Abstract: A fluid heater, comprising a tank having an inner chamber for accommodating a fluid to be heated, a heat exchanger having a tube bundle and a shell surrounding the tube bundle, the shell being introduced in the tank so that a wall of the shell is in contact with the fluid in the interior of the tank, the shell having an inlet for introducing a heating fluid, the tube bundle having an outlet which communicates through an upwardly raising pipe with the interior of the tank and an inlet for introducing a fluid to be heated, so that when the heating fluid is introduced through the inlet of the shell into the shell heat of the heating fluid is transferred through the wall of the shell to the fluid in the interior of the tank and also heats the fluid inside the tube bundle so that the heated fluid from the tube bundle is supplied through the upwardly raising pipe into the interior of the tank, providing a heat transfer between the heating and heated fluid by forced and natural convection simultaneously.