I don’t recommend going out and paying $6000 for a wood-fired hottub, but Dutch Tub is a pretty genius idea for anyone looking for sustainable decadence. Works on simple convection: you build a fire inside the spiral of pipe, and the warmer water rises. Cooler water gets sucked into the bottom. You can insulate the fire for additional efficiency. Temperature is controlled by controlling the height of the fire-basket that is inside the spiral: raise it and less of the heating coil is heated. Granted, wood, while renewable, isn’t a resource that an entire global economy should depend on. But anyone who knows me knows I’m not interested in having a centralized global economy anyhow.