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Another possibility...!

It was very interesting to see all respondents to the original
question assuming that the effect of under floor or in-floor heating
would immediately lead to dryness.  This is not always the case though.

A friend of mine had his central heating system altered to in-floor
heating, with tiles on top.  The area under his floor however is quite
damp.  What happens here is that the concrete floor sucks up the
moisture from beneath, which is because of the heat, then evaporated
through the cement separations between the tiles.  His relative
humidity (R.H.) is now about 95% in winter!  So beware of speedy
conclusions.

It is essential to do R.H. measurements, so start with a reliable,
calibrated hygrometer mounted under the piano, and go from there.
But only after proper diagnosis.  If high humidity would indeed be
the culprit, check via Google for a product called "Moisture King".
It's cheap and mounts easily under the piano and keeps the R.H. 'round
about 45% year around.

Rob J. Buskop
the Netherlands