The EU-21 controller (with LED display) is designed to control the CH water circulation pump. The controller's task is to switch the pump on when the temperature reaches the setpoint and to switch it off when the boiler cools down below the hysteresis setpoint. This mode of operation prevents unnecessary switching on of the pump, which saves energy, extends the lifetime of the pump and improves its reliability. The EU-21 controller can also act as a thermostat, switching the pump on when the set temperature is reached. The controller also has an anti-stop function that prevents the pump from standing for out of season periods (the pump switches on approximately once every 10 days for 1 minute). The anti-freeze function is an additional protection designed to prevent the water in the appliance from freezing (when the temperature drops below 6 °C, the pump is switched on permanently).