The general received wisdom about the relative humidity (RH) level for pianos is that it should be held at a constant 42%, at a temperature that does not cause condensation of moisture on any surface in the room. However, that’s very difficult to achieve in the typical home unless you have museum-quality central air-conditioning. Much has been made about the horrible things that might happen to your piano if you deviate from this magical number and/or allow large swings of RH in the micro-climate around your piano.
Actually, pianos are really very hardy and resilient things, especially the expensive high-end ones! Even the large Chinese OEM piano manufacturers are learning what it takes to help a piano be less susceptible to absolute RH levels and RH variation. I spoke at some length with Ulrich Sauter about RH control, and his basic message is to just use your common sense. There is no need to panic nor be overly anxious. Some short-term variation is perfectly fine, even the large ones that I experience in my home. It’s not as if your piano will fall to pieces or seize up unless its really abused.
There are actually three aspects to this, as far as I can see.
The first is the control of RH on a long-term basis, where the micro-climate around the piano should vary as little as possible, on average, over weeks/months.
The second is the fact that there will be short-term RH variation on a daily basis. In my case, for example, if I have friends over and the air-con is on for a few hours, the RH can drop down to the low 40’s. But once the aircon has been turned off the RH slowly creeps back up to between 65 to 70% over the course of a few hours. In any case, Sauter (and I guess most other manufacturers of ‘tropicalised’ pianos) use a varnish on their soundboards that dramatically slows down moisture absorption. So short-term RH variation is typically not an issue.
The RH in the living room sometimes spikes up into the high 70’s, for example when I open the patio doors to go do some work on the potted plants, or if the exhaust fans in the kitchen have been on for a while and outside air is being slowly drawn through the small gap under the front door. But getting it back down to the 65-70% range is simply a matter of turning on the air-con for about 30 minutes or so.
These short-term variations are unavoidable unless you can run your air-con practically 24×7. The tuning will also drift a little, so it will have to be tweaked (which is why I tune my own piano). But once the piano has acclimatised you will find that the tuning will drift less. By the way, in the tropics, using a dehumidifier rather than air-conditioning is not a good idea unless you think that your home is not sufficiently warm and stuffy!
The third aspect is the need for air circulation within and around the piano, to prevent pockets of dampness from forming for extended periods of time. If you play your piano regularly then there is usually no problem. But if your piano is closed for a lot of the time, then having a heater bar turn on and off under the control of a humidistat is probably a good idea because it induces some circulation around the action (and to an extent the keys). You will also need air circulation around a piano, particular if it is backed up close to a wall.
And yes Sauter prefers control of the room’s micro-climate rather than in-piano heaters or humidifiers. Others have heard the same from Grotrian-Stienweg, Steingraeber, etc.
There are a couple of absolute no-no’s:
- Never ever locate the piano where direct or strong indirect sunlight falls on it. This is simply asking for trouble.
- Never ever let the RH stay at or above 75% for extended periods of time. An average of around the mid- to high-60’s is good. This means that you almost always have to close off the room from the outside for most of the time, and use air-conditioning for some of the time.