Not enough airflow??

[butte pit crew]

I just built an 8 ft. reverse flow smoker this spring.  I seem to be having trouble getting the thing hot.  I burn only natural lump (Lazzari) and I cant seem to get it above 225 degrees.  I think the problem lies in the size of the exhaust.  I only used a 4" pipe.  I'm thinking of adding a second pipe to this to increase the draw and airflow into the chamber, but I dont want to have to keep cutting the tube.  Any ideas on the size of exhaust needed for this?  It is 8ft long and has a 30" diameter.  Other than the low heat it works very well.  When its up and running all 4 temp guages are within 10 degrees of each other.  Let me know what you think. 

 

[bruno994]

225 at the door, or 225 in the center of your bottom cooking grate?  Mine shows 200 at the door, but temp in center of bottom grate is 250.  Could be a number of issues, exhaust being a major one of them.  Did you use the pit calculator to get all your sizes correct?  If not, punch in your numbers here and see if you have undersized anything (firebox, intakes, exhaust):

http://www.feldoncentral.com/bbqcalculator.html

To quickly find out if it is air intake, just crack your firebox door, if your temp increases, then you have a lack of air getting to the fire, but with a 4" stack, I would say that sounds more like your problem, especially with a pit that size.  Adding an exhaust or going to at least a 6" diameter stack should help. 

By the way, welcome to the forum and we love pics (qview), if you have some of the pit, post them.

 

[bruno994]

With the pit calculator, if your pit is 96 x 30, your exhaust, if it is 4" diameter, would have to be 92" tall.  If you goto 6", then you would only need 40", 8" would require only 23".  I do believe you have your problem identified. 

 

[JckDanls 07]

Also what about the hole from firebox to smoke chamber.. make sure that is of proper size.. along with the end of the reverse flow plate away from the end wall...

 

[smokinjoe1970]

also with an 8' machine especially 30" Its hard to get to temp anyway, How big is the firebox? The 4" Pipe Could be Your problem or at least a big part of it. I bout a 30" X 7' with 2 4" exhaust stacks But they didn't run deep enough into the Chamber I added 5 1/2" to the inside length and that fixed it. ( ya see heat rises and follows the least resisted path so The higher up your exhaust stack is the faster the heat escapes sounds Odd but its true. I went from 200 -225's to 250-300 on same wood, so then I could  dial back ( use less wood) and cook longer more controlled. I sold that unit to the Cleburn county  Boy Scout troop  They Love it. Good luck ..

 

[smokinjoe1970]

also with an 8' machine especially 30" Its hard to get to temp anyway, How big is the firebox? The 4" Pipe Could be Your problem or at least a big part of it. I bout a 30" X 7' with 2 4" exhaust stacks But they didn't run deep enough into the Chamber I added 5 1/2" to the inside length and that fixed it. ( ya see heat rises and follows the least resisted path so The higher up your exhaust stack is the faster the heat escapes sounds Odd but its true. I went from 200 -225's to 250-300 on same wood, so then I could  dial back ( use less wood) and cook longer more controlled. I sold that unit to the Cleburn county  Boy Scout troop  They Love it. Good luck ..

 

[butte pit crew]

The firebox is 2'x2'x2'.  it is made out of 1/4" plate and the lid is 3/8" to help hold the heat.  Also the exhaust stack is dropped down to grill level but only sticks out of the top 3".  I really knew better on the stack but it is my first build and mistakes happen. Here is a pic of the pit just after painting.  On the other side of it is a 15 cf freezer converted to a fridge and a hand washing station that gets hot water from a propane portable shower with a pump.  Thanks guys I will be adding 2 new stacks to this and we will see if that clears up the heat problem.

pit crew 4.jpg

• butte pit crew
• Sep 17, 2012

 

[daveomak]

Pit Crew, morning....   The members, previously, gave you some very good advice....   The pit calculator will let you know where the initial design can be improved....  Run the numbers through it and then you will have a good starting place for improving your smoker....  Get back to us and let us know how it is going...  If you have any questions, we are all here to help....   Dave

http://www.feldoncentral.com/bbqcalculator.html

 

[sqwib]

I would have to agree that a pit that size may be hard to get hot with lump.

Have you tried wood?

It does sound like the stack is undersized for prroer draft.

Do you have air coming underneath the lump or over top of it, check the size of your firebox to smoke chamber inlet.

I would swap out the 4" for a 5" piece and follow the pit calc for proper height, if possible.

 

[ribwizzard]

you have any better pics?

 

[unclejim]

I agree that there may not be enough airflow. I have a MC 120P and wish it had 2 stacks. Both open to warm up the tank then close one while cooking.