Abstract: A portable oil field heater heats a liquid water-petroleum mixture by passing it through a single continuous path primary coil, and then a single continuous path secondary coil. The continuous path primary coil extends along the walls of the heater, and the secondary coil is a bundle above an LPG burner. The air for combustion is introduced from the top of the heater through false walls to beneath a false floor beneath the burner. Thus, it is not necessary to have insulation (particularly fire brick) along the sides of the heater; thereby greatly reducing weight and maintenance. The safety of the unit is greatly improved by introducing the combustion air from a height well above the level of the burner.