[Greenbuilding] heating water with wood stove - heat transfer/efficiency calculations?

[Corwyn]

On 11/9/2011 2:44 PM, Reuben Deumling wrote:
> Interesting-thanks for the calculation, Corwyn.
> So if I run this another way, I find that I can raise the temperature of
> 10 gal of water by 60F (from 45F to 105F) with 0.125 lbs (2 oz) of wood.
> That seems fishy.

And it is.
60F increase * 10 gals * 8.3 lbs/gal * 1 BTU/lb = 4,980 BTUs.
4,980 / 60% efficiency (a guess for a good wood fired boiler) = 8,300 
BTUs.  So 1.0375 lbs of dry wood.

> I wonder if what we're neglecting here is that the fraction of the
> usable heat generated by the wood burning (60% here), only xy% is
> actually entering the coil of copper tubing. If that fraction is, say,
> 1% then the amount of wood required would be 100x more or 12.5lbs of
> wood. That strikes me as a more realistic figure. Anyone care to comment?

If instead of a wood fired boiler, you are using a wood-stove to heat 
water as a side effect of heating the house, yes, the efficiency of the 
wood -> water heat transfer might be quite low.

I should note, at some point along the progression of improving 
efficiency of your house, you are going to reach the point where you are 
using more energy to heat hot water, than to heat the house.  I suspect 
I am already there.  Given that, if one intends to use a wood-stove to 
heat hot water, it should be efficient at that at the expense of heating 
the house.

Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
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