Cob: Cob Bench Stoves

[Shannon C. Dealy]

On Wed, 9 Jan 2002, Robert Bolman wrote:

>     A number of people have done heated cob benches which I've always
> seen as a poor persons's masonry heater.  They use a rocket stove type
> design to do the initial combustion and then run the combustion gases
> through (typically) six inch stove pipe embedded in the bench after
> which it goes up and ultimately exits the building.
>     
>     My understanding is that they suffer from a few problems.  For one
> thing they can draw very poorly because of all that horizontal
> flu.  Also, I understand that six inch round stove pipe isn't the most
> conducive shape to transfer the heat to the cob.

I have done some work with rocket bench stoves, and have spent quite a bit
of time talking with Ianto and reading about the design of wood fired 
stoves (I'm building a wood fired foundry).  Based on this experience, I
think a few observations may be helpful:

   1 - If the stove draws poorly, it is because of either a design or
       construction problem with the specific stove, there is nothing
       inherently wrong with the bench stove design.  I have seen bench
       stoves that didn't draw properly, but in each case, once the
       problem was diagnosed and corrected, the bench stove worked fine.

   2 - All that horizontal pipe in the bench is simply an exhaust pipe,
       it is not a chimney, the chimney which drives the system is located
       in the large barrel at the end of the bench.

   3 - While it is true that a round stove pipe isn't the most conducive
       shape for transferring heat, if you use alot of pipe (either a
       longer bench or running the pipe back and forth through the
       bench), you can extract pretty much all of the available
       exhaust energy from the stove.  The bench stove built at Cob
       Cottage's Coquille, Oregon site last Fall used 37 feet of pipe and
       had an exhaust temperature of 105 degrees F. (taken at it's warmest
       point in the center of the pipe).  Another stove I have heard about
       with an extremely long exhaust run has an 85 degree exhaust 
       temperature.  Alternatively, a fairly simple heat exchanger could
       probably be designed and built using simple sheet metal tools (or
       possibly adapted from an existing commercial unit) to more
       efficiently extract the heat from the exhaust and transfer it to
       the bench with much shorter runs.

>     Now that we're "post Y2K", I want to design a heated cob bench
> using a little blower to facilitate the combustion within a cast
> refractory "rocket elbow".  Then I would plan on smaller diameter pipe
> traveling a greater distance to fully transfer the heat into the cob.

Based on my experience, I wouldn't bother with the blower, if the stove is
properly designed, it won't gain you anything but it will waste some
electricity (not to mention requiring electricity which could be a problem 
during a power outage).  In addition, it might be possible that the blower
could actually decrease the efficiency of the stove by adding to much air
(cooling the combustion chamber) as well as by forcing the exhaust gases
through the system faster so they don't have as much time in the exhaust
pipe to dump their heat. 

You might consider taking one of Cob Cottage's Pyromania workshops (the
bench stove version), I found it very helpful, and I think you might find
it useful with your own stove design.


Shannon C. Dealy      |               DeaTech Research Inc.
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