I bought a government surplus diesel fired engine coolant heater last week for $10, new in the box. It is 23,000 btu input and 15,000 output to the engine. It hooks up with a thermosiphon setup using 1/2" npt fittings. The heater runs on 24 volts dc. Amp draw when starting is 11 amps (fuel pump and igniter) and 1.1 amps run current once the fire is lit. It works slick. These were used on trucks, tanks, and the like. The junk guy says he can get more of them. I'm planning to use this one with my off grid pv setup for domestic water heating. The boiler drum is stainless steel. It is about 8" diameter and around 18" long, including fuel pump, solenoids, thermostats, ect.