This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
Obviously, no one in their right minds would want to go
through the hassle of doing a boiler descaling at monthly or bimonthly
intervals. Fortunately, one can reduce the descaling requirement to
virtually nothing, even when running the 90 mg/l hardness, 50 mg/l
alkalinity water best for coffee by using this trick.
If the boiler autofill only runs to replenish water lost by
steaming, it only amounts to a few ounces per day. Suppose the water
in the boiler starts out very soft. Then these small additions of
moderately hard water won't bring it to scaling levels for at least a
week or so.
So once a week, put very soft water into your tank and flush
the boiler with it until the water has been softened down. The
flushing technique is the same as that described in the descaling
section. Then put your regular nuetral to moderately hard espresso
water back in and enjoy your shots with complete confidence. If you
regularly use boiler water for Americanos, etc., or do a lot of
steaming, adjust the frequency of soft water flushing to match [see
section 1.7]. If this is impossible due to very high use or boiler
inaccessability, you will have to go with a softer non-scaling water.
Straight RO can be used for this flushing since enough
minerals will generally remain to operate the autofill sensor. If the
pump doesn't go off due to low minerals, simply shut the machine and
replace the RO water with the regular water. It will only take a few
seconds of pumping to raise the mineral content enough to deactivate
the autofill. Alternatively, add 5% to 10% tap water to make it
Even if boiler safe water is used, the boiler needs to be
flushed occasionally. The frequency can be calculated using the data
in section 1.
If these procedures are followed, there's technically no need
to descale the boiler at all. Nevertheless, an annual or biannual
descaling is wise to pick up any stray scale that may have formed due