DECALC is not a button. It is a light indicator that reminds you to perform EasyDecalc. Performing EasyDecalc helps maintain your appliance so that calc particles do not build up inside the boiler. It is highly recommended that you perform EasyDecalc every time the DECALC light blinks. When the iron is cool, usually 2 hours after ironing, open the EasyDecalc knob and let the water flow out into a cup or into the sink.