What distance

[supersapper]

How close should I bring the back wall to the bottom of the pans. Right now I have a 5"- 3" front to back in the ramp. This year it will be natural draft. The arch is 7' in length with an 8" chimney.

[TerryEspo]

I am not sure if I understand your question correctly but will give an answer.

I was told here from others that when the heat and flame leaves the back of your pan heading towards the chimney stack, it should be going through a 2" gap with 8" chimney.

So, if your pan is 6 ft long, right at 72" the heat and smoke should be passing through a 2" gap the total width of your arch.

Hope that helps.

[Super Sapper]

You want your area under the pan to equal your stack size or a little larger. An 8 inch stack would be 50 square inches so around 2 inches on a 2 foot arch and a 6 inch stack would be 28 inches or a little over 1 inch on a 2 foot arch. This is for a flat pan, for a flue pan you would go up to about 1/2 inch below the pan. This should be from the top of the ramp to the stack or a little befor for a raised flue.

[eustis22]

yeah, I'm not following this at all....I changed from batch pans sunk into my 2 X 3 Hobby to a continuous flow (flueless) sitting on top.....I sloped my arch up to my stack at roughly 45 degrees...leaving about 20 inches of firebox.....so the distance underneath my pan varies from 18 to 8 inches of clearance. Do I need to alter the shape of my arch?

[supersapper]

You want your area under the pan to equal your stack size or a little larger. An 8 inch stack would be 50 square inches so around 2 inches on a 2 foot arch and a 6 inch stack would be 28 inches or a little over 1 inch on a 2 foot arch. This is for a flat pan, for a flue pan you would go up to about 1/2 inch below the pan. This should be from the top of the ramp to the stack or a little befor for a raised flue. thanks! That s what I needed to know.

[eustis22]

I still don't get how you got an 8 inch stack is 50 sq inches....that's nowhere near the area of a cylinder.

[Big_Eddy]

Cross Sectional Area of a circle is pi R^2
8" stack. R=4
3.14x4x4=50.24 or roughly 50sq in

[eustis22]

thanks....I thought I was going senile. The volume of the stack is not a factor?

[gouldjs]

I have the same question. I have a 3x5 flat pan. We are in process of building the arch. I have a 2x3 fire box and then a 3 foot run under the rest of the pan to the stack. I plan to have 4 inches of clearance between the pan and the bottom of the arch in the back but plan to lay some fire proofing board over the metal to protect it and that should lower the gap to 3 inches or less. At the end of the firebox I have 8 inches between pan and bottom. Final welding is set to occur wed so I have time to modify yet. I have a 6 inch chimney with up to 10 foot of stack if I want it. Should I narrow the gap between pan and metal further?

[psparr]

yeah, I'm not following this at all....I changed from batch pans sunk into my 2 X 3 Hobby to a continuous flow (flueless) sitting on top.....I sloped my arch up to my stack at roughly 45 degrees...leaving about 20 inches of firebox.....so the distance underneath my pan varies from 18 to 8 inches of clearance. Do I need to alter the shape of my arch?

Withba short pan you will have a steep ramp, but you should only have 2" between the back of the pan and the ramp. Does your stack exit out the back or out the top behind the pan?