BBQ Pit engineering question..

[Guamaque]

Hello, I built a custom BBQ pit myself awhile ago, and have since made some modifications. I posted some pics of the new mod, the old pit pics are on here somewhere about a year ago. My problem is I have problems keeping the temp up to my target temp, 250-275 in the cooking chamber. I converted my over under BBQ to an over under - offset BBQ. Probably the only one to have such a beast. The reason was, the over under was always too hot for low and slow. So I converted the storage chamber to the left as the pit, as I originally planned to do, but was too lazy to go through with it at the time. So before, I used an open draft system, had a fully open chimney and a draft air inlet control in the pit chamber. It worked ok, but ran too hot. So I converted the BBQ to an offset design. But, I also added a new pit temp controller, and it works fine. You can see the copper air intake tube, bottom left of the pit, that the blower attaches to. The problem is that in the top cooking chamber, on the right in the pic, it never gets to where I want it at 250. I have the control temp probe about 1 foot from the top of the chamber. I also have quality analog probes in place and they all agree. I would like to know if anyone has any ideas on how to improve this. Should the chimney be closed down with a damper when using a power draft to keep the heat in. Vs. an open draft where you want full flow out for heat and airflow in . . And with such a vertical pit, where do you think the best place to measure temp for the pit controller should be. It varies tremendously every 6 inches. I even have a analogue dial in the chimney which sometimes runs higher than the cooking chamber...

[Guamaque]

Actually I think I realize the problem. I will just tear it down and start over !! My wife always says, you never get it right on your DIY projects, until the third try.. LOL.. This will be my third try.. It does seam to be a historical fact for some odd reason..

[QTEX]

IMO yes you need to put a damper on the flute to slow down the "draft" so the whole are in the cooker equalizes temp better. You can start a bigger fire and choke it down and it should hold temp well. I sort guessing though the pics too small to really tell whats going on. You have to find the balance between fire and airflow to get it to work correctly. If you look at Backwoods and Pitmakers and WSM's they all operate a a very little airflow concept in comparison to normal stick burners. I'm sure others will chime in shortly so dont tear it down yet.

[Flying_Spaghetti_Monster]

Beautiful doors man :)

[smokeyokie]

Dont Tear it down!!! just lift the whole thing up and send to me I think I can tinker with it and make it work!!!LOL....

[Just BS]

I'm no expert, but I would think that this is a common problem. You don't state what the different temps are. or even how many degree differences there are between each rack, so its kind of hard to say.

Personally, I would just keep tweeking it. Find the sweet spot and cook there when you're not loading that thing up to its gills...and then when you do load it up, just rotate as needed.

Nice work.

[Guamaque]

Thanks. I screwed up the design so I'm trying to salvage it. Some good questions so maybe more can help. The pit temp, on the left lower door, runs hot as expected from a burning wood fire, 500' roughly. I have an analog probe in there. The blower runs most of the time and the wood I consumed last night was insane, a ton. So the fire, air and heat are being produced as needed in the pit. I planned this design to use with a blower only, as there are no air vents present, just the 1 inch metal tube through the wall of the pit for the blower. So its definitely airtight. The grill racks are on the right. About midway in the top metal door is my therm probe, about a foot from the top of the chamber. Probe enters from the side through a hole drilled in the cement. I also have an analog there and the temps are consistent with each other. I can not get that temp to get close to 250' at all, it stays at 180-200 ' , even after burning a ton of wood and the pit temps going 5-600 ' So the problem is I cant get any heat to where it needs to be. I do get great smoke, it would be a great cold smoker, but I don't have the patients to BBQ a chicken for 10 hours, like it took last night. A few of my ideas, please chime in. The meat chamber is too tall, the smoke gets cold by the time it gets to the grill rack. I did dampen the chimney, it just seamed to make it smokier, not hotter. Or, maybe I just suck it up and move the fire box back to the right lower door area, that is where it was, but then it ran too hot. I wanted it to be offset. I could put an aluminum sheet metal insert to guide the heat from the firebox up to the grill rack also. One last point, the entire pit / grill combo is cement and firebrick, so even with the fire blazing , the top of the cement fire pit on the left doesn't even get hot, the thermal mass must be overwhelming as well. Very lost at this point. Thanks.

