Thermal fluid heater employs a fired burner and two helical coils, one inside the other, to provide up to twice the life of other three-pass heaters while providing the required amount of heating. Hot burner gases pass through the center of the smaller coil and double back between it and the larger coil. They then travel back between the outer diameter of the larger coil and the heater shell. Its lower flux rate means that its helical coils will be subjected to much lower thermal energy, especially radiant energy, than is usual in a three-pass heater. This also means that thermal fluid flowing inside the coils will have a much lower film temperature. To achieve the lower flux rate, the heater is somewhat larger than a three-pass heater with higher flux rates.