Last spring we built an above ground pond using an old billboard sign as a liner [billboardtarps.com] which cost us 1/10th what a 'regular' pond liner would have. In winter, the temp can dip below freezing here & we don't want to loose our fish, some of which have gotten quite large. So being a free energy mad scientist of sorts, I came up with an idea to heat the pond for next to nothing by pumping pond water through a heat exchange in my chimney. This whole project cost me under $50 and is keeping a 750 gallon [2839 liter] open air pond a temperate 70 F [21 C].
This project was a smashing success! Every hour that I run this, it saves us 1500 watts of grid electricity per hour which is what our 'old' pond heater consumed. The sky is the limit on this too. I could route part of this to heat our garage, my workshop, etc. Next year, I plan to route a leg of this to heat our green house - fresh veggies all winter!
Step 1: Aluminum vs. Copper
I found a 25' [8.2m] coil of 3/8" [0.952cm] Aluminum tubing online, shown in fig1 with a cig lighter as a size reference. I then measured the inside diameter of the stove pipe - my chimney is insulated & has a clean out plug in the bottom. I determined that my actual stove pipe was 6" [15.4cm] wide and I wanted to keep at least 1" [2.54cm] clearance between the heat exchange coil and the stove pipe.
I want to pause here for a moment, in case it isn't evident why I chose to use Aluminum tubing over Copper tubing - the same reason you don't throw copper coins into a fish tank.. using copper tubing will kill the fish.