Batch-box Bell RMH

I am planning on replacing my small, top-loading, inefficient, and otherwise crappy wood stove this summer. I would like to use the existing chimney, which is centrally located and the space around it will not accommodate an Evans style RMH because the bench would block a door or walkway. So I have been looking into a more vertical bell type of thermal mass. I would also like to build it as a horizontal feed batch style RMH for ease of daily use and to reduce the chance of smoke back (our region has some wild windy weather and I've heard from another RMH builder in the area that his J-tubes always had occasional smoke back so he now only does horizontal feed). The house is a rental and to get permission for this I need to build a heater that is user-friendly for future inhabitants and does not need to be repaired/rebuilt every so often.
To summarize, my design goals are:
1) vertical thermal mass
2) batch-box feed
3) durable materials that won't burn out
4) limited troubleshooting/repair for future users

I would be willing to spend a little more money on materials to achieve my goals such as ingredients for cast-able refractory or ceramic blanket type insulation or some such. But I don't want to purchase a zillion firebricks, or some fancy-pants NASA engineered materials though I know they could make a stupendous stove. Is there a middle way here that balances cost with ease-of-use and durability/ease-of-care? Is cob a feasible material for a bell type heater? I've been staring at images of brick-based bells and am having a little trouble visualizing the details for a cob version.

The house is 800 sq ft with decent insulation in a zone 6 climate. Any thoughts on system size?

Has anyone done a stove like this before? Batch-box Bell RMH using ceramic/cob/mineral type materials?

Of course I would love to also incorporate an oven, a hotplate, a yogurt shelf, and a little nook that makes ice cream and does my taxes, but as the first built-entirely-by-me RMH I should probably keep it simple.

Getting sufficient structural strength in a tall narrow vertical mass bell style heater using only cob is problematic. I'd build the stove Russian style using good ole' brick. If you like the cob look, the stove can always be plastered later on.

It's a lot of work, however. For an idea of about what you're getting yourself into check out this example. Here's a mostly step by step MHA workshop build of a compact footprint bell heater, with hot plate, and a black oven:

http://www.mha-net.org/docs/v8n2/wildac12g.htm

Garnet Morgan wrote:I am planning on replacing my small, top-loading, inefficient, and otherwise crappy wood stove this summer. I would like to use the existing chimney,

Your situation sounds almost exactly like mine!
We can race this summer, how's that sound?

In any case, here's one that's just what you're after. Big single bell, run by a batch box. Lots of photos.
http://batchrocket.hostoi.com/html/foto.html

I've started building a practice one in my barn. I'm using fieldstone and clay mortar. (Because I've got an awful lot of both.) Let's see if I've got a photo? Hmm, no,doesn't look like it. I'll try to get it later and upload it tomorrow.

Byron Campbell wrote:It's a lot of work, however. For an idea of about what you're getting yourself into check out this example. Here's a mostly step by step MHA workshop build of a compact footprint bell heater, with hot plate, and a black oven:
http://www.mha-net.org/docs/v8n2/wildac12g.htm

That's a pretty one! I'd say it's on the complicated end of things, with the double bell design and the bells and whistles.
There are performance gains available by going to a double bell, but for me, there's so much to gain by going from a bad woodstove to an adequate masonry stove, that it's enough. The additional gains by going to a more complex double-bell arrangement with flues... that's a ton of additional work and possibility for error, for just a modest amount of improved performance. Not worth it for somebody in my situation.

Now, to be totally fair, here's my experience: I've built some batch boxes outside. I've got a batchbox built indoors, and the bell around it is partway done. So I haven't finished a complete heating system, whether complex or simple, with its chimney, at all yet.

So what do I know anyway?

Byron -- that was my hunch about the cob not working well for a bell. I had actually just been looking at that Wild Acres project and it certainly is a nice little unit. I get the gist of it from the photos but I would need a more detailed plan to be sure about the brick layout. A white oven sounds more useful to me, but I suppose a black oven is better than no oven. The amount of quality mason-work with all that brick is definitely a consideration (I've got some folks who could be very helpful at a cobbing party, and not so much at a brick laying party).

Mike -- You're on! That's a great set of step-by-step photos. So, the gasses exit the heat riser and fill the bell, then drop down to the bottom where they enter those two towers that go up to the chimney? Why do they have openings on the front? For clean out? I would love to see pics of your project.

