Step 1: Build the Heat Exchanger
The heat exchanger is basically a coil of copper tubing that spirals up the inside a vertical chimney. We're trying to get as much heat from the fire as possible, including the hot air and smoke that rises up from it.
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.