Abstract: A heat exchanger is provided which has a heat exchanger coil made from a continuous length of tubing arranged in an alternating series of conical helical and reverse conical helical winds wherein the medium to be heated flows downwardly through the coil and the heated medium from which heat energy is to be transferred flows upwardly around the coil in counter flow fashion to provide for a compact and efficient heat exchanger. Two methods of constructing such a coil are disclosed which utilize one or more mandrels to form the tubing into the approximately shaped coil.