A water heater comprising a heat pump device and a water tank is provided. A temperature controller operates the heat pump device based on a comparison of a measured water temperature and a set temperature. The water heater further comprises a source temperature sensor measuring the temperature of a heat source serving to provide heat energy to the heat pump device process. The source temperature sensor measures a source temperature and provides a source temperature value to the temperature controller. The temperature controller adapts an activation temperature in response to the measured source temperature. The water heater provides an improved accuracy of the temperature controller without unnecessarily reducing the life expectancy of the heat pump device. Furthermore, a corresponding method for controlling a heat pump device in a water heater is suggested.