The best thing you can do for a piano is keep the temperature and
humidity in the room containing the piano as constant as possible.

The best thing is a climate controlled environment, but many of us
cannot incorporate this into our heating or cooling systems.  Instead,
I suggest piano owners purchase an inexpensive gauge for measuring
temperature and humidity and place it on your piano.  Keep a record of
the readings.

If you notice drastic changes, it is important that you make an attempt
to stabilize these readings.  In my case, I purchased a de-humidifier
and a humidifier and placed them in the room with the piano.  I keep
the doors and windows to the room (where the piano is located) closed
at all times, and I try to keep the humidity close to 40% and the
temperature approximately 72 deg. F. year-round.  During very hot and
humid weather an air conditioner is very helpful also.

In my case, our house is very humid in summer and extremely dry in
winter.  These conditions are very bad for a piano and can cause
numerous problems with rusting, sticking actions, cracked sound boards,
damaged glue joints, and tuning stability.

I do not recommend the addition of electrical attachments to pianos
that add or subtract moisture to their fragile soundboards and actions.
It is far better to control the climate of the entire room than to
"spot-steam" or "toast" the interior or underside of the pianos.

In regard to an outside wall: It was important in the old days when
insulation and vapor barriers were non-existent, but in modern homes
it is doubtful if placing a piano on an outside wall will do any harm.

Bruce Clark
Piano technician, retired