This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
This section does not deal with what commercial espresso
people understand by descaling, that is, disassembling the machine and
removing large amounts of scale using picks and long, strong acid
baths. It does deal with flushing home use machines relatively quickly
with mild acids to dissolve small amounts (5 grams or less) of scale.
Since the term "descaling" is also used for this procedure, and since
this has caused some confusion in alt.coffee conversations between
pros and amateurs, I'm going to call it "preventive descaling"
First I'll describe how preventive descaling is done on
various machines to give readers an idea of how much work is involved.
Then I'll describe various water treatment options to get the coffee
taste and preventive descaling frequency you want.