All offset vets I need your help for a friend

[cmwr]

I got a good friend who started doing comp BBQ a year ago. He bought a homemade trailered offset from a fella and it has been working fine. This week he built a new firebox for it cause the old one was rusting out and it is not drafting at all. He called me down to help him but I use UDS and have limited knowledge on offset smokers. So I snapped a bunch of pics and want to know if anyone knows what is going on by looking at the pics.

I will tell you what I know. His old firebox was a small round barrel connected to the large cooking chamber via a steel pipe right in the middle. It was not built as reverse flow and he is aware that reverse flow would be better but that is not the issue here. The old firebox had the transfer pipe exiting the apex of the firebox into the middle of the cooking chamber. This new firebox, as you will see in the pics, is square and has a large volume of the chamber higher than the transfer pipe. I suspect the heat is rising to the top and not transferring through. Another issue I saw was that his intake door, as seen in the pic, is higher than his grate. I think that may be a problem as well.

So what happens is he gets a roaring fire going with 4-5 logs and as soon as he closes the door, intake wide open, within a few minutes the fire goes out and smoke starts coming out the intake really bad. It is drafting backwards apparently. I also am not sure if his intake is big enough for the size of the chamber he has. I thought it looked pretty large but maybe it is not. He said he can crack the door just 1/4" or so at that point and the fire will start going again. So ???? Anyone got any knowledge to help him out? He has to have it ready by this next weekend. Thanks guys! Cory

 

[ClintHTX]

I'm not sure about reverse flow smokers but my offset is half the size of that and the hole from fire box to smoker chamber is much bigger. I followed feldons smoker calculator for mine. You might do a search for it and punch in your info and it'll tell you how big everything needs to be to be a fine tuned machine. Also, I leave my door open at all times so I can get a clean burning fire. More air flow cleaner fire.

 

[cmwr]

I'm not sure about reverse flow smokers but my offset is half the size of that and the hole from fire box to smoker chamber is much bigger. I followed feldons smoker calculator for mine. You might do a search for it and punch in your info and it'll tell you how big everything needs to be to be a fine tuned machine. Also, I leave my door open at all times so I can get a clean burning fire. More air flow cleaner fire.

If you leave your door open all the time what's the purpose of an intake door?

 

[Bludawg]

Without the dimensions of everything to run through the pit calc it's hard to say. Just by your pictures from what I can see the connection between the FB & the Main needs to be Much bigger, the stacks are to short and to small in dia for a proper draft. Basically your trying to pack 20 lbs of Stuff in a 2 lb sack through an eyedropper sized openings.

 

[ClintHTX]

My door has no intake. The door is either open or closed. Look at Aaron franklins fire box. It's built the same way.

 

[dwfisk]

My initial observation is the pipe connecting the firebox to the cook chamber it WAY to small and coming off the firebox at the bottom of the firebox is not ideal. The configuration in the pics is forcing exaust gasses/smoke back down into the fire and the small exit to the cook chamber results in slow/poor air flow.

Just my $0.02.

Edit: Just noticed those tractor flappers on the exhausts. Does he have a way to open those suckers up? FWIW I ALWAYS run my exhausts wide open regardless of what kind of cooker
I'm using. If he can't get the exhausts open he will never get air flow regardless of the other dimensions.

 

[cmwr]

Without the dimensions of everything to run through the pit calc it's hard to say. Just by your pictures from what I can see the connection between the FB & the Main needs to be Much bigger, the stacks are to short and to small in dia for a proper draft. Basically your trying to pack 20 lbs of Stuff in a 2 lb sack through an eyedropper sized openings.

But even if your right, and I am sure you are bludawg, it worked ok with the old chamber. Not saying it can't be improved a bunch but whats wrong with this firebox that you can see if anything?

 

[JohnnyB]

That firebox needs to be dropped a ton. The opening from the box to the smoke chamber needs to be at the very top. Looks like the design is an attempt at a reflow system to burn the smoke a second time, but it needs extra vents at the back to achieve this. You drop that box, make the opening to the chamber the size the calculator recommends, and you're in MUCH better shape. Like Blu said, get those stacks taller too.

 

[cmwr]

That firebox needs to be dropped a ton. The opening from the box to the smoke chamber needs to be at the very top. Looks like the design is an attempt at a reflow system to burn the smoke a second time, but it needs extra vents at the back to achieve this. You drop that box, make the opening to the chamber the size the calculator recommends, and you're in MUCH better shape. Like Blu said, get those stacks taller too.

