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re: environment problem with outdoor furnaces?
28 jan 2004
ecnerwal   wrote:

>the typical waterstove is a creosote factory. they get loaded up with 
>wood in a combustion chamber which is directly surrounded by water, so 
>there is always a cool (200f or less - cool to a flame) surface 
>condensing creosote around the wood; this prevents efficent combustion. 

i've heard much the same from the masonry stove association. a clean burn
seems to require a hot firebox. an efficient burn seems to require a low-
temp or (ideally) condensing chimney.

>they are controlled thermostatically to smoulder until the water cools, 
>then blow for a bit to burn wood, then smoulder, then burn. they often 
>have a smokestack which is all of 5 feet off the ground, and tend to 
>spew smoke sideways.

thermostatic control is a good idea, in general. forced draft makes lower
chimney heights possible, with lower initial cost. these days, why waste
lots of heat and chimney materials to make a draft that is better provided
with a small fan that's also responsive to the amount of heat needed?

>intelligent wood-burning water-furnace designs combine a heat store of 
>several hundred gallons with burning all the wood loaded full-bore, and 
>have very low emissions, through use of high-temperature secondary 
>combustion in refractory tunnels.

it seems to me that the inherent thermal mass of a typical house can store
enough heat for a reasonable overnight setback, eg 70 f at 8 pm, and 60 f
at 8 am. and for a given heat exchanger, burning wood at a low rate could
be more efficient than "full bore."

>a qualified indoor wood furnace could (with use of antifreeze, etc) be
>installed outdoors if preferred, so as to not drag wood into the house...

outdoor woodstoves have other advantages, eg indoor space savings, less
indoor mess, a higher firebox that's more convenient for people, bigger
chunks of wood which require less cutting, and greater fire safety.

i suggest a horizontal 55 gallon drumstove in an box near a house window
with a fan to circulate air between the house and the box through two 8"
round ducts in the lower part of the window, eg grainger's 61 w 550 cfm
4c847 $72 10" round fan and 2e158 $14 room temp thermostat. at its upper
limit of 158 f, the fan might move about (158-70)550 = 38k btu/h.

the box might be about 2'x2'x4' long, made from $11 4'x8'x1" sheets of atlas
energy shield "r7.2" double-foil polyiso board (which begins to discolor at
about 400 f), with a 9'x6" chimney with 7'x6" and 7'x8" heat exchange pipes
inside an 8'x1'x1' downdraft passage of an 8'x1'x2' vertical foamboard duct.
a 6"x5' pipe costs $5.98 at home depot, 8"x5' pipes are $7.97, 8"x2' pipes
are $3.25, 8" elbows are $3.48, a 6" chinese hat for the top costs $7.35,
and a 10" to 8" reducer to mount the fan costs $5.75. we might put the box
on top of 4 stacks of 3 8" blocks and tie the chimney down to the blocks
with guy wires.

the fan could blow room air from the lower part of the house window into an
8'x1'x1' updraft chimney flue passage, then down past the heat exchange pipes
in the other 8'x1'x1' passage, around the drum and back into the house window,
providing more combustion air through the drum door when house heat is needed.

if 10 cfm of smoke flows out of the drum at 1200 f and out the chimney
at 100 f, after condensation, we need a heat exchanger effectiveness
e = (1200-100)/(1200-70) = 0.973. with 550 cfm of room airflow and a rate
ratio z = 10/550 = 0.02 and e = (1-e^-0.98ntu)/(1-0.02e^-0.98ntu) = 0.973,
ntu = 3.67, and a = ntuxcmin/u = 3.67x10/1.5 = 24.5 ft^2, approximately.

we might put a 6"x7' steel pipe with end caps inside an 8"x7' pipe, and
unsnap 2 2"x6' edges of each pipe and bend 1"x6' ls along the edges and
screw the flat parts of the ls together with some stove cement, like this,
looking down, viewed in a fixed font:
8" pipe .          . 6" pipe
        .  smoke   .
        .          .
        .     1"   .
        . ..........     
        . .
      1".s. (screw)
  1"    . .
......... . ---

        gap  4"       inside

......... . ---
  1"    . .
      1".s. (screw)
        . .
        . ..........     
        .     1"   .
        .          .
        .  smoke   .
        .          .

so room air can get inside the 4" gap to the inner pipe, and attach an 8"
to 6" reducer to each end of the 8" pipe to make smoke travel up between
the pipes, with about 7pi/12(6+8) = 25.7 ft^2 of heat exchange surface.


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