Basically, we took an inflatable hot tub out to the Ocean Beach here in San Francisco, filled it with salty ocean water, built a bonfire, set a heat exchanger up on top of the fire, and pumped the saltwater through the exchanger until we had a nice, hot tub to lounge around in.
Here's how we did it!
Note: Thanks to our photographer friend Loren for taking some pictures at the beach. The well-lit nighttime photos are his. Naomi Dabby took our intro photo, below, at Priceless.
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.