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help with water heat using a wood stove

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Posted by Mike on November 22, 2003, 4:08 am
Hello-Has anyone tried using a interior wood stove to heat water and pipe it
to anther room or building.  I am trying to do this and having problems.  I
am running a .75 hose (also 1/2 copper) line to a garage and back.  total of
300 feet of hose.  To heat the water I bought a coil of soft copper 5/8
tubing which I simply put inside the wood stove.  I am having problems
getting hot enough temps from the tubing.  Max seems to be about 40 degrees
Celsius.  I insulated the buried lines with 1 inch of Styrofoam.  The return
line and heating line are side by side in the insulation.  I simply cut a
groove in strips of 2 inch insulation and trapped the line within two pieces
of the Styrofoam.  I have two radiators from a hospital in the garage-each
being about 4 feet long.  I have them connected in series.  Total amount of
water is about 5 or 6 gallons.  Questions:

Am I expecting too much?
Is it too little water to take the heat to the garage?
Should I build a tank and somehow fasten it to the wood stove instead of the
coil of pipe?

Any help would be greatly appreciated as I expected this to work a lot
better.  A lot of people around here (Manitoba Canada) heat their entire
farm buildings using outside wood stoves but they have 1.25 buried pipe with
a lot more water.

Mike Annetts

Posted by Ecnerwal on November 22, 2003, 4:20 am

Congratulations - you've created a heat exchanger. Hot water from the
hot coil heats cold water coming into the hot coil, and becomes cool by
the time it gets to the far end...

You need to insulate the hot line from the cold line, as well as
insulating both from the ground. You probably also need to use somewhat
larger pipe to get a respectable flow rate over 300 feet. What sort of
pump are you using?

Cats, Coffee, Chocolate...vices to live by

Posted by Mike on November 22, 2003, 5:50 am

I am using a pump that pumps about 5-10 gallons a minute.  I don't know if
there would be enough contact between the two hoses to exchange much heat
but maybe.



Posted by Ecnerwal on November 22, 2003, 12:35 pm

This is the rating on the pump, or this is an actual measurered output
WHEN CONNECTED to 300 feet of small-bore pipe/tubing? If you go out and
read a bit more, look into "total dynamic head" - you have a lot of it.
It's the friction loss from pumping a given flow rate through a given
length of a given size of pipe. Your actual TDH is a bit fiddly to
figure since you have a mixture of sizes and lengths of each size, and
we don't know the exact length of each, but for instance, to get 10GPM
through 300 feet of 1/2" pipe, you need 600 feet of head, or 259 PSI at
the output of the pump. For 300 feet of 3/4" pipe at 10GPM, it's 84
feet, or 36 PSI.

Depending on the pump, you could very easily connect a pump rated for 10
gpm and get less than 1 gpm flow in practice. Perhaps very much less.

Cats, Coffee, Chocolate...vices to live by

Posted by Harry Chickpea on November 22, 2003, 6:07 am

Yes.  Way too much.

Among other things.  

Frankly, you should abandon the idea. A 150' run is too long, 1/2" hose or pipe
is too small, you will have a difficult time getting useful heat out of a stove
using any sort of retrofit, and even if it did work, an unvented coil of hot
copper pipe in your stove could develop a steam pocket that would ruin your

Grab a high school physics book and do some quick calculations.  You'll find
the problems are quite apparent.

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