Stack size on my new smoker

[fredouellette89]

Hey everyone I just built myself a new smoker and I think I might of messed up on my stack. I feel like it isn't drawing enough. So I am not too sure if it's to short do it doesn't draw or that it's too long and creating back pressure. The collector is 1 foot to centre of the stack and the stack is a 4" pipe and 3 1/2 feet long. I am rather worried so did I mess up or could it be something else? Also the cook chamber is 24"x48 "and the fire box is 24"x24". Any help would be greatly appreciated.

 

[SmokinEdge]

Picture of your air intake and grate level would be helpful.

 

[fredouellette89]

Picture of your air intake and grate level would be helpful.

I can also adjust the door to be slightly open to give more air to the fire .

 

[civilsmoker]

Would need some more pics and dimensions of the exit. I can say right now the stack being over 12 inches from the end of the cooker isn’t helping. There are a lot of losses happening through that narrow opening. I haven’t done the calcs but I would have put a 6 inch dia stack 36 - 42 inches high. You can always reduce the intake to control air flow on a larger stack but you can’t fix a restricted stack....... I always double (from calcs) stack size on patio sized cookers.....

 

[fredouellette89]

Would need some more pics and dimensions of the exit. I can say right now the stack being over 12 inches from the end of the cooker isn’t helping. There are a lot of losses happening through that narrow opening. I haven’t done the calcs but I would have put a 6 inch dia stack 36 - 42 inches high. You can always reduce the intake to control air flow on a larger stack but you can’t fix a restricted stack....... I always double (from calcs) stack size on patio sized cookers.....

The collector is 4 inches high and 22 inches wide.

 

[JIMSMOKES]

I don't know the answer but where you find the pipe?

 

[SmokinEdge]

The collector is 4 inches high and 22 inches wide.

The trouble here is heat wants to rise more than fan out or spread. I’ve seen these more lately, I think the whole Franklin craze started this. What works better is a plenum, like this, we are creating a large funnel that goes up to the stack, sort of force feeding the exhaust.

I would pull the fire grate one time and burn a fire. See if that helps. Your grate is right in the middle of the intake blocking it partially. Also helps to have two air inlets. One low to feed the fire, and one high just under cooking grate height to move the heat from the fire to the cook chamber. In this way you can control the heat of the fire and cook chamber separately.

This is how I modified my cheap New Braunfels cooker

 

[Ringer]

The stack looks too small and possibly too short. Like the others have said the collector is restricting alot from the looks of it due to spacing off the chamber and lack of plenum. A combination of two or 3 things going on it would seem. I haven't run the numbers but have you plugged those into Dave's calculator? It will give you the exact size and length of stack you need. It is for an rf smoker but it should work for an offset.

Standard Reverse Flow Smoker Calculator... by DaveOmak and others... Ready to use.. rev5.. 6/19/15.

Here's to "Alien BBQ" and the folks that first put the calculator together and "1728 Software Systems" for this great "Circle Calculator" and all the folks at "SmokingMeatForums", with a special shout out to Ribwizzard, that contributed their ideas and feedback while this thing was coming...

www.smokingmeatforums.com

 

[fredouellette89]

The stack looks too small and possibly too short. Like the others have said the collector is restricting alot from the looks of it due to spacing off the chamber and lack of plenum. A combination of two or 3 things going on it would seem. I haven't run the numbers but have you plugged those into Dave's calculator? It will give you the exact size and length of stack you need. It is for an rf smoker but it should work for an offset.

Standard Reverse Flow Smoker Calculator... by DaveOmak and others... Ready to use.. rev5.. 6/19/15.

Here's to "Alien BBQ" and the folks that first put the calculator together and "1728 Software Systems" for this great "Circle Calculator" and all the folks at "SmokingMeatForums", with a special shout out to Ribwizzard, that contributed their ideas and feedback while this thing was coming...

www.smokingmeatforums.com

according to his calculator I have more cubic inches in the stack then needed.

 

[fredouellette89]

The trouble here is heat wants to rise more than fan out or spread. I’ve seen these more lately, I think the whole Franklin craze started this. What works better is a plenum, like this, we are creating a large funnel that goes up to the stack, sort of force feeding the exhaust.
View attachment 632443
I would pull the fire grate one time and burn a fire. See if that helps. Your grate is right in the middle of the intake blocking it partially. Also helps to have two air inlets. One low to feed the fire, and one high just under cooking grate height to move the heat from the fire to the cook chamber. In this way you can control the heat of the fire and cook chamber separately.

