Some years ago I joined the Piano Technicians Guild and got their RTT rating, since I've always tuned my own pianos and have worked professionally on many others, especially here at the University. I have used and installed a number of Dampp Chaser systems, and they do work very well. The major problem by far is that people forget to keep the water reservoir refilled, with the result that the heating element can come on and bake a soundboard dry during low humidity because it has no water to evaporate; then customers say it's useless.
The unit consists of a heating element, a water reservoir and a sensor which automatically switches back and forth from humidifying to drying mode, depending on the room conditions. There is also a warning light that alerts you to a low water level.
My advice is, first, to be sure that you have a qualified technician who knows how to install the system and just where to place the components for maximum effectiveness; second, take the trouble to use distilled or purified water for filling the reservoir--this will prevent buildup of residue on the pads. If you have a filter on your kitchen sink, that water would do fine.
Technique is 90 per cent from the neck up.