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

nick pine nick at early.com
Fri Nov 11 09:29:08 CST 2011


Gary BIs <gary at builditsolar.com> writes:

Reuben Deumling wrote:

>> * 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.

If the wood releases 20lbx8K Btu/lb = 160K Btu when burned and that all goes 
into 10x8.33 = 83.3 lb of water, it will raise the water temp 160K/83.3 = 
1921 degrees :-) Raising 10 gallons of 62 F water to 212 takes about 
(212-62)83.3 = 12.5K Btu, leaving 160K-12.5K = 147.5K, enough to boil away 
another 17.7 gallons. This could make for an interesting explosion.

>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.

A 400 F stovetop under a 200 F flat soft copper pipe spiral inside a 
1"x2ft^2 R0.2 reinforced concrete plate on top (fiber concrete under wonder 
board?) might put (400-200)4ft^2/R0.2 = 4K Btu/h into the water.

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

OTOH, the stove can burn hotter and cleaner and more efficiently if heat is 
extracted via a forced-draft chimney vs the firebox. The chimney might have 
a heat exchanger 
http://www.tractorsupply.com/wood-heating-coal-heating-accessories/united-states-stove-miracle-heat-6-in-heat-reclaimer-3195359 
blowing hot air through a car radiator, with some pressurized draft air for 
the airtight firebox.

Then again, wood is work, so solar is better.

Nick 





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