K guys I will pass the info on. what about the fact that the draft door is higher then the fire grate?

 

[cmwr]

My initial observation is the pipe connecting the firebox to the cook chamber it WAY to small and coming off the firebox at the bottom of the firebox is not ideal. The configuration in the pics is forcing exaust gasses/smoke back down into the fire and the small exit to the cook chamber results in slow/poor air flow.

Just my $0.02.

Edit: Just noticed those tractor flappers on the exhausts. Does he have a way to open those suckers up? FWIW I ALWAYS run my exhausts wide open regardless of what kind of cooker
I'm using. If he can't get the exhausts open he will never get air flow regardless of the other dimensions.

Yes he runs them wide open. Those are just to keep rain out when it is sitting idle.

 

[BBQ Freak]

I am expert on building pits but heat rises and it will not the way it goes to the smoke chamber , it is positioned to high and too small of pipe for air flow .

 

[cmwr]

I am expert on building pits but heat rises and it will not the way it goes to the smoke chamber , it is positioned to high and too small of pipe for air flow .

That's what I told him from my own common sense. The heat is getting trapped at the top cause the transfer pipe us in the middle

 

[Grif054]

The problem is in the pipe from the firebox to the pit. It is nowhere near big enough to handle the amount of air movement from the firebox to the cooking chamber. Also, the firebox itself could stand to have a little bit bigger intake as well. The firebox ends up starving for oxygen and that is why it is trying to also draw it from the cooking chamber because both directions are limited flow. Even when you are cooking with the door open there is only so much heat that could travel from the firebox to the pit. There is a lot of wasted energy going on there.

 

[Bludawg]

But even if your right, and I am sure you are bludawg, it worked ok with the old chamber. Not saying it can't be improved a bunch but whats wrong with this firebox that you can see if anything?

The old FB was smaller and mounted lower so the air was moving faster in a natural draft. The way it's designed now all the heat is hitting the top of the FB once it gets hot the fire can't breath because there is no place for the hot air to go.
If it was me I'd move that FB to the end cut a hole in the main and the end of the FB and zip it together and plug the old hole in the main, close off one stack and add a bigger one on the opposite side. It looks to have enough intake to run proper.

 

[cmwr]

The old FB was smaller and mounted lower so the air was moving faster in a natural draft. The way it's designed now all the heat is hitting the top of the FB once it gets hot the fire can't breath because there is no place for the hot air to go.
If it was me I'd move that FB to the end cut a hole in the main and the end of the FB and zip it together and plug the old hole in the main, close off one stack and add a bigger one on the opposite side. It looks to have enough intake to run proper.

I will run this stuff by him bludawg

 

[ButtBurner]

for a test, tell him to try building a fire in the smoke chamber to pre heat it as he gets his fire going in the firebox. Once its hot and a draft is going it might breathe then.

But that would just be a crutch for now.

as been stated the design has a lot of issues

 

[SmittyJonz]

Why does he use a transfer tube ( which I agree is way too small and should be at top of FB)? He loses a lot of radiant heat benefit vs firebox cut into cook chamber.

I agree FB needs to be lowered with a bigger tube at top or raise cook chamber and bigger tube at top of FB.

 

[oldbill]

yeah, I don't know how you could ever get good temps in the cooking chamber trying to get enough airflow through that little pipe!:shock: Everyone who's already chimed in has got you pointed in the right direction though, the great and unique thing about this forum is that there's always a very knowledgeable person (or two or three) on the spot to help.:thumb:

 

[landarc]

I would scrap that firebox, it is a terrible design.

1. way too much head room, the area above the outlet serves no purpose, but, will trap heat and smoke, creating an area where the gasses build up and serve to starve the fire.

2. The inlet is too small relative to the volume of the firebox, the real balance of a smoker is that the exhaust, intake, and all areas where the heat and smoke move, have to be balanced.

3. He doesn't have enough volume of firebox below the fire to have decent airflow around the fire as ash develops. He can neither raise his fire grate, nor lower the bottom of the firebox in this design.

4. The connection between the fire box and cook chamber is too small for the new size of the firebox, and honestly, with the two exhausts, it would still be the restriction. You really want the connection to provide as much air flow as is possible to leave the exhausts