The filling volume of the circulation chamber in water heaters of the above-described type is between 450 ml and 200 ml, preferably between 400 ml and 220 ml, and particularly preferably between 300 ml and 230 ml. This filling volume also comprises the filling volume of the nozzle. Each nozzle as such has a filling volume of about 7 ml. The illustrated embodiment is normally integrated in a cooling water circuit in a vehicle which has a volume of about 3 to 6 liters. At least one heat exchanger for heating air in the passenger compartment can be integrated into this cooling water circuit. In addition or alternatively, the cooling water circuit may also have heat exchanger surfaces for technical components of an electric vehicle to give these the necessary operating temperature in case of cold ambient temperatures.

The partition 6 separates the previously mentioned circulation chamber Z from a connection chamber A, which serves the electrical connection of PTC heating elements 12 in the way to be described later. Heating ribs 14 project from the partition 6 into the circulation chamber Z. The heating ribs 14 extend at a right angle from the partition 6.

FIGS. 2 to 4 illustrate details of these heating ribs 14.