A core piece (3) includes a number of tubes (6) which are located in parallel and a number of rib elements (7) which are located between a pair of neighbouring tubes. An inlet tank (1) is provided at one end of the tubes to distribute hot water into each tube. An outlet tank (2) is provided at the other end of each tube to receive the hot water. An electrical heating device (9) is provided at a certain position of the core piece and contains positive and negative electrode plates and a heater located between them. A covering (20) for the electrical cabling is mounted on the inlet or outlet tank and has positive and negative cabling elements. The positive and negative cabling elements are connected to the positive and negative electrode plates respectively.
