4" Cookstove

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4" Cookstove

Postby mannytheseacow » Mon Jan 20, 2014 1:59 am

Waiting for some red curry sweet potatoes and banana to cook up on the RMH and dreaming about a cook stove up in my kitchen. I'm looking for some input from the experts here. I roughed out a 4" J mold today today to cast a test core for it.
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The primary purpose is for cooking and secondarily for heat. I've got the perfect spot, with a good exhaust location straight up through the roof. The major limitations are that I have 36" x 19" to work with for a foot print, and ideally I'd like to keep the height to 36", too.

I've got a plan that I think will work, using a 4" square J tube, using a 2" refractory core on all sides (riser cast right into the core). The outside of the box would be made of brick stacked on edge (3"), allowing for another 1.5 - 2" around the core that I could fill with zonolite for more insulation. The cooking surface would be cast iron or sheet steel (whatever I can scrounge up) measuring about 19" x 14". Coming out of the J, the flue gases would be directed down and into a second bell directly behind (4" x 4") and exit straight up into a 6" duct pipe. This design would allow a 2" air gap on surrounding the stove (between the wall and cabinets).

Any thoughts? Is the J too small to work? Would red bricks suck too much heat if used for the outside?

I plan on doing a test with the form I made, first out of clay and lime. But I'm not sure how the whole masonry part will affect the performance.
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Re: 4" Cookstove

Postby hpmer » Mon Jan 20, 2014 2:29 pm

4" is not too small to work, but it is borderline. If you have 6" flue, why not go with a 6" stove? MUCH more heat and it is always possible to run it less than full out if less output needed for cooking.

You'd have to possibly adjust the rest of the build to accommodate the extra 2" here, but it would be well worth it in my opinion.

I say this because I have built stoves ranging from 4" to 8" and am amazed at the improvement as you go up in size. In fact, I have a 4" powering a bbq at the moment that is entirely disappointing. I'm going to try one final adjustment before I rip it out and build a larger one, probably 6".

I don't think I'll ever build anything less than 5-6" again.

You could add additional dimensions outside your existing mold before you pour.

As long as you insulate well between the core and the outside facing, your material (brick in your case) shouldn't much matter.

Cool idea, looking forward to your updates.
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Re: 4" Cookstove

Postby mannytheseacow » Mon Jan 20, 2014 2:46 pm

Thanks for the input hpmer. I came up with the 4" square working off the dimensions of the space I have to work with. Basically I just reverse engineered the whole thing this way. The major factor is the width of the stove where a 4" square gives me a good balance between size and insulation of the riser. I also have very little riser height to work with. I hate to be the guy that asks for help and then just goes and does his own thing anyway so I'll go back to the drawing board and see if I can't possible squeeze a 6" in there.

On a separate note, I watched a Ernie and Erica video where Ernie was talking about heat Taurus and top gap distance after the riser. He said larger gaps are better for moving heat down to the mass while the small 1.5" gap is better for focusing heat at the top of the riser. Any thoughts? Might be useful for focusing on the cooking surface?
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Re: 4" Cookstove

Postby hpmer » Mon Jan 20, 2014 3:09 pm

I hear you on starting with the outside dimension and working inward. That's how I ended up with a 4" bbq that doesn't work worth a ....

Better, in my experience at least, to start with a core that's going to give the output you need and adjust from there. You could, for example, finish with something narrower than your 3" bricks, like tile on backer board or something.

I'm just saying the most important part is the core, not the outside finish. Lessons learned the hard way in my case.

Heck, even 5" will be much better than 4".

Finally, there's a delicate balance when trying to optimize the distance from heat riser to top of the barrel. It needs to be a minimum calculated based on your system size (2" for an 8" system, and less for smaller ones). The minimum will send more heat to the cooking plate and create a hot spot. You lose the hot spot as the distance increases, which means less going to the cooking surface and more downstream.

The calculation is basically the area of a cylinder that matches your system size.
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Re: 4" Cookstove

Postby matt walker » Mon Jan 20, 2014 4:55 pm

Ohhh, cool project Manny. I'm on the fence on size. If you just want it to heat up a cooktop, I think a 4" might work. On the other hand, there's a huge difference as hpmer states, between 4" and 6". Backerboard and tile makes a nice surround, and it's dead easy to box in. Use some plumbers tape to strap the corners, super simple to do any shape. As for the top, your stated size is perfect for a cast iron griddle as a cook top. Lodge makes one that size, and there's a much more expensive Camp Chief griddle that is really nice. A brick top with a little rabbet where the griddle can inset, and a piece of fiberglass stove gasket and you are there. Just set the griddle in the rabbet on the gasket.

