[Cob] rocket stove

[Shannon C. Dealy]

On Mon, 1 Nov 2004, Lisa King wrote:

> I had a question about the longevity of the rocket stove. I have  concern
> about the metal barrel burning out over time, I know my old barrel wood
> stove eventually gave out. It just doesn't seem like it would be very easy
> to replace the barrel.
> I'm wondering if there are any thoughts on
>     * how long it would take for a barrel to give out.
>     * How one would replace it.

As one of the people who rebuilt the bench stove in Linda and Ianto's
house a few years ago after about six or seven years of operation (it
still worked, it just wasn't drawing as well as it used to), here are my
observations:

  - Main barrel appeared fine then, and is still in use today.  Much of
    the oxygen inside the stove should have been used up by the
    time the gases reach the barrel, so the rate of deterioration is not
    as high as one would expect, but in time, it will have to be replaced.

  - You do have to chip out/break apart the cob around the barrel in order
    to remove it, but I don't remember it being that difficult to get the
    barrel out, and to reset it just requires a batch of fresh cob and
    some finishing work once the cob has dried.  It's probably a two day
    job for one person to pull the barrel and rebuild the stove, but of
    course this will depend on the stove design and the person.

  - The primary problem with the system was a build up of ash and broken
    bits of cob partially blocking the exhaust pipe area, right where it
    comes out of the barrel.  The stove pipe or ducting that was used for
    the initial section of pipe into the bench from the side of the barrel
    had been eaten away over time (it was much thinner metal than the
    barrel).  This means that a good design should plan for some means of
    cleaning out the entire exhaust system all the way back to the barrel,
    though if it's not feasible to handle the whole system, try for a
    setup that will at least allow cleaning the first several feet of pipe
    next to the barrel, as this is where ash build up will be the
    greatest.

>     * Or if there is a material that might last longer.

Other materials might last longer, but will they radiate heat as well?
Are they readily available?  Ultimately, any fire based heating system
will require periodic maintenance as the heat and air slowly eats away
at whatever materials it is made of,  and even if it can't be damaged by
oxidation, heating and cooling most materials over and over again will
usually break them apart.  If you really need it to last longer, your best
bet may be to either make a replacement for the barrel out of much heavier
sheet metal, or dispense with the barrel completely and replace it with
some kind of cast ceramic material (though the heat cycling might break
this up faster than the barrel would burn through, I don't know).

> On the same vein, in the rocket stove book Ianto talks about the bricks and
> how no matter what type you use eventually they will break down. I wonder
> if there is any idea on how long this could take and how we would go about
> replacing or even knowing such a thing happened.

When it happens, bits of the brick will fall in and start blocking the
interior of the stove, at which time (depending on the amount and how much
it blocks), the stove will either stop working, or just not work as well
as it once did.  As far as replacing it, you would have to pull the barrel
and rebuild the interior.

>
> One of my concerns is things breaking inside where I can't see it and some
> type of smoke filling the house while we sleep or some type of oxygen
> stealing event like you hear on the news.

As with any combustion based appliance in your house, get both a
carbon-monoxide detector and a smoke detector.  A few years ago we found
my grandmother was being poisoned slowly by a gas furnace (there was a
recall out on it, but nobody informed us), her health improved noticably
in the few weeks after the furnace was replaced.  Combustion of any
sort inside a house, particularly the tightly sealed houses we make today
is inherently dangerous, so take precautions when using gas or wood
burning appliances.  As far as "oxygen stealing" events, I guess I haven't
heard about these, though there are low oxygen sensors in some gas
appliances, so maybe there are low oxygen detectors similar to smoke and
carbon monoxide detectors.

> My other concern is that this might only be a viable stove for maybe less
> then ten years and if that is the case I'd like to think on that in my
> design if possible.

I'm not sure I understand what you mean by viable, do you only consider it
viable if it doesn't have to be worked on after ten years?  All systems
that I have encountered require maintenance, the only question is how much
and how often.


Shannon C. Dealy      |               DeaTech Research Inc.
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