[Just BS]

What's "too hot" (when the fire is below the food)?

Different foods cook at different temps ~ use both. If it's too hot, you could use a heat diffuser or maybe some type of tuning plate.

Fire bricks on the bottom and up the sides in both chambers? How big is your opening between the offset and the pit?

[Guamaque]

Originally Posted by Just BS

What's "too hot" (when the fire is below the food)?

Different foods cook at different temps ~ use both. If it's too hot, you could use a heat diffuser or maybe some type of tuning plate.

Fire bricks on the bottom and up the sides in both chambers? How big is your opening between the offset and the pit?

Thanks, I think those are the issues I will try, the opening could be bigger, I sledged it out bc is was a solid cement wall before. I'm guessing about 1 ft by 18 inches. I was thinking of the firebricks as well, to narrow down the cooking chamber. Too hot was 350' with a broken pizza stone as a diffuser. Thanks for the tips, I'll keep experimenting.

[chad]

Nice build.
As you have probably noticed, offset is not as "efficient" as verticle. But, it is "traditional".
I would measure the temp at the cook grate(s) as it makes not a rat's ass of difference anywhere else in the cooker.
Now, this is just personal observation, but you may be depending too much on technology and not enough on learning management of the fire. I have a sneaking feeling you need to spend a few learning sessions actually sitting there monitoring the and temps...you may need a bigger stack or a bigger vent at the fire box...with any decent fire you should easily maintain 250-275...I'm thinking you need a bigger air inlet. But, I'm sitting here arm chair quarterbacking. I've got a build of my own to get on with so thanks for the reminder. Again, that's a nice looking build.

[mallorjc]

I'm gonna have to sleep on this (and meybe sober up to read your whole issue). For now, Nice Build, bro. Can't imagine (didn't read) u have a problem.

[Guamaque]

Thanks dudes. I think Dave may have got me thinking WTF did I try to convert it to an offset, I should have just tried to figure a way to tame the fire down with the over under. I got on a kick about all these cool offsets and said, hey I can still do that. lol. Thats me, overfixing everything. Thanks for the advise everyone. I will experiment today. I may use it as a combo by shifting wood from under , to the side based on heat or something like that.

[BigBellyBBQ]

think about the heat asbortion rate of a masonary build and in my opion, the fire box should be sealed with plate and under your main cooking chamber because as you stated "consumed a large amount of wood.." you must keep flues open....the steel plate will help transfer the heat and the masonary will protect / insulate..I used 1/2" boiler plate in mine.. if you have a 500 fire box and cant hold 250 tells me you are not efficient in transfer...smoke really does not cary heat unless your moisture is high..ghood luck..

[Guamaque]

Originally Posted by BigBellyBBQ

think about the heat asbortion rate of a masonary build and in my opion, the fire box should be sealed with plate and under your main cooking chamber because as you stated "consumed a large amount of wood.." you must keep flues open....the steel plate will help transfer the heat and the masonary will protect / insulate..I used 1/2" boiler plate in mine.. if you have a 500 fire box and cant hold 250 tells me you are not efficient in transfer...smoke really does not cary heat unless your moisture is high..ghood luck..

Ok, thank you, I think you are on to an answer, but please explain to me in detail what you mean. Are you saying that I should line it with metal on the interior. I was thinking about that, the fire now just goes straight to the brick and cement with no radiator deflector. Would a simple sheet metal deflector keep temps inside rather than absorbing into pounds of cement and brick ?

[mallorjc]

I like the idea of fire-box reflective insulation from heat absorption by the brick and think you should consider the 1" tube being too small to allow proper flow / hence cold smoke at top of cooking chamber. This just looks too "almost perfect" to tear down!