Awesome work, I am very interested in your project. We have 3 Mack trucks that get the oil changed every 4 months. That is 33 gallons of oil every 4 months.
I'm thinking a copper water coil inside the pipe with the fire or just blasting onto a large tank of water, then two heat exchangers, one air
20x20 Water to Air Heat Exchanger New Front Mounting | eBay and one water to water, either homemade or bought. Like coil in coil, or like a solar water heater tank.