Hybrid Car – More Fun with Less Gas

Re: Distributing wood burning stove heat

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Posted by nicksanspam on September 29, 2004, 8:20 pm

Wood is work. More insulation means less work...

NREL says Peoria is 27.0 on an average December day, with a 34.6 F
average daily max... 830 Btu/ft^2 of sun falls on a south wall.

Maybe you have a 10'x30' south wall. With a layer of polycarbonate glazing
(about $.50/ft^2 in 4' wide rolls, with a 10 year guarantee), you might
collect 0.9x830 = 747 Btu/ft^2 and lose 6h(80-31)1ft^2/R1 = 295, for a net
gain of 452, or 135.5K Btu for the wall. You could keep the shed 65 F for
8 hours if 135.5K = 8h(65-31)G, ie if the shed's thermal conductance G
= 498 Btu/h-F or less. G = 900 ft^2/R makes R = 1.8. Not much :-)

That might come from Grainger's $0.55 4C941 450 F 136 cfm blower with
their $6.38 4WZ05 speed control pushing cooled flue gas up a chimney.

A forced draft might go out a window...

You might control the heat with a thermostat (eg Grainger's $3.25 2E158)
that turns off the draft blower when the room's warm enough. Another in
series might turn on the blower when the stove is hot.

So long as you don't need a natural draft, why not suck more heat out of the
fluepipe? The draft blower might be near the outdoor chimney connection (if
any) and its long 6" fluepipe could draw air from the stove outlet. Meanwhile,
the middle part of the fluepipe could be inside a 10" pipe with a T and a 10"
to 6" reducer at the chimney end to make an air-air heat exchanger, with the
T mounted horizontally, with Grainger's $0.85 4C847 550 cfm 10" fan at the
far end of the T (also controlled by the room temp thermostat) pushing room
air into the T, through the space between the inner and outer pipes. The room
air would emerge warmer from the end near the stove.

Meanwhilst, the flue pipe would slope towards the chimney, and flue gas would
enter the blower from the 6" vertical pipe below the T, via an elbow with a
pinhole to let condensation drip into a bucket and exit the blower into the
outdoor chimney connection at X. A condensing chimney might produce at least
15% more latent heat than one without, with the same wood consumption.

The setup might look like this, in a fixed font like Courier:

            10" pipe  ______________________________
                   ___________________________      f
         6" pipe / -------------------------- \ <== a
                 ||   ----------------------\||/----n    --------
                 ||            10" elbow --> ||_________|   X    |
                 ||                          \  -->       blower |
                 ||                           \---------|________|
                 ||   |<--       L        -->|
draft    |         |
inlet--> |  stove  |
          ---------                       | drip |
           |     |                        |bucket|

If 20 cfm of 600 F flue gas (about 12K Btu/h of sensible heat) enters the L'
fluepipe and the fan pushes 400 cfm of 70 F room air into the 10" pipe and
we want a 212 F exiting flue gas temp, E = (600-212)/(600-70) = 0.73. Z
= Cmin/Cmax = 20/400 = 0.05 and E = (1-e^-(1-Z)NTU))/(1-Ze^-(1-Z)NTU)), so
NTU = 1.347 = AU/Cmin, = 3.14L/20 in this counterflow heat exchanger, and
L = 8.57 feet, or less, with condensation. L = 10' would be convenient.


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