Fill the water tank with coffee machine descaler and water to 12 cups, the ratio will depend on the descaler instruction. 
If you do not have a descaler suitable for coffee machines you can instead use white vinegar or lemon juice, as they are natural descalers. Ratio for this is 4 cups white vinegar/lemon juice 8 cups water (filter water is preferred but tap will also be fine)
Position the carafe in place, select filter 12 cups then start, when you start seeing water come through to the carafe stop the brew and let it sit for 30 minutes to an hour.
Finish the brew by selecting filter then 12 cups. 
Then run 2 more cycles of filter 12 cups with just water to get all the limescale out and descaler out.

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