The more I look into masonry bells the more questions I have. I'm not sure about casting a large piece for the top and am thinking about multiple smaller bells (dragon heater style) with smaller refractory caps that could be purchased. Anyone here try for a DIY dragon heater? Or maybe I could just use Matt Walker's castable refractory formula for the bell tops. Thoughts?

The bell and masonry stove shown use a lot of fire brick to line their insides. To me, that is where cob might be a useful choice, as a lining.
My plans include cob over barrels or filing cabinets. The outside cold be brick or even tile.
As for capping bells, I have been investigating backer board, and slate tile or counter tops.
The woodfired oven enthusiasts have a lot to say about materials and heat.

First, check thoses links.

https://permies.com/forums/posts/list/43528#344410


And if you want a cheap bell, my opinion is to get either barrels, or square/rectangular metallic tanks, like old home heating fuel tanks, or tractor fuel tanks. And use thoses as single or multiple bells,. Covered with whatever material you can. If protected by a thin sheet of metal inside, even concrete can work well enough. So a whole lot of choice are availlable to you. Stones, bricks, even cheapish ones, you don't need firebricks in that case. Urbanite assembled with cob or the dreaded concrete. For europeans, a cheap way to get mass, which has to be layered with some binder, is roof tiles. Often they go for dead cheap in big lots, in secondhand ads.

Hth.

Max.

William Bronson wrote: The bell and masonry stove shown use a lot of fire brick to line their insides. To me, that is where cob might be a useful choice, as a lining.

That would sure save some cash! My understanding is that firebrick is used on the inside above the level of the top of the heat riser for its thermal shock resistance and common brick is used everywhere else for its cheapness. Would cob or a particular mix of cob stand up to the heat in lieu of the firebrick lining?

William Bronson wrote: The woodfired oven enthusiasts have a lot to say about materials and heat.

Of course! If they use cob to make wood fired ovens then it should be fine for inside the stove above the heat riser. Except that the super efficiently burned gasses of the heater are much hotter than the smokey fire of the cob oven. Again, can anyone vouch for the use of cob in such an extreme thermal environment? Or does it cool off enough by the time it hits the cob lined walls (relative to within the heat riser) that it's no biggie?

Satamax Antone wrote:First, check thoses links.

https://permies.com/forums/posts/list/43528#344410

Wow, this link (http://technologieforum.forumatic.com/viewtopic.php?f=19&t=27) provided me with a lot more clarity, Thanks Max. My German isn't so good though, I just looked at all the pretty pictures. Do you suppose the cast pieces for the top of the bell are using the same (presumably) insulative refractory mix that the core was cast from? What about the material for the slabs of the bench seat and the pieces that line the wall behind it? I'm wondering about the thermal shock needs of the materials for those locations. Will any cob/stone/earth type thing work just fine or does it need to be more heat resilient?

Satamax Antone wrote:And if you want a cheap bell, my opinion is to get either barrels, or square/rectangular metallic tanks, like old home heating fuel tanks, or tractor fuel tanks. And use thoses as single or multiple bells,. Covered with whatever material you can. If protected by a thin sheet of metal inside, even concrete can work well enough. So a whole lot of choice are availlable to you. Stones, bricks, even cheapish ones, you don't need firebricks in that case.

OK, so get that I can put basically whatever material I can find for mass on the outside of the bell(s). I am concerned about the durability of using large metal containers for the insides of the bell. I've read a lot about metal parts failing over only a few years. Granted, most if not all of that is for the heat riser and firebox. Is there a longevity concern for using a 55 gal drum as the bell form?

Garnet, the rocket 177 made from Peter's plans, at technologie forums is made out of refractory mortar, not insulative refractory. Nor is the top. They've insulated with kaowool or superwool 607. And it's not german, but dutch! A language which has few words in common with yours Some stones are refractory, like white quartzite. Some other ones are tough enough. Like slate/shale. You can make a "refractory cob", out of clay, sand ans sawdust, or finely chopped straw. Or even wood ashes, or coal ashes. Proportions are hard to get tho. Or you can also add grog (chamotte) to toughen the mix. The insides of a bell rarely exceed 600C° i think. That's too high for concrete, but well within the range of clay mixes.

A metal bell will may be rust, but that will take some time. Several years or decades.

But for the innards, feed tube, burn tunnel and heat riser; "metal is doomed!"

A 55 gal drum as a bell form is perfectly adequate. http://s65.photobucket.com/albums/h228/mremine/NYC%20Rocket%20Stove%20Build/