For my grand pianos I use the Dampp Chaser systems that I purchased
from John Tonyan ( http://tonyanpiano.com/ )  See

  http://www.pianolifesaver.com/english 

I live in the high desert and the indoor humidity changes from winter
to summer are severe (using a wood stove in the winter, evaporative
cooler in the summer).  The systems keep my pianos at a constant
humidity level.  The system is very easy to install, and is worth every
penny in my opinion.  That reminds me, I need to go water my pianos. :-)

Rich Ingram