A heating system for a fluid includes a double helical coil of continuous tube, a burner, and a containment structure. The double helical coil has an inlet and an outlet disposed at a bottom end and a plurality of external radial fins spaced apart along a length thereof. The burner is centered in the double helical coil and disposed above the inlet and the outlet. The containment structure has a closed bottom end and a flue opening at an upper end. The containment structure is positioned above the inlet and the outlet to enclose the double helical coil and the draft-type burner. The burner heats the continuous tube to evenly transfer heat to the fluid while the fluid flows through the continuous tube from the inlet to the outlet.