I do think the 4" will be easier to hit your desired height and stay in the realm of the right ratios. No way to get a 6" that low without cheating the height or going angled with the riser, which grows your dimensions either way.
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Re: 4" Cookstove

Postby mannytheseacow » Mon Jan 20, 2014 6:57 pm

Great input, guys! The height is the big limitation, which prompted me to the 4" to begin with. I think even using an angled riser or a swooped riser I would still be limited to maybe 32" from the bottom of the burn tunnel at most, and that would be focusing the flame where I don't really want it.

I hate tile ( ;) ) but I like this backer board surround idea. That would really give me some room to play with and let me size down the other dimensions a bit. So forgive my ignorance, but what do you mean by plumber's tape? You mean that silicone thread tape stuff? and then just wrap it all around the backer board?

I could just smear the whole outside with a layer of mortar and give it some kind of texture or something; I think I'd like that much better. Would I have to use refractory cement for that to resist the heat? Last silly question for now- how would you connect brick at the top with the backer board?

My stupid questions aside, I see this developing a lot faster and cheaper than I imagined!
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Re: 4" Cookstove

Postby hpmer » Mon Jan 20, 2014 7:47 pm

I think any tape will do to hold it together while you work the rest. I'd probably just use masking tape to hold the corners. Matt may have meant painter's tape, the blue stuff that's maybe 2-3" wide.

And the insulation should keep temps way down this far from the flames, so no special refractory stuff is needed (read: use the free stuff like cob) ; )

Finally, you could use a shorter feed tube to make the rest of the ratios work. Would mean shorter fuel and therefore more attention, but you'll probably be standing right there anyway as you cook.

But really, it should work. If you start with 36" high and subtract 2" for your cook top plus gap, and another 2" for the bottom insulation, that leaves you with 32". Subtract 6" for the height of the feed tube leaves you 26" for the riser. 3:1 is the minimum suggested ratio, but this is a bit over 4:1.
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Re: 4" Cookstove

Postby matt walker » Mon Jan 20, 2014 8:23 pm

Hpmer, the difficulty I see with the dimensions is that when you calculate the way you did, it looks possible. Then you add enough insulation and material underneath it to be safe and pretty quickly you are well above counter height. I don't think 2" is enough of anything underneath if it's on a wood floor. Not to say it can't be done, it just gets tight real quick trying to keep a 6" system under 32" H. Not to mention then you have to fit the downdraft section in the same footprint. Tight.

By plumbers tape I meant that metal strapping with the holes in it. You can get 100' at Home Depot for $14. Strap and screw all joints with that stuff. And yep, you could skip the tile and just go with natural plaster like you finished the downstairs RMH with. Or tiny pebble aggregate, or broken glass mosaic, or...whatever. You shouldn't need refractory for this, natural plaster or even type S thinset mortar will work. At least, it did on my outdoor system that gets really freaking hot.

For the top, well, I'm assuming there's some infill between the core and skin that you could set the bricks on. Otherwise, hard to say without drawing up actual plans, but I'd probably build the core, build the surround box with backerboard, then infill with cob or cob/perlite and set the brick top on that. Or skip the bricks and use cob. There's probably a million ways to do it.
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Re: 4" Cookstove

Postby mannytheseacow » Mon Jan 20, 2014 9:40 pm

Hmmm. I think for now I'll start by casting the 4" square core and play with it with a barrel on it outside and just see what kind of fire it has. If it doesn't work I can use it for an outdoor bread oven or something. If it works well enough and I end up using it inside, this is what I've come up with (roughly; this might all change in 15 minutes). It would go on a ceramic tile floor, set on 3/8" of thinset over 1/2 durock, on top of 3/4" subfloor. If I put 2" of zonolite down under a 3" core, that will leave me 28" of riser. I feel comfortable with the insulation but not sure about my dimensions. Easier to change those now than later!
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Re: 4" Cookstove

Postby hpmer » Mon Jan 20, 2014 10:59 pm

Matt,

I think you're right about needing more than 2" underneath to protect the floor. All of mine have been for outdoor use and I failed to account for that factor.

Makes great sense to pre-build it outdoors and see what that puppy can do.
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