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

[Gary BIs]

On 12:59 PM, Reuben Deumling wrote:
> Do any of you know a source for info on the following question?
>
> * design of simple wood-fired DHW calls for a coil of copper tubing either 
> wrapped around the stove pipe or configured into a plate that would be 
> attached to the outside of the firebox thermo-siphoning to a 10 or 20 gallon 
> storage tank in the attic. That tank of course would be plumbed into the 
> household water system. The question is if there are basic calculations that 
> would allow someone to know what sort of heat transfer rate he could expect?
>
> I.e., 20 lbs hardwood/160,000 BTU burned yields XY deg F increase in 10 
> gallons of water.
>
> Anyone?
>
> Thanks.

Hi,
There are some articles here: 
http://www.builditsolar.com/Projects/BioFuel/biofuels.htm#FireboxCoils

The articles give some hints at heat output -- like this external coil one:
"How much hot water will the exchanger produce? Well, during a typical 
seven-hour cycle we would load 55 to 60 pounds of wood into the Atlanta 
Catalytic, which would boost the 42-gallon tank's contents to just short of 140° 
F. This eight-pound-per-hour burn rate is probably somewhat higher than most 
folks use, so the volume of hot water that you might get from a similar unit 
could be slightly less. Of course, if you maintain a strong burn all day long, 
the total over a 24-hour period should still be more than 100 gallons per day of 
plenty-hot water. And even if you often operate your stove in a "closed down" 
condition, the system will significantly reduce your utility bills."
Read more: 
http://www.motherearthnews.com/do-it-yourself/woodstove-water-heating.aspx?page=3#ixzz1dF0uYXN2

The outfit than makes the inside of box coils gives some hints at how much water 
each model will heat up.

I think that anything coupled to the outside of the stove or the outside of the 
chimney is going to have low efficiency, but if the wood stove is on a lot 
anyway, maybe that OK.

Some people don't like the idea of coils on chimneys as it may effect draft to 
the point of making problems.

Gary