Step 1: Build the Heat Exchanger
We made our heat exchanger out of 3/4 inch tubing, various fittings for the ends, and 12" diameter cylindrical sheet metal duct. The whole thing ended up being about six feet tall. Click through the photos to see the exchanger under construction. Whit coiled the tubing, stuffed it up into the duct, and then brazed the connections onto the ends.
Whit says the copper got work-harded as he coiled it, so that first step was more annoying (and resulted in a less-regular coil) than we expected. But it worked just fine.