A heating device for heating of a fluid, particularly in a vehicle or vessel, comprising at least one heating element in a housing with a house wall and a fluid inlet and a fluid outlet. The wall or a part of this or a part of the house wall is a heat conductor for conducting heat between the heating element and at least one of the fluid channels. At least one chamber is defined by the at least one wall and a portion of the housing wall, or by walls in pairs between the fluid channels. The heating element is mountable in the chamber, which has an opening towards the outside of the housing. The opening is not fluidly connected to the fluid channels. The heating element can be a PTC element.