Abstract: A fluid heater may include an inlet operable to receive a working fluid at a first temperature, a heat exchanger coupled to the inlet and operable to heat the working fluid to a second temperature, a burner operable to supply heat to the heat exchanger, and an outlet operable to receive the heated working fluid and inject the heated working fluid into a hydrocarbon wellbore to reduce a viscosity of the hydrocarbon. The heated working fluid at the outlet may be pressurized to a pressure above atmospheric pressure, and the second temperature may be higher than a boiling point of the working fluid at atmospheric pressure.