Second post here. I am attempting Ernie and Eric's 6" Annex heater.
I am playing with my bricks to practice layout and see where I am in terms of having enough bricks or not.
1) my fire bricks are a different size than the clay. (fire: 9", 4.5", 2 5/8; clay: 8 5/8", 4", 2 3/8 " ).... How do you combine use of them w/o compromising stability.
2) Is there typically a lot of brick cutting...?
My fire feed needs to be 13", but stacking the fire bricks gives 14 3/4 ".... How to I fix this?
(I Know I laid the tower wrong, bricks should be turned the other way after course 6)
Don't know if any of this will help. Looks like your riser will be about 4" stacked like that. Here's pics of mine so far. I'm by far no expert I'm learning from these forms here. As for diff bricks try to keep them in the same layer you can see mine I have large bricks mid way of the riser and large bricks on the very first layer. I don't know what your floor is made of but it will probably burn it if it's not concrete. I'll take a picture of a piece of wood I was testing on outside.
Assuming you will pack some insulating material around this and then finish it out to the size you want, so as long as the internal feed is the proper dimensions you can place bricks as needed. The feed isn't facing the same temperatures as the heat riser, so you could use the clay bricks at the feed and have 2 layers flat and 2 layers on edge for (2-3/8, 2-3/8, 4, 4) 12-3/4" which is pretty close.
If I remember correctly, a 6" system has a 25" CSA? So however you can position the bricks so that the internal space is 25" square and the opening is as close to a square as possible. If it's an option I would buy the half thickness bricks for the riser, so your brick and insulating wrap combo isn't too thick. Easier to cut pieces cleanly on thinner bricks if I had to guess too. But your 9" fire brick minus 2-5/8" is 6-3/8" so you can overlap them enough to leave you 5" per side interior instead. It's just a matter of tweaking positioning and having enough of one kind for each section, rather than mix and match.
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If you're making a 6" system, the brick core can be 6" x 6" inside. A factor overlooked by early experimenters is that square channels have more drag than round channels of the same cross section; 6" square and 6" diameter are about equivalent in actual drag. A 5" x 5" square channel would have significantly more drag than a 6" diameter channel.
For brick stacking, you generally want them on edge instead of flat both to minimize material expense and to make the dense material thinner. You can't make a brick stack exactly 6" high without cutting, but one firebrick flat at 2 1/2" plus one on edge at 4 1/2" gives 7", and you can make the feed and burn tunnel say 1/2" narrower. This will allow for a half inch of ash to stay on the burn tunnel floor which will insulate the floor some more. You can then go to a 6" x 6" heat riser. The little steps outward on the sides will, if anything, add a touch of turbulence which is good.
The burn tunnel and lower third of the riser see the highest temperatures, so those are the places to concentrate firebrick if you have to choose.