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

Reuben Deumling 9watts at gmail.com
Wed Nov 9 11:44:08 PST 2011

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.
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?

A (water heater) pilot light is typically 400BTU/hr. I know that over the
course of 24 hours I can heat 20 gal of water in my water heater (not sure
by how much, but by this calculation--again assuming 60% efficiency it
would be 35F)
9,600 BTU * 60% efficiency of system = 5,760 BTU
divided by 166 lbs of water = 35

Lots of BTUs in that stick of firewood!

On Wed, Nov 9, 2011 at 10:02 AM, Corwyn <corwyn at midcoast.com> wrote:

> 20 lbs of wood @ 8,000 BTUs per pound = 160,000 BTUs.
> times efficiency of the system (e.g. 60%) = 96,000 BTUs.
> divided by 10 gallons of water * 8.3 lbs per gallon = 1,156 degrees F rise.
> Alternatively, 96,000 BTUs divided by (say) 60 degree temperature rise =
> 1,600 gallons of water
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