Abstract: Water temperature sensors in the form of thermisters are in physical thermal contact with the inlet and outlet pipes of a water heater and may be surrounded by insulating material to make them more insensitive to the ambient air temperature and more sensitive to the temperature of the respective water pipes and the water contained therein. These two thermisters are connected in a bridge circuit connected to an operational amplifier whose output will be of a given polarity only if the measured temperature difference exceeds a predetermined value. The operational amplifier's output is then used to adjust the effective thermostatic setting of the hot water heater in a two-valued control arrangement.