Melitta ME12DSB Owner's Manual page 15

Mineral Deposits / Decalcifying
Your Coffee Maker must be cleaned when you begin to notice excessive
steaming or the brewing time increases considerably. This condition is
caused by a buildup of lime and mineral deposits from your water. If the
pumping action stops before all the water has been pumped out of the
machine, then the appliance requires decalcification.
The frequency of decalcification depends on the hardness of water used.
It is suggested that this unit be descaled according to the following
intervals:
• Soft water: every 80 brewing cycles
• Hard water: every 40 brewing cycles
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