New build, bad draw?

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fobar22

Newbie
Original poster
Oct 23, 2010
18
12



I have ran two cooks on it so far, and I am curious if anyone can help diagnosis issues or if it is just learning the new smoker.

A few details, all of the ratios work out with the online calculators, roughly a 80 gallon CC. I also have a warming box that the heat can flow through (I have internal dampers). The exhaust is 3" diameter and about 38" long, and the firebox is a 18" cube. Everything is 1/4"  steel except the RF plate, that is 1/2".

I am able to get it to run around 250, but I have to have the inlet, and both outlets completely open. I don't think I can get any higher temperature than that. I originally planned on no warming box for all of the dimensions, so I am thankful that it is there to provide some extra exhaust flow. When I close the warming box exhaust, the temperature drops a good bit.

I can think of two potential issues, I'm not burning with a good bed of coals, the logs are always burning. Or my chimney needs to be a larger diameter. 

As a side note, I am not surprised by how much wood this thing consumes, but I did think once warmed up it would maintain temperatures better. The temperature at the grate can vary a great deal. I figured with all of the thermal mass it would be rock steady.

Any thoughts?

Thanks
 
I had used the Feldon calculator, but just ran the numbers through the one linked. They were relatively close to what I had previously calculated.

CC volume = 18400 in^3

                                recommended / actual

narrowist RF airpath - 73.6 / 104 in^2

air inlet -                   18.4 / 35.42 in^2

exhaust volume -       405 / 275 in^3

So it looks like my exhaust volume is low, which was my intuition since opening the warming chamber exhaust helped.

I also think I am loosing a lot of heat through my air inlet. Before my next cook, I plan on installing a heat shield on the back side of the FB door. It will redirect most of the air down/around, and allow some to pass over. But its main purpose is to block the heat loss due to radiative heat through the air inlet.
 
For the FB, upper and lower intakes are recommended...   The lower intake provides air to the fire for heat control....   The fire should be on a raised wood grate 3-5" above the floor for ash accumulaton...   The upper air inlet moves the heat from the FB to the CC...    If your FB seems too hot, the upper air inlet will resolve that issue...  When operating properly, with a large bed of coals, the FB should not be too much warmer than the CC, letting you know you are using wood efficiently...  

The FB/CC opening should be non restricted... allowing for good heat flow...

 
Yep toss that 3" stack and run a 5" or even 6" . Take pics of all the openings between. Fb/cc. Rf plate and also cc/ warming cabimet. I would not bother with a deflector plate for the firebox as most other smokers dont have it.
 
Is that some sort of damper the end of the tape is touching...    You don't need it...  I'd cut it out...  It is restricting the flow....


You sure cut that close to the desired opening...   One thing....  The air/heat etc. needs to take a 180 deg. directional change...  I'm thinking that flow is restricted from the 3" opening....  the "effective opening is probably about 1 1/2 -2" for the heat and air to move through...

 
The plate you see at the top of the FB is a baffle to restrict airflow from the main CC when flipped down to be flush with the FB wall. Above that there is a second opening that can allow all of the heat to flow directly to the warming box, in effect a smaller vertical smoker. I am confident that very little smoke flows directly into the warming box when the baffle is in the proper position. I say this because there is a second baffle inside the warming box that must also be opened and it has a very good seal. If I have the warming chamber closed off from all airflow it doesn't get but 150 degrees, probably from being right over the FB. I know the current design, with baffle in the FB, may cause a restriction, but I want to leave it in place for now. I will remove it if other ideas don't help improve performance.

My plan at this point, new exhaust with wider diameter. If that doesn't cause much change I will increase the area of the CC entrance by removing some of the RF plate. I will probably test the firebox door baffle as well, I have plenty of scrap and it can easily be removed if it doesn't seem to help.
 
In my opinion your exhaust is to small and your openings are to small. I built a 60 gallon reverse flow last year and my FB to CC opening is a little bigger than yours. First thing I think you should do is open up the opening at the end of the plate. This should improve the flow and put at least a 4" ID exhaust. That should help get the heat out of the FB and into the CC.

Kyle
 
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I agree with socal. I see from your first pic you have a hypertherm plasma. Would only take a few mins to zipp off another 2" from the RF plate to open it up and weld on another flat bar for grease dam.
 
One other possible problem is your firebox is on the small side. I used an 18" square firebox for my 60 gallon. I think you should be able to make the smoker work but you need good flow and good fire management.
 
Thanks for the advice, I will be sure to report back how things progress. 

In the same of being a good scientist for the community, I will only change one thing at a time. 

While I love my plasma cutter, it took more than a few minutes to cut through the 1/2" plate. Of course I had about 60" of linear cut to make and my unsteady hand is what made it challenging. It made quick work of 1/4" for sure.  
 
I would go 4" ID on the exhaust 24" tall. Anther thing to consider is the R/F plate being 1/2" thick is going to take a while to heat and since a reverse flow depends primarily on the radiant heat for temps you are at somewhat of a disadvantage until the plate heats up. You might try putting a weed burner in your tunnel and preheating the chamber to get a jump start on heating the plate.
 

I am happy to report that changing to a 5in chimney and raising the fire grate using bricks has made this into a smoker to brag about. The rate of wood consumption has dropped and I get much better control of temps as low as 225.

Thanks for the help.
 
Short big exhausts work. :) Glad you got her breathing properly. :)
I have a 6" x 24" on my 24" x 55" cooker.
 
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