Boiler/BBQ/Oven/Mass Heater/Firepit

[matthewwalker]

Okay, well, I had an awesome weekend. My head spun around like five times I had so many ideas I tried to incorporate. Most of 'em, no surprise, originated with a kernel of an idea here, and blew up in my head into this impossible system.

I started on Thursday night, wanted a quick "L" chamber style, simple rocket cooking unit. I used only cob from my property, which I had a bit mixed up already from an earlier project, a 2' section of 6" flue for a riser liner. I molded up a quick and dirty L burner, using a 5 gallon bucket with the bottom cut off for an outer mold for the riser. Just pulled it up and off as I stuffed the cob/perlite down and formed the riser that way. I didn't really expect it to last long, but I didn't care.

I formed a crude burn tunnel with some scraps of wood and cobbed the thing up. Cobbed a small Weber grill to the top, after I cut a hole in the bottom. Put some clay bricks in there to diffuse the heat, and baked bread and bbq'd some burgers in there. I was eating an hour or so after starting building the thing. It was awesome.

That's when I started dreaming, and the trouble started. Next morning, I got my big "cob knife" out, which is just an old bread knife, and cut in a top feed, and cut away one whole side of the burn tunnel. Put a 14"x9" window along one side of the burn tunnel, and lit it off. So freaking cool!

Oh, crap, more ideas. I put a barrel on it, I built 4" thick cob walls into the surround of a bench at the exit of the barrel. I stuck a 4' section of 6" pipe down into that bench chamber, about 5" deeper than the barrel exhaust. I put a bunch of small sticks across the top as little trusses and cobbed over the top of the bench chamber. Yep, Donkey, I did a big version of your 6" stove chambered end. I just made the whole bench the chamber. I lit it off. It rocks.

I cut a hole in the top of the barrel. I cobbed the Weber grill back up there. Lid on, outdoor rocket mass heater. Snuggle on the bench, by the barrel, in front of the really warm, bright window at your feet. Bake bread in the Weber. Take the lid off, rocket cooker. BBQ food, stick a big pot of water in there, I boiled about 4 gallons of water for hours while I dried out the bench.

Yeah, my mind is totally blown today.

[matthewwalker]

Oh, I have another window. I think I will put it on the other side and make the Firequarium!

[hpmer]

Wow, that is fantastic!

How will you protect it from the elements? Also, how does it heat the bench if you've cut a hole at the top for the bbq attachment?

[matthewwalker]

Thanks, yeah, the protection thing....well, I'm not entirely sure but my current thoughts are some thin slate or the like for a bench top and top of the feed. Just need to give it some overhangs and I think it'll be good for a while. I built this thing in a very short amount of time, and with very little labor, so I'm also not opposed to just getting it through the summer and doing it again better when it needs some attention.

The hole on top is one of the innovations I'm really pleased with. Depending on how much you restrict the outlet up there, you can control what goes to the bench. Last night I got the webber grill cobbed in on top and put in a pizza stone, that just about completely closes the grill off, but has a 1/4" gap all around or so. To my surprise, the stove was drawing air IN there, and pushing it to the bench! Weird I know, and once it got really, really hot it stopped doing that. But, to answer your question directly, put the lid on the grill, the gases travel to the bench. Take the lid off, and most of the heat blasts past the grill, but still the bench flue shows some flow. It's really neat, and again the idea kinda came from Donkey's home system, where he said he could remove the iron burner over the riser and the flames would still follow the original path. It's really neat. Of course, it's a design for outdoors only, so tolerances for leaks and stuff are really loose.

[matthewwalker]

Here's an update on the outdoor system's new window layout...

[woodburner]

I must have one. I think it would be possible to fit a wooden "swing away" shelter over the top for protection, and with some VHT paint on the barrel it should look good for years.

Eek! is that smoke coming out of the stack or just steam?

[peterberg]

This is interesting, your bench do qualify as a bell with a dead end. The barrel and the chimney stack are very close to each other. Because you've placed the exit at a lower level than the entry, this is a good example of a bench which isn't a stream channel, at all. Very nice!

[matthewwalker]

Thanks very much you two.

Woodburner, just steam. I've rebuilt it almost every day, so you are seeing it running with soaking wet clay everywhere in the burn chamber/heat riser and bench. In that last video it's probably only an hour old, so there's a lot of steam.

Peterberg, it's amazing how fast I can heat the bench, and it heats fairly evenly out to the extremities. I'm very excited about the possibilities, it would be so very useful for folks who want a RMH on a suspended floor, or need quick response, or more even heat over a surface, and on and on. Again, I got this idea directly from Donkey's Six Inch Stove In a Tiny Hut thread, so all credit to him and you as well for the knowledge and inspiration you have shared here.

[hpmer]

What's the temperature of the bench? Is it too hot to sit on when firing the stove?

[matthewwalker]

No, not even close, but the latest configuration is weird due to the windows. When I just had a straight path to the riser, the bench was hot. It was still wet, so hard to tell how hot, but I suspect it would get too hot pretty fast. With the windows, I'm blocking off the burn tunnel to route the flame path past the window. That robs a LOT of heat from the system, and the bench is not too hot. I think it could be tuned to whatever the operator desired, and if you had the potential to over heat the bench, I think it could be controlled with the lid of the BBQ. Open it up, not much heat to the bench, close it down, hot bench. I think you could regulate it fairly well with some experience.

[matthewwalker]

One more update with some technical discussion of the window and burn tunnel configuration.

[woodburner]

Apr 23, 2012 6:22:34 GMT -8 peterberg said:

This is interesting, your bench do qualify as a bell with a dead end. The barrel and the chimney stack are very close to each other. Because you've placed the exit at a lower level than the entry, this is a good example of a bench which isn't a stream channel, at all. Very nice!

Can you explain how it's working Peter? I just don't see what's happening. To me it looks like a normal RMH but with the pipes under the bench missing, in which case I can't see how the gas flow is defined.

[peterberg]

Apr 26, 2012 14:13:47 GMT -8 woodburner said:

Can you explain how it's working Peter? I just don't see what's happening. To me it looks like a normal RMH but with the pipes under the bench missing, in which case I can't see how the gas flow is defined.

OK, I'll try...
Down here you'll see a link to a schematic draft of the bench, barrel and exhaust duct. Top view without the bench deck, side view into the bench. The bench is entirely hollow, exhaust duct is reaching to about 2" or 3" from the bottom.
The barrel is on top of the bench, the hot gases are streaming down. When entering the open space, these do very much want to stay in the upper regions. To the left there's a lot of open space and the hottest gases will drift that way. As a result, the cooler gases close to the bottom will stream out the exhaust.

That's all there is to it.

[woodburner]

Thanks, I understand now. Just need to make sure the bench top won't collapse.

[Donkey]

Is it just straight up cob?
As an experiment you could linseed oil it. Just keep painting it on till it won't soak in anymore.. I've got some wattle and daub outdoor shower walls exposed directly to weather, prepped in this way. So far so good.
One thing to consider is how the oil will react to the heat.. Might screw up, might not. Don't REALLY know.. You'd have to try a little test patch and see.