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re: how to keep wood stove fired at maximum output?
18 nov 2003
jonathan mau  wrote:

>got a wood burning fireplace insert recently.  it has a maximum output of
>75,000 btu/hr.

you might ask the manufacturer how that was measured.

>my house has a heat loss of 50,000 btu/hr (+-30% say) at -40 degrees c.

i wonder how you know that. have you tested it with a kwh meter and
an electrical heat source (turn on all the lights and appliances for
12 hours overnight?) and indoor and outdoor thermometers? air leaks
come to mind... 

>the temperature was no worse than -10 last weekend and i could only just
>get the wood stove to keep the temperature up.  this would have been
>a heat loss of 1/2 the worst case 50,000 btu/hr.  the problem is that
>one only gets decent output when burning smoke - the charcoal phase
>is probably only 1/4 to 1/10th of maximum output.  if i keep feeding
>new wood, eventually after 12 to 24 hours the charcoal bed builds in
>height and i can't get new wood into the stove.

a different draft arrangement might help. i saw several simple wood gasifier
designs at the sustainable resources conference in colorado last month. one
was a $60 coffee-can campstove with a aa cell and a fan that blew a little
(preheated?) air in under the fuel to gasify it and blew more air in above
the fuel to burn the gases... you might add a grate and a separate air inlet
below, and another air inlet above to burn the wood gases, with some way to
vary how much air goes to each place.

you might recover more stove heat and preheat combustion air with two
concentric stovepipes, with stove gases going up the inside pipe and cool
outdoor air coming down between the inner and outer pipes or down through
the space between a new stovepipe and an original chimney, with a high temp
blower (eg grainger's $80 4c941 136 cfm 102 w blower with a 450 f max input
air spec and a speed control and a thermostat to turn it on) sucking hot air
out at the bottom.

if 10 cfm of flue gas enters the inner pipe at 600 f at the stove and
100 cfm of 30 f outdoor air enters the space around the inner pipe at
the top of the chimney, the heat capacity rate ratio z = cmin/cmax = 0.1.
with 16' of 6" inner pipe with 1.5 btu/h-f-ft^2 of inside and outside
air film resistance, a = 25 ft^2 and ntu = au/cmin = 25x1.5/2/10 = 1.9
and x = e^(-ntu(1-z) = 0.185 and e = (1-x)/(1-zx) = 0.83, ie this heat
exchanger would be 83% efficient, and, theoretically-speaking, the flue
gas would exit at 600-0.83(600-30) = 127 f, with thermally-desirable
condensation inside the (stainless steel) pipe. the flue gas would lose
about (600-127)10cfm = 4.7k btu/h of heat, and outdoor air would emerge
near the stove at about 30+4.7k/100 = 77 f.

cooling the flue gases will decrease the stove draft, but the combustion
temp might actually rise, with preheated air, and the blower will slightly
pressurize the house, which will tend to make up for the decreased natural
draft. pinholes in the inner pipe could be a problem. i wouldn't try this
without a good co detector. this could also function as an air-air heat
exchanger, without a fire. in a perfectly airtight house, you might turn
on the blower with a humidistat.

nick




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