This is how I modified my cheap New Braunfels cooker
View attachment 632444

I am confused as to what you mean by pull the fire grate?

Would making the collector higher the 4 inches and shortening it make a big difference in draw? Because from where ever I've calculated the stack is big enough.

 

[JckDanls 07]

ok then.. How high is the exhaust stack from the top of the cook chamber... Calculator suggest/requires it to be 36" above the TOP of the cook chamber @ the right diameter (or a lil bigger) per calculator ...

 

[SmokinEdge]

I am confused as to what you mean by pull the fire grate?

Take the grate out of the firebox entirely and build a fire on the floor of the firebox. See if it draws better. Your grate is currently setting about in the middle of the air inlet. This effectively cuts the air intake in half.

 

[fredouellette89]

Take the grate out of the firebox entirely and build a fire on the floor of the firebox. See if it draws better. Your grate is currently setting about in the middle of the air inlet. This effectively cuts the air intake in half.

That will be impossible seeing as I put a plate to have a semi insulated fire box.

 

[fredouellette89]

ok then.. How high is the exhaust stack from the top of the cook chamber... Calculator suggest/requires it to be 36" above the TOP of the cook chamber @ the right diameter (or a lil bigger) per calculator ...

Okay this is helpful,. Mine is 42 inches total and only 27 1/2 inches above the top of the cook chamber.

 

[civilsmoker]

Is your football opening the welded perimeter? Assuming so, the collector is experiencing losses with the food grate in the middle, secondly, like JD said above the stack height above is measured from the top of the cooker. I would double the calc’d volume on the stack.

If it were my smoker I would make the collector 8 inches in height (leave the bottom just go higher, above the grate), and use a 6 inch stack (a foot longer) that is closer in on the collector (ie chop about 3 inches off the collector end). Then I would add a second air intake above the door of the fore box. These adjustments assume the foot ball is the full welded perimeter. You should have no problem getting a OS run from 225 to 500 degrees like that with the right flow openings.

 

[fredouellette89]

Is your football opening the welded perimeter? Assuming so, the collector is experiencing losses with the food grate in the middle, secondly, like JD said above the stack height above is measured from the top of the cooker. I would double the calc’d volume on the stack.

If it were my smoker I would make the collector 8 inches in height (leave the bottom just go higher, above the grate), and use a 6 inch stack (a foot longer) that is closer in on the collector (ie chop about 3 inches off the collector end). Then I would add a second air intake above the door of the fore box. These adjustments assume the foot ball is the full welded perimeter. You should have no problem getting a OS run from 225 to 500 degrees like that with the right flow openings.

Okay this is very helpful. Just because of how wide the collector is the most I could make it with my cutting upwards would be 7ish inches I could maybe get more if I went down a bit more. The rest of it I could do easily (the size of pipe and collector overall length).

 

[civilsmoker]

Okay this is very helpful. Just because of how wide the collector is the most I could make it with my cutting upwards would be 7ish inches I could maybe get more if I went down a bit more. The rest of it I could do easily (the size of pipe and collector overall length).

3 inches up should work good. Going down more won’t add much value to the flow you want. Because it is in the lower 1/2 the upper air is causing turbulence at the collector as it try’s to go down and that conflicts with the lower air.....opening it up higher helps deduct this friction in the overall system. The upper air vent in the fire box also helps smooth out the air flow.

As a rule of thumb you want the stack opening in the upper 1/2 to 1/3 for best air flow. If you always run low and slow the lower is better (between) if you like to roast (375) @ 1/3rd is better or even 1/4th..... Raising it up the 3 inches will put you in the middle of that and higher than to top of your fire box.

 

[civilsmoker]

This is what your air is doing now.....when you open up the collector it will reduce the “down draft” effect.....

 

[fredouellette89]

View attachment 632496
This is what your air is doing now.....when you open up the collector it will reduce the “down draft” effect.....

Is possible that's why I saw a bit of smoke coming out of the fire box when I opened the fire box door the whole way as it was having a hard time going out the collector and being pushed back?

 

[civilsmoker]

Correct the flow as is is causing back pressure, the larger collector and more stack draw should correct this.