At a wedding a few weeks ago I met a guy (husband of one of J.’s friends) who is building a system to heat their domestic hot water via the wood stove. His set-up involves running a steel water pipe right through the stove (he has welding skills– similar off-the-shelf versions are available, though). A pump then circulates the water from the hot water tank, through the stove, and back. A controller senses temperature differential and only turns the pump on when it’s needed. (In summer, they use the same controller to run a solar system.) I’ll ask his permission to post a few pictures here– it’s pretty cool.
I love this idea. I’d like to build a similar system. And I think I can & will. But, my ideas sometimes don’t work out in practice– so my M.O., now, is to require on-site proof-of-concept at every stage that involves either (a) irreversible damage to something expensive, or (b) buying parts. So, before even thinking about drilling holes in the wood stove (which I doubt I can ever bring myself to do), I want to test a version that just uses the stove top for heat exchange. Step 1 was to estimate how much heat could, reasonably, be extracted from the top of the stove. Step 1a was to guess: the Luckily, because it’s almost Thanksgiving, it was easy to procure a cheap aluminum pan that almost exactly fits the stovetop.
A quick test revealed that water can easily be heated in the pan at a rate of about 2,200 BTU/hr, with the stove at medium-burn. At that rate, it would take about 5 hours to heat 20 gallons of water from 50F to 120F. That’s encouraging. We don’t typically burn the stove 5 hours a day, of course. But neither do we need to produce all of our hot water from wood heat– the idea would be just to make a big dent in it. And in fact, I don’t really know how much hot water we use– it’s probably less than 20 gal/day.
The next test was to put a 10-foot coil of 1/2″ copper tubing on top of the stove, and run water through it for a while to see whether this would be suitably efficient as a heat exchanger. Unfortunately, it was not nearly so good, capturing only 940 BTU/hr. At that rate, we’d only manage to heat perhaps 5 gallons per day. But, there is much room for improvement. I have given a lot of though to means of increasing the heat conduction. For example, the coil could just be set into a water bath in the aluminum pan. Some heat would be lost to evaporation, but since the cool water in the coil would keep the water bath from ever exceeding about 120F, this would be minimal. Another idea is to put the coil in the pan, then fill the pan with steel shot (or even, and my guru on this matter suggested, pennies). What I’d really like to have is a slab of copper with the coil cast in the middle of it– but that is not a DIY project! More tests to come, stay tuned.
Total project expenditure thus far:
Copper coil $22. Turkey pan $2.