how much space under flat pans?

[Stamford sugarmaker]

I'm getting ready to archboard/firebrick my new 2 X 4. The arch consists of a rectangular firebox connected to a "tunnel" leading to the stack. There is no ramp leading to the tunnel, just a vertical surface. The distance between the bottom of the tunnel and the bottom of the flat back pan is maybe 8 inches.

I have read several different possible ways firebrick this section. Once I put the archboard on the bottom, how much clearance should there be between the top of the firebrick and the bottom of the pan? Would one inch of space along the entire distance from the firebox back to the stack be about right? Or should I just use the thickness of a full brick and put one or two vertical walls in the tunnel to squeeze the heat up onto the bottom of that back pan?

thanks, Peter

[foursapssyrup]

2-4 inches. every arch is different, so play around with it a little to find what works best on yours.

[C.Wilcox]

I used half bricks to get about two inches of space below the pan and then right at the far end of the pan I used a single row of full bricks to reduce the distance to about an inch. Seems to work pretty well, but definitely play around with it to find what works best.

[Cardigan99]

Peter, I would say twice the area of your smoke stack. Ex: If your stack is 6", it has an area of 28.26" (3.14 x radius squared), so something like 2.3 inches from the bottom of the pan (28.26 * 2 = 56.52) / 24" wide = 2.33). You'll get both a good draw and heat transfer.

Todd

[Sugarmaker]

Yes stack area x 1.5 or x 2 seems like it would work.
Regards,
Chris

[Stamford sugarmaker]

Thanks for all of your suggestions.

The stack eventually gets reduced to 6" so I guess inch and a half to two inches sounds right. I won't cement them so I can try a few different arrangements.

Peter

[Stamford sugarmaker]

Here's an update after testing. First of all, I converted to a 7" stack. I then arranged the brick to have about an inch and a half clearance all the way back to the stack- a distance of about 2 feet under the back flat pan. Bad idea, it couldn't breathe.

When I cleared all the brick out and just left the arch board on the bottom of the tunnel, man o man, what a difference. That 4 inches of space created an inferno in there and I was quite relieved. Stack temp went from 550 degrees to buried at 900 degrees.

But the next prob...no, opportunity to learn, is that there's a glowing halo at the top of my 10' stack. I had read that it is an indication of unburned gases, which is undesirable. I'd like to try putting a couple of vertical bricks, staggered, in the tunnel to slow things down a bit and burn off the gasses. Does that make sense? If not, what should I do?

thanks, Peter

[holey_bucket]

Peter, I am by no means an expert. BUT I can tell you what I would try. My local refractory warehouse (Industrial Firebrick in Grand Rapids, MI) sells refractory tile rated for... 2200 degrees or so. I believe their minimum sized piece is 2x4x2" and is around $60 or so for that piece. I would think you could prop the tile up at various angles very easily to test where you get the best heat retention with the least amount of valuable heat going out your stack. Good luck!

[1badsapper]

Is this a rule of thumb as far as the formula you described to Peter. I'm building a masonry arch & have had some concerns in this area. If this all works out I will have plans posted. Bricklayer for 28 years.
Peter, I would say twice the area of your smoke stack. Ex: If your stack is 6", it has an area of 28.26" (3.14 x radius squared), so something like 2.3 inches from the bottom of the pan (28.26 * 2 = 56.52) / 24" wide = 2.33). You'll get both a good draw and heat transfer.

Todd

[maple flats]

My 2x6 only had about 1" maybe 1.25 clearance and it did very well. I now use a 3x8 raised flue and have only 1/4", but the raised flue then conducts the heat and exhaust to the stack. Notice, I don't say smoke, once you get burning you will have ZERO smoke.

[SeanD]

I'm going to try closing the distance under my back flat pan to get a better boil. Right now the bricks are dry-laid flat along the bottom. I wonder, has anyone ever tried laying them in with a slight angle up like this ///// or this /---/---/ to give the bottom a washboard effect? Would that slow the gasses rushing toward the stack and push more heat up against the pan?

Sean

[C.Wilcox]

They're not at an angle, but that's essentially what I did. I have half bricks on the flat under the pan all the way until the back where I have a single row of full bricks. I put them in thinking that it would cause the unburnt gas/exhaust to roll and burn better, but I also have a constant flame at the top of the stack when I'm firing.

[Dennis H.]

You really don't need anymore room that the opening of your stack.

Is this a raised, drop or flat pan?

Flat pan just figure out how many square inches the flue is then convert that to what you will need to have under pan.

As for a raised or drop flue pan you should be able to place your brick really close to the bottom of the pan. That way this forces the heat up into the flues.
I feel this is more important with a raised flue. If you leave to much under the flues the heat will just bypass the flues and just go start to the back and up the stack.