The Dampp-chaser Humidity Control System
We recommend to all our customers the installation of a Dampp-Chaser in their piano. A principle benefit is to stabilize the piano's soundboard and with that, the tuning that sensitively rides on it by way of the bridges. If the soundboard swells with an increase in humidity, the pitch of the instrument is incrementally raised. But not evenly: the soundboard is like a diaphram, and the bridges over which the strings pass start near the rim at one end, go out across the middle, and then down to the other end. Plus the bass strings have their own bridge. And the wound wire of the bass is more flexible than the plain wire of the tenor and treble. Basically, the less change in the moisture content of the soundboard, the more consistant and musical a tuning will remain during the course of a year.
Here is some counter-intuitive pragmatism. The best time to tune your piano is when it sounds its best (not when you're in pain because it sounds terrible). The less the wire has to be moved and the more it's being refined in the same place, the more solid the end result will be. So, with or without a Dampp-chaser, it's arguable that you'll have more musical days per year if you tune it once a year in the same moderate humidity each time, than if you have it tuned twice a year changing it back and forth. But in New England, the weather could be almost anything on the day of the tuning, even if it is the "temperate" season.
So. The best approach is to support that temperate strategy with Dampp-chaser insurance. The closer your soundboard's weather-of-the-day to, say, 42% relative humidity when the tuner appears, the better the job that will be left behind. And that's just where the Dampp-chaser tries to keep things.
For an upright, the Dampp-chaser has the added benefit of helping to keep the action screws tight. They become loose largely by the wooden parts swelling against their screws and compressing themselves in the damp season and retaining enough compression in the dry season to eventually cause noise, changes in spacing (with parts potentially hanging up on each other), uneven wear, and eventually stripped screws.
For a grand, the Dampp-chaser Company offers the Undercover, made of acoustic speaker cloth, which is upholstered to the under side of the rim, covering the system and buffering between what the Dampp-chaser's trying to do and the ambient atmosphere. This about doubles the efficiency of the system and makes a very tidy, inconspicuous presentation.
Little maintenance is needed. Watering the humidification part (by a tube attached under the rim and a can provided at installation) every two or three weeks, with a cap of additive (also provided) to keep it clean. Because minerals are left behind as water evaporates to provide humidity, however, the pads in the system gradually clog and eventually cannot wick the water called for by the humidistat.
We advise servicing the Dampp-chaser each fall when we come to tune. Then the humidification part of the system is most effective when your piano needs it most.