This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
There's basically three choices: Using boiler safe water and
never descaling. Using water close to 90 mg/l hardness, 50 mg/l
alkalinity and descaling less frequently. Or using harder water and
descaling more frequently. The choice mostly depends on how much the
machine is used [see section 1.6], and how hard it is to descale.
Commercial operators have no choice. They must use boiler safe
water, otherwise their machines would accumulate 5 grams of scale in
anything from a few hours to a few days. Preventive descaling at these
intervals is impossible. Home machine owners who never want to descale
should follow the same course. The simplest way is to use RO water
spiked with enough tap water to get to a 30 - 40 mg/l level. This
course is absolutely safe provided the boiler is regularly flushed.
The hose end softeners sold by many espresso machine manufacturers do
not, according to my measurements, make for scale free operation; but
they will reduce descaling to quarterly frequency. My guess is that
the cartridge style inline ion exchange units for plumbed in machines
will be more effective, since they don't operate at elveated water
I personally think home machine users are better served using
neutral or harder water and preventively descaling. At home use
levels, the 5 gram scale accumulation criterion will only require
monthly to quarterly descaling, and the coffee taste will be at its
best whenever the water is at 90/50 or above, since the machine will
reduce it to that level by heating it. Whether or not to soften the
water down to this level depends on how hard the tap water is; if the
water is very hard, descaling could become a weekly chore, and such a
partial softening could be preferable.
There's several ways to do this. The conceptually simplest is
to boil the water for a few minutes; this removes hardness in excess
of coffee levels. I use my Brita jug for all drinking water, since my
water is cloudy from old pipes; its incomplete softening, or that of
the hose end softener in a warm tank, also makes for a tasty espresso.
Finally, tap water can be diluted with RO water to reach any hardness
level one wants.