I live in an apartment in Singapore, and the piano is located in my living room. The doors and windows are closed all the time. The piano is closed and covered overnight or when not played for extended periods (e.g., when I am away on business trips).
What I used to do up until a couple of days ago was to turn on the airconditioning (I have Panasonic split units) whenever the RH rose above 60%. I would set the temperature at one to two degrees below the outside temperature. This allowed me to take the edge off the heat and also keep the RH at a comfortable level for humans and piano.
I don’t use standalone dehumidifiers because they raise the ambient temperature by a couple of degrees. Too stuffy.
The problem with this method is that control over RH was very crude. I had to adjust the aircon’s temperature setting as the external temperature rose during the day, and fell during the night. Huge RH swings between 38% and 65+% were common. Made the tuning drift a lot too, particularly audible in the top 2+ octaves (goes flat when RH drops). Even the RH inside the closed and covered piano would fluctuate tremendously, albeit with a slower rate of change than ambient RH.
Well, I think I’ve found a solution to the wild swings — use the aircon’s dehumidifier function! I checked the user manual. It says that in summer the aircon tries to keep the humidity between 60% and 75%, and in winter between 55% – 70%. The air temperature is typically a couple of degrees below external air temperature. So two days ago I set the aircon units to ‘dehumidify’ and left them running all the time.
The result? Indoor RH always around 50+% RH, comfortable indoor temperature that more or less tracked external air temperature minus 2 degrees, and very very slow change of RH inside the piano. For example, the piano was closed and covered last night with in-case RH reading at 59%. The RH reading has only just dropped to 58%, more than 14 hours since the piano was closed. Excellent! Better yet, the blower unit and compressor are not running all the time. They turn on only when the RH rises. I suspect the relatively low RH reached in the room could be due to the fact that the blower units are high up on the wall, and measuring temperature/RH ratios different from the rest of the space.