This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
High alkalinity and low hardness water protects a Gaggia,
since the alkalinity won't be reduced by scaling; while low
alkalinity, high hardness water is deadly, since even if the original
alkalinity is above the 50 mg/l safety limit, it will scale out,
making the water and steam corrosive [section 1.8]. This leads to the
For those who are willing to be painstaking: 1. Use good
coffee water (see above) 2. In the presence of calcium, steam
temperature will reduce the alkalinity to a corrosive level. If you
are using very high calcium water, even the brew temperature can
reduce the alkalinity to corrosive levels. Therefore, AFTER EVERY
ESPRESSO MAKING SESSION, FLUSH THE BOILER AND IMMEDIATELY TURN THE
MACHINE OFF. Descale on a regular schedule as explained above.
For those who want it foolproof at the slight expense of
taste: Start with hard and high alkaline water (higher than 50mg/l),
then soften it with an ion exchanger. The ion exchanger removes the
scaling calcium and magnesium, but leaves the high alkalinity intact.
If you use a small ion exchanger connected to the machine's hose,
don't forget to recharge it weekly. Avoid the Brita or Pur jug filter
for this, since their use of hydrogen ion replacement makes the water
slightly acidic, and leaves much of the calcium in.
If you have very soft water, or an RO system in the house, add
a small pinch of baking soda (sodium bicarbonate) to every liter of
water when you refill the tank. This will not be tasteable, and will
supply about 70 mg/l of non-scaling alkalinity, more than enough to
protect the boiler.