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. Thus, the gases make three passes through the heater, transferring heat to thermal fluid that flows in the helical coils.

Heatec 3-pass heaters are available in horizontal and vertical configurations. Capacities range from 0.7 to 60 million Btu/hour.