The black-and-white grid is an analog temperature display, which is not very accurate. Red light means heating to the temperature you set. What ability do you use? Washing one person for 50 liters is no problem. The thermostat is not broken. If the temperature setting is not twisted in the wrong direction, you are using it improperly. A 50-liter water heater can make up cold water while producing hot water. It is good to really produce 60 or 70 liters of hot water.