Player failure due to under floor heat is likely be the precursor
of piano soundboard and pinblock failure.  Subjecting pianos to under
floor heating is akin to putting stewing beef into a slow cooker.
The constant stream of warm, _dry_ air rising from the floor can cause
very serious damage to anything wooden: antique furniture, even doors
and door frames.

There is no simple solution, but installing a humidification system
would be a good place to start.  Unfortunately there is no space under
the belly of a player grand.  So I suggest a humidifier directly under
the piano during the heating period.  Otherwise, the owner won't just
be kissing the player goodbye...

Jurgen Goering, piano technician
http://www.pianofortesupply.com/