Rocket BBQ

[hpmer]

Hi Manny,

The exhaust is a 5' high, 7" piece of HVAC duct. Not super high, but higher than the grill itself. I've built a few cores (all have been pure cook stoves where the pot is placed at the top of the heat riser and used for maple syruping), but this is the first stove I've made with the riser enclosed by a barrel and this introduces variables with which I have no first hand experience, hence my questions for those of you that have some.

The core itself is made from the mix I've used successfully for several stoves and is mostly a mix of clay and ash and the dimensions are mostly good, although I did have some sagging of the riser as it was drying out.

I think most of the problems lie in the issues I've laid out above, and I'll work through them one at a time.

Finally, don't misunderstand, I'd love to have a heater in my home, but there are others that need to be convinced before it would happen. I haven't given up, but am also trying to be realistic, so the BBQ version is a way for me to use it more frequently than the 3 weeks of sap boilin' season.

Cheers!

[mannytheseacow]

Right on, hpmer. I think you've got a solid plan there, and I hope that thing starts burning properly for you.

[hpmer]

So, this weekend I loosely closed up the oven portion of the firebox with a bunch of insulative, broken off pieces from other experiments. That got most of the air flow moving in the right direction and really improved the quality of the burn. Still only reaching 300* in the grill so I think Matt's idea of expanding both openings to 4" is going to be needed, but for now I'm more concerned with the quality of the fire. 300* is enough to cook on so it is usable as it is.

Next step is to officially close off the oven and add an extension to the burn tunnel to get that relationship back to a more normal one. The extension should also improve the fire creep/smoke back issue as I really think the metal handle is causing the temps in that area to rise substantially.

I have cast the patch bricks out of my ash:clay mix with a bit of chopped up ornamental grass leaves in as my straw alternative. They are air drying now and I plan to fire them in my other stove before installing them over the next couple of weeks.

Making progress, although I found a chipmunk living in my exhaust channel and making escape holes through the wet cob. If it's not one thing it's another...

[matt walker]

Cool man, thanks for the update. I'm excited to hear about others using this method to cook, it really is a game changer in my opinion. You will get a huge improvement from enlarging the openings, 3" means that what little flue gas can stream through the center is all pulled outwards like a cone of water in a fountain, so you are getting almost nothing in the grill. Go bigger and you'll be burning your food, I guarantee it.

[hpmer]

[matt walker]

[hpmer]

Ok, here's an update. I want to work through the issues one at a time to see what I can learn in the process.


The stove in question.



The barrel handle, cobbed in, helped until the bottom of the patch fell off, confirming it is part of the problem.

[hpmer]

Next step is to add space to separate the feed tube from the barrel.


Here I cut out a section near the barrel to accept a "patch" brick.


Patch brick ready for firing in my other stove.


Patch installed, and handle cobbed over. A piece of rebar holds wood away from front of the feed tube. Fire creep is now gone, I think due to the combined impact of moving the wood further from the barrel by extending the burn tunnel with the patch brick, and insulating the barrel handle. Quality of the fire is much better, but temps in the bbq still maxed out near 350*. That might be partially because the patch needs to dry out. I noticed the steam from the drying cob being sucked down into the burn tunnel, which I'm sure lowered the temps of the fire. Still, quite a nice improvement.

[matt walker]

Hpmer, your dimensions look all sorts of wrong on the burn chamber overall. Is it as shallow and wide as it looks? That's going to be a big part of your low temps. Dimensions matter, a lot. If you can, get that burn tunnel to be square rather than short and squat, and build up the feed to be a proper feed tube. That would be a good starting point, anyway. Burn tunnels that are wide and low are notorious for performing poorly. Forgive me if I'm wrong, it's possible the photo angle is making it look that way.

As for temps in the BBQ itself, another possibility is your exhaust. How tall is that? You might shorten it a bit, or put a damper in it, or just obstruct it at the top, partially. That will do a lot to send more gas through the bbq.

[hpmer]

It's much better than it looks. It is 9" deep, 3" insulated mix above a 6" burn tunnel. I have to cob over the oven on the far end and square it off; I just have temporary pieces of old bricks covering it at the moment. Good thoughts on the exhaust, I'll try that next.