A fluid heat exchanger for use in a fluid heating system is disclosed that includes a rapidly heatable inside tube surrounded by a hollow outside tube for heating a fluid flowing between the inside tube and the outside tube for circulation through the fluid heating system. The substrate section includes doped regions that have a heavily doped N-type region and a heavily doped P-type region adjacent to one another. The identifier may be an email address or user identifier. In addition, since integral action is not effected in the adjusting portions, the term of 1s2 is not included in transfer functions of inputs and outputs, thereby realizing stability of control. In a preferred embodiment, the heat absorber comprises a planar heat pipe.