I encountered two issues on the bull note- first the concrete was too hard when I started. I should have just let it be. Second, I had covered the forms with painters tape, and when I used the bulldoze trowel it torn up the tape on the edge of the forms and embedded it into the concrete. If I had a do over I would not use the tape on the top 3/4 inch of the walls.

One you get below the mess with the bull nose things look pretty good! Minor air bubbles, no major holes, and the cove trim around the bottom edge is virtually perfect all the way around!

I suppose I can patch this up with some mortar. The top still feels a little gritty so I'll let that dry and brush it off. Maybe some leveler, or just mortar?

cheers!

[Macrinehart]

The question of depth of oven opening, flue gallery and flue size has had me thinking about the details on this part of the oven. I've read some anecdotes of differential configurations, but not why's. So today I decided to do a little research.

First call was to our local sheet metal fabricator. They don't have directly applicable experience but their opinion was that for forced air systems, which is their specialty, the crossectional area for transitions needs to be consistent. So for an 8" pipe, that 50.27" of area, which suggests an flue gallery opening with the same area. Some options: 4" x 12.6", 5" x 10", or 6" x 8.4". In theory, a reduction in crossection area could result in a negative pressure, pushing smoke back down the chimney and out the front of the oven.

A second design consideration comes from firebox design and cod specifications, where the following ratios are recommended from fire box to flue opening:
Square: 10 x 1
Round: 12 x 1
Rectangle (with aspect ratio > 2:1) : 8:1

So based on a 41" diameter oven, a rectangle flue gallery with 2:1 or better aspect ratio needs 173 sq opening. That looks like a 9" x 19.25" opening to the flue gallery! But of course that needs to shrink to an 8" pipe, which could lead to a negative pressure. Seems like a lot more than I've seen discussed so far.

I'm also thinking that because the actual fire is not going to be using the entire oven floor, maybe the firebox specifications are overkill?

Welcome any comment on this from the pros. Thanks!

[david s]

yes. You'll notice how much extra space you get between the gallery and the dome.
Let these pictures do the explaining.

[Macrinehart]

Hi david s - I drew this crosssection view, is this what you mean by setting the flue gallery inside the oven perimeter?

[david s]

To make the flue gallery shallower it can be set inside the oven perimeter. As the dome rises it gives you more space for the gallery vent opening. Building in brick units reduces the ability to make the gallery thinner and thereby shallower.

[MarkJerling]

.....Actually, as I look at it I realize that I presume on my initial plan that the chimney landing has to be "outside" the door reveal so that the oven can be fully sealed when the door is closed, right?...

100% correct.

....
As I was walking on a beautiful spring day I saw a vision of my future oven clad in a mosaic tile depictions of the park, reservoirs and forest from each of the cardinal directions.

Dang! Sounds like a real work of art. And I was happy when I painted mine the other day!

[Macrinehart]

For visual perspective I arranged a few brick over the floor template to illustrate the outline of the oven opening and chimney landing.


I put the 36" T there to visualize reach with a pizza peal. The plan calls for a decorative arch outside the chimney landing which I planned at 4.5" deep (another half brick beyond this model) but I suppose that is not required to be that deep.



The landing is not to scale, but gives a close idea of my thoughts for eventual placement and size. Actually, as I look at it I realize that I presume on my initial plan that the chimney landing hs to be "outside" the door reveal so that the oven can be fully sealed when the door is closed, right?

In other news, a walk this afternoon inspired me to name this project after or neighborhood Park. The entrance to Mount Tabor Park, which I am told is the only dormant volcano inside the city limits in the US is located about 3 blocks from our house. It's 176 acres and features three beautiful open reservoirs that used to supply city drinking water for Portland, plus old growth forest, and a public concert venue located inside the volvanoe's caldera!

As I was walking on a beautiful spring day I saw a vision of my future oven clad in a mosaic tile depictions of the park, reservoirs and forest from each of the cardinal directions.

https://www.portland.gov/parks/mt-tabor-park

[Macrinehart]

I just wanted to say thanks for all of your posts. I am grabbing information from your build as we work on ours.

Thank you, Miles! Also thank you to Mark Jerling, David s, Gulf, Mongo, UtahBeehiver, NC, and others that have contributed their experience and insight! I really appreciate all the help, and hope to pass what I'm learning forward. This is definately an excellent community!

[Macrinehart]

That's a crazily deep entry you have planned. The deeper the entry the harder it is to work the oven.

david s thanks for this feedback. It reminds me that the floor template was originally drawn without the door stop reveal, and that line I drew on the bricks also is missing the reveal. The depth of the opening is about 22" to the interior of the oven, but after the opening arch the width should be an additional 3" (1.5" reveal on each side).

That said, I am concerned about open depth too. I will dry stack some bricks to play with this some more.

[Doodbird]

Hi Mac,

I just wanted to say thanks for all of your posts. I am grabbing information from your build as we work on ours. I don't think that I will go through the trouble of cutting bricks for the wall parameter on the floor. We haven't decided on diagonal vs herringbone yet. I hope to layout floor bricks in a couple of weeks.

[david s]

The best way to cut curves is to cut straight(s) and then take off the excess. Obviously, binding a grinder blade in a cut is never a good idea. I used diamond blades and that worked very well. Work in a well ventilated space and wear a mask.

And make sure you cut them using a wet brick saw. That's a crazily deep entry you have planned. The deeper the entry the harder it is to work the oven.

[mongota]

When you cut your floor you might already be aware, but you don't have to cut it "as is" as shown in your photo. Meaning, the offcut shown in red can be cut up into smaller pieces to make the infills shown in green. That'll save you from having to cut up those two whole bricks just to get the two green triangular infills. Look through your floor pattern and you'll see how offcuts from one area can be used as the infills in others.

[MarkJerling]

Thanks for these tips, will give the diagonal a try.

UtahBeehiver, your picture raise another question I've been wondering about - cutting rounded edges for the boarder of the floor, as well as the intersections between the interior dome wall and the opening arch?

I've seen videos of folks using an angle grinder to cut these curves, plus a stone chisel and hammer for finishing, but I keep imagining the spinning grinder wheel getting bound in the curved cut and exploding into flying bits of shrapnel.

Is that how people are actually doing those cuts? Any safety protocol aside from the standard precautions (gloves, face shield, mask)?

The best way to cut curves is to cut straight(s) and then take off the excess. Obviously, binding a grinder blade in a cut is never a good idea. I used diamond blades and that worked very well. Work in a well ventilated space and wear a mask.