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re: heating water with a woodstove
12 may 2000
georges  wrote:

>>>it's not useless because it works.
 
>>well, maybe it's just expensive...
 
>...i'm still not sure which part you think is expensive,
>certainly not the coil...

maybe "expensive" is the wrong word.

extracting heat from the flue vs the stove itself can be more
energy-efficient, with a lower-temp flue gas going up the chimney
and a cleaner and hotter fire, since no heat is directly removed
from the stovebox. we might even heat water in summertime this way,
with a well-insulated stovebox. but (as with solar hot air systems),
removing a large fraction of the stove's heat output from the flue
means having lots of "expensive" gas-to-solid heat transfer area.
i keep thinking about these problems:

1. putting the coil around the flue pipe makes it higher up in the room,
which means the tank has to be still higher for good thermosyphoning, so
the tank becomes more difficult to support and we run out of headroom.

2. cooling the flue pipe encourages creosote buildup, which adds
thermal resistance to the inside and tends to block the chimney. 

3. we need lots of flue pipe area to collect heat from the flue gas.

4. that part of the flue pipe (4' to 8' of it--another potential
headroom problem?) needs to be thick in order to conduct heat (sideways,
parallel to its surface) into the spaced copper coil, and in order
to be strong and durable. steel isn't a good heat conductor compared
to copper (8x better.) decreasing the coil spacing increases the cost.

5. a copper coil that is simply wrapped around the flue pipe has 
a metal-to-metal line contact at best, if it's kept tight and
there's no out-of-roundness. not much heat transfer area. tubing
is often soldered to solar collector plates in order to raise the
metal-to-metal contact area and lower the thermal resistance. that
might work here, if the stove is never fired up without water inside
the tube. how else can we raise the fluepipe-to-tubing conductance? 
squish the tubing into an oval shape around a very strong fluepipe
using special equipment, as in a gfx greywater heat exchanger?

i'd like to make a hydronic top (simpler and cleaner and more useful
and universal in kit form than trying to custom-fit pipes inside stoves)
with a thermostatically controlled pump and a nearby unpressurized
hot water store (eg 8 plastic 55 gallon drums under an insulated
workbench surface.) a basement version of an outdoor wood boiler,
with lots of thermal storage.

in wintertime, a room thermostat would turn on a fan to circulate
room air among the drums. in summer, that wouldn't happen, and
the stove would be infrequently fired with wood, newspapers, or
certain non-toxic plastics like polyethylene. heating 8 drums to
150 f and letting them cool to 100 before firing stores 180k btu
of heat, 3 or 4 day's worth of summertime hot water.

(with a $75 high-temp blower from grainger providing suction near
the chimney entrance, the flue gas might also run through a 4"
serpentine smokepipe under the workbench. if it cools enough,
we could dispense with the chimney or add an electrostatic
precipitator, eg an ion generator inside the pipe.)

the dhw heat exchanger might be 150-300' of 150 f garden hose coiled
in one of the drums (with a float valve to shut off the water if it
begins to leak) or a couple of 4" pvc pipes above the drums with some
bulkhead fittings and garden hose adapters. cold water would enter
this heat exchanger from a cw basement laundry tap and exit into the
hw tap or into the drain fitting of a conventional water heater.

overheating the heat store should not be a problem, provided there's
enough thermal capacity in the drums and the room air circulation fan
turns on when they get too warm... 

nick




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