I have been in the business of buying, restoring and selling pianos
for 43 years.  Every time a piano comes in to me with one of the
add-on humidity devices installed, I immediately throw the unit
and everything associated with it in the trash.  I do the same with
the aluminum tubes installed in pianos which are supposedly sold to
homeowners with the idea that they will be good for the piano.

Any device installed anywhere in a piano cannot do any good; just
the opposite, in fact.  The entire piano must be surrounded by air
with elevated humidity in the dry winters of the Northeast, the dry
deserts of the Southwest, or anywhere else.

All the parts which are made of wood -- including action parts,
sounding board, pinblock and the rim itself -- must be humidified,
and dehumidified in damp environments, at the same time and same rate.
Otherwise, uneven expansion and contraction will occur, severely
damaging a piano, or any other instruments made of wood for that
matter.

In addition, over-humidification, especially in a localized area of
the piano, which can occur with these installed devices, can adversely
affect older pianos in an even worse way.  Since these pianos were
built using hide glue, which is not only water soluble but hygroscopic
as well, all the joints of any wooden parts of a piano are likely to
come apart over time.

Tuners like to install these humidifier gimmicks to make extra money.
We advocate instead the use of room humidifiers or systems installed
in the heating and/or air conditioning ductwork of the house.

Rick Smith - The Piano Exchange
Glen Cove, New York