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re: help with water heat using a wood stove
22 nov 2003
mike wrote:
>...i am running a .75 hose (also 1/2 copper) line to a garage and back.
>total of 300 feet of hose.
...150' one way.
>to heat the water i bought a coil of soft copper 5/8 tubing which i simply
>put inside the wood stove.
...212 f?
>i am having problems getting hot enough temps from the tubing.
>max seems to be about 40 degrees celsius.
...104 f at the far end?
>i insulated the buried lines with 1 inch of styrofoam. the return line
>and heating line are side by side in the insulation.
oops.
>i simply cut a groove in strips of 2 inch insulation and trapped the line
>within two pieces of the styrofoam.
like this, with a 16" perimeter, and 150x16/12 = 200 ft^2 of outside surface?
4"
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>i have two radiators from a hospital in the garage-each being about
>4 feet long. i have them connected in series.
big old cast iron jobs?
>total amount of water is about 5 or 6 gallons. questions:
>
>am i expecting too much?
maybe not.
>is it too little water to take the heat to the garage?
the amount of water isn't important, but the flow rate is. you mentioned
5-10 gpm, which sounds more like a guess than a measurement. you might
measure this with a bucket and stopwatch at the stove end. you might lose
(212-50)200ft^2/r5 = 6500 btu/h to 50 f soil, so more insulation seems
useful, as well as separating supply and return pipes... 5 gpm is 300 gph
or 2400 pounds of water per hour, ie 2400 btu/h-f, so the water temp might
only drop 6500/2400 = 2.7 f because of the heat loss to the ground.
>should i build a tank and somehow fasten it to the wood stove
>instead of the coil of pipe?
well, hchickpea thinks the coil may blow up, which doesn't sound good, but
it might gather more heat in the fire than outside the fire. then again,
it might gather carbon or creosote or burn through or undesirably lower the
wood combustion temp. i'd check the flow rate and measure some temps before
changing this. boiling the water inside the coil would raise the heat transfer
rate. an open tank above might condense steam bubbles and limit explosions.
my woodstove has a flat top, with a vertical steel 55 gallon water drum on
top with several plastic film drum liners inside. the water only gets up to
130 f, with no insulation around the drum, and that takes a few hours...
(130-70)22ft^2x1.5 = 1650 btu/h. not much. the drum weighs about 450 pounds,
which helps ensure a good thermal connection between the drum bottom and
the stove top. the drum bottom isn't perfectly flat. the center part dishes
down and touches the stove, but there's an air gap near the rim. perhaps
i should empty the drum and put a piece of plastic film over the stovetop,
then a 1/2" layer of sandcrete (mixed with ground copper?), then put the
drum back and fill it with water again to try to eliminate the air gap.
i put 150' of high temp garden hose in the drum to preheat cold water for the
electric water heater. you might circulate unpressurized water with no heat
exchanger, and some insulation around the drum.
i've considered replacing the drum with a 2'x2'x2" thick ferrocement pad with
some copper tubing inside, which might make a nice stew-simmering surface,
never much over 212 f, but i worry that the thermal resistance of the stove-
concrete interface and the concrete itself would lower the heat transfer rate
a lot, and making a pad like that isn't easy, and it would still only have
4 ft^2 of surface to transfer heat from the stove.
how would we get heat from a horizontal 55 gallon drumstove with a cylindrical
top surface into water? could we find a flexible material to make a "water
blanket" to drape over the top surface, or use the top surface as a mold for
a hollow or pipe-lined ferrocement saddle?
nick
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