A device for detecting and signalising a leakage in a double-walled conduit system of a heat exchanger for heating fresh water by way of heat from waste water in the sanitary field as well as a heat exchanger including such a device. The device has an elastic element and a signalisation unit, wherein the elastic element is essentially liquid-tight and the signalisation unit is configured to generate an acoustic signal and the signalisation unit is activatable by way of a mechanical deformation of the elastic element.