[Cob] wood stove - Clarification

[Shannon C. Dealy]

On Thu, 26 Aug 2004, Quinn wrote:

> There has been so much talk about trying to make a stove do something
> it's not intended to do: heat a bench.  It doesn't sound like a safe,
> environmentally sound, efficient or useful idea.
[snip]

In a couple of the most recent postings as well as this one, it is not
entirely clear if the poster is refering to all wood stoves or just
conventional wood stoves (as opposed to the rocket/bench stove).  So to
clarify or correct the postings:

  - Trying to use a conventional wood burning stove design to heat a
    cob bench (without some significant changes to the way it works)
    is probably a bad idea from a pollution, safety, and environmental
    perspective.

  - A properly designed and built rocket bench stove (which is also
    a wood burning stove), is:

        Safe

        As environmentally sound as is possible given that the fuel
        you are using is wood.

        Not only the most efficient wood burning stove system available,
        it is for all practical purposes, the most efficient one that is
        possible, limited almost exclusively by how much thermal mass
        your "bench" has to dump the heat into and how much piping you
        run through it.

I know that a number of people on this list will probably want to argue
the last one, but I am not exagerating, so to put an end to the
argument before it starts, the only correct way to define efficiency in
this context is the percentage of the total energy available in the wood
which is actually delivered to the interior of the building.  This is
measured by the amount of the fuel that is oxidized (burned) and can be
measured by examining the components of the exhaust leaving the building,
with 100% combustion, all that will you will see is ash (with no carbon
content) and steam, you will also have no creosote build up anywhere in
the exhaust system.  The second part is heat delivery (which is where
conventional stoves fail to even come close to the bench design), this can
be measured by the exhaust temperature as it leaves the building, the
closer it is to the interior temperature of the building, the less heat
you have lost/wasted.  With a rocket bench stove design and a large enough
bench, you can easily drop the exhaust temperature below 100 degrees F.,
where conventional stoves even when "shut down" in order to burn slowly
can still exhaust at a minimum of a few hundred degrees F. (and this is
when their combustion is least efficient, so you get better heat transfer
while increasing pollution and wasting fuel).  When you open up the
conventional stove, their combustion efficiency is much greater, but their
exhaust temperature is MUCH higher as well, so a large percentage of the
heat is being dumped outside the building.

The rocket bench design achieves for all practical purposes 100%
combustion efficiency, and while the heat transfer efficiencies are not
quite as good as the combustion efficiencies, that is more a function of
design choices rather than a limitation of the system.  Most people don't
have room to run hundreds of feet of ducting through benches in order to
get every last bit of heat out, though 30 or 40 feet folded up inside a
smaller bench does a pretty good job and can still give you exhaust
temperatures in the 100 deg. F range.

NOTE: the only wood burning stove systems that come close to the rocket in
terms of total efficiency are some of the russion and northern European
mass stove designs which use a high temperature fire and a folded flue to
achieve similar heat transfer results, though these are very expensive to
build, and still can't match the efficiency of the rocket bench design.

I know we have wandered a bit off topic here (even though these stoves
are built into cob benches), and I apologize to those of you who aren't
interested.  So let's try not to post to much more on this unless you
truly have something new to add.  You are welcome to email me privately
with questions, though buying the Cob Cottage book may be simpler.


Some of you know this stuff, but to be up front about it:

I have designed a number of these stoves, both conventional and
experimental, and worked on quite a few more designed by others.  I do not
receive any money from Cob Cottage, though they are friends of mine, and I
did help them with the stove book (reading and commenting on several
drafts of the book) as well as writing a review for it.

FWIW.

Shannon C. Dealy      |               DeaTech Research Inc.
dealy at deatech.com     |          - Custom Software Development -
                      |    Embedded Systems, Real-time, Device Drivers
Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
   or: (541) 929-4089 |                  www.deatech.com