Yamaha C7 tuning instability

[Mark Dierauf]

I take care of an older Yamaha at a recording studio that has a tuning
instability problem that I've never encountered on any other piano - the
pitch fluctuates by as much as 3 or 4 cents over the course of only a
few minutes. It took me quite a while to realize that this fluctuation
ran in sync with the forced hot air heating system. Now that it's
summer, the air-conditioning from the same vents is having the same
effect. The piano has a complete DC unit installed with a bottom cover,
is tuned at least 20x annually, and is very pitch-stable in the long
term. It is not located in direct sunlight and the temperature doesn't
vary much, certainly not within seasons. In the winter there is a
humidity control system adding moisture through the hot air ducts. The
studio is in an old New England barn that had several hundred thousand
dollars worth of renovation, and so is very well finished and insulated,
but in the time that it takes me to work through the temperament the
first notes tuned have already moved substantially. Once after chasing
the pitch all through a tuning I repeated A4 for several minutes and
watched the pitch rise about 2 cents above the starting point then fall
about 2 cents below. This cycle kept repeating as I watched. Has anyone
ever seen such a problem? Did you find any solutions?

- Mark