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

[Frank Tettemer]

Reuben,

I like what you're doing; but it is really challenging to come up with 
hot water from a fire that only needs to burn brightly for a few hours a 
day.
Here's three different experiences of setting up DYI wood fired hot water.

 >> During the seventies, I tried 100 feet of soft copper, wrapped 
around six inch single walled flue pipe, with the 40 gallon tank 
positioned so that there was about ten feet total rise, from the lowest 
portion to the highest portion in the closed loop.  With the stove 
burning during mid winter, the water got warm only, not hot. we tried 
wrapping the outer portion of the copper coil with fiberglass and 
builder's aluminum foil, which helped. But it still wasn't great. There 
was hot water only during the minus 20 degree days, with the fire 
roaring day and night.

 >> What has worked well is a custom made double walled flue pipe, made 
from heavy stainless steel, and filled with water. The water column is 
three feet high, inner flue is six inch diameter, and the outer tube is 
eight inch diameter. Thus there is close to five sq. ft. of surface 
area, of the inner flue pipe in direct contact with the water. The heat 
exchange area is functional.

Compare this to the coiled copper pipe, wrapped around a flue pipe, 
which touches the flue pipe only on the copper pipe's cross sectional 
tangent. This set up has less than a sq. ft. of surface area. The heat 
exchange is very slow.

 >> In a different example, from my own stove, which is a Waterford Wood 
Stanley (cook stove), I first wrapped copper pipe back and forth, 
covering about three sq. ft. of the back of the cook stove. The copper 
pipe also only touches the stove along it's tangent, which is a very 
small area, even though there is over fourteen feet of copper pipe. This 
area is covered with fiberfrax, (high temperature insulation), and then 
covered with a steel shield.  The water gets warm, only, not hot. so an 
additional length of pipe was added, after this pre-warming pipe, and 
this pipe enters the firebox, There is about three feet of 3/4" pipe, 
directly in the path of the flame. Now, it gets Hot!

So a fire lit in the morning, and burning all day will result in a 
fourty gallon tank of hot water by evening or late afternoon. This is a 
rise from 20 degrees C, to 45 to 50 degrees C.  It takes about four 
firebox loads to achieve this, approx. ten to fifteen sticks of wood.  
In mid Winter, when the stove is firing harder, (minus 20 degrees C 
outside), we get 60 degree C water by evening.

The down side of my very successful hot water heater is that there is no 
CSA or UL sticker on this apparatus.  That means, that should I ever be 
inspected, or require an insurance claim for problems related, I'd be up 
a creek without a paddle.  Thus, I've never built such a thing for most 
customers, as most customers either want insurance, or have a mortgage 
which requires it.

In Canada there are very few factory installed hot water pipes, coils, 
or boilers that are UL or CSA approved. Pacific Energy stoves, Heartland 
stoves, and Esse are the only ones I know of with CSA stickers.  It is a 
very small market.  Only us off-gridders or sustainable greenies seem to 
want such a feature.

Frank

-- 
Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884