Since the most obvious solution has not been yet been mentioned,
I guess I will finally put my oar in: simply shut off the heat under
the floor.

Under floor heat systems, for the most part, are a somewhat new
innovation, having fairly sophisticated control systems, and usually
are arranged with thermostats for a number of separate zones.  Shut
down the zone involving the piano.

If that would involve too large a zone, then the heat system should
be re-plumbed to either permanently remove the area around the piano
from service, or create a separate zone for that area, to remain off
as long as there is a piano in that spot.  In the meantime, I would
disable that zone rather than risk expensive damage to the piano.
This should not be a huge project if the floor in question in located
over a basement.

Re-plumbing a floor heating system could be fairly simple and
inexpensive or a considerably more challenging and expensive project
depending on location, type of materials involved, and the level of
plumbing ability of the homeowner.  I would guess the most challenging
system to alter would be of the type embedded in concrete slab.

In any case, the homeowner who is up to doing the job himself is going
to be able to save big bucks.

Tom Lear
San Francisco, Calif.