I have mentioned this before, but it bears repeating.  If the tuning
pins are reasonably tight and the piano goes out of tune, usually
the biggest problem is a wide variance in humidity.  This is usually
seasonal in areas that depend upon artificial heat in winter and with
windows and doors open in summer.

Obtain a good hygrometer and keep track of the relative humidity in
the room where the piano or pianos are located.  If necessary, close
off the room from other rooms, and place a de-humidifier in the room
during the humid months,  It may require air conditioning too.
(Climate control is the key to tuning stability.)

The problem is not so much heat or cold, but the moisture in the air.
Too much moisture expands the wood in a piano, including the frame
and pin block.  When the wood expands the tuning goes in all
directions.  Yes, you can have it tuned to compensate for this
variance, and it should hold until the next seasonal change and then
the piano will go in the other direction and require tuning again to
compensate for shrinkage during the heating season.

I have told people this for years, and few seem to understand the
importance.  Our local church has it backwards -- they add humidity
in the summer and turn on the de-humidifiers in winter to keep their
pianos and pipe organs "warm".  Those old ladies in charge just will
not listen!  I caught one pouring water into a humidifier when the
humidity inside the church building was 85%.  I gave up trying to help
them!

My own piano resides in a climate controlled room, where the doors and
windows are closed year round.  The hygrometer reads an average of 38%
relative humidity.  The piano stays in tune nicely, and only requires
a minor touch up on the tuning after a lot of playing, perhaps once
a year or so.

Bruce Clark