Looking for Firebox help

[Sunnyacres]

Yes that makes sense but my stack is not 6”. My stack is a 6x22 rectangle at the base.

[berkshires]

Yes that makes sense but my stack is not 6”. My stack is a 6x22 rectangle at the base.

You say "at the base". Is the whole thing 6x22? If so, then I hope you have a monster firebox LOL. But I assume you mean that's just the connector, and most of the stack has an internal open area much smaller than that. Right?

[n8hutch]

Yes that makes sense but my stack is not 6”. My stack is a 6x22 rectangle at the base.

Your base is a over sized rectangle like that because it's a transition/change in direction of flow, your flue gases are changing direction from a horizontal path of Gravel to a vertical path, if it was only 28 cubic inches as your 6" pipe would be it would be much too restrictive to promote a good natural draft because the gases would be trying to go through a sharp corner if you will, with a 6" opening the gases travel in a arched curve rather than a square turn up. Does that make sense?

I would go with a 1" gap between the pans and your arch rail, then when you add your gasket material you should be somewhere between an inch and an inch and a half.

[Sunnyacres]

You say "at the base". Is the whole thing 6x22? If so, then I hope you have a monster firebox LOL. But I assume you mean that's just the connector, and most of the stack has an internal open area much smaller than that. Right?

The stack base is 6x22 tapering up four feet to an 8" pipe coming out the back. I had to come out the back because I could not go thru the roof since it's there's another level above our shack.

[berkshires]

The stack base is 6x22 tapering up four feet to an 8" pipe coming out the back. I had to come out the back because I could not go thru the roof since it's there's another level above our shack.

Okay, so if the inside diameter of the pipe (less than outside if it's double-wall) at its narrowest is 8 inches, that's what you want to use to calculate the size of your stack. If it really is 6 inches inside then you want an inch and a quarter between pan and bricks. If it's 8", here are the new calculations:

The area of your 8" stack is pi x (radius squared).
R = 4"
R squared = 16"
16 x pi = 50"

So if your arch is 24" wide and you want to calculate how many inches you want between brick and pan to equal 50 square inches, you divide 50 inches wide by 24 inches, to get about 2 inches high.

Cheers!

[Sunnyacres]

Ok I'll try 2" my first year we had like 6" and each year I shrink it down and inch but still think I have too much space. I built by evaporater with a deep ramp since we are adding a flue pan this year. (Hopefully if time permits). Thanks for the help.

[Super Sapper]

If you are going with a flue pan you will want no more than a half inch under the bottom of the flues to force the heat up into them.