The one I built many years ago I used the burner off a Oil Furnace, had a coil rolled at the local fab shop from triple wall gas pipe 1" and mounted the burner to blow through the center of the coil. Not a steam cleaner but it heated the water to about 220 at 4.5 gal per min (almost steam), used a 4,000psi Cat Pressure Washer Pump to push the water thru the coil. Had a friend who worked on washers and he provided all the safety switches, these are a must because you can easily blow a unit up and someone could get burnt bad(Scalded). Never really liked steam to clean with, damages gaskets and painted surfaces, whereas hot soapy water cleans without as much damage.
