Tom, I was told a relative humidity between 40% and 60% would be optimal by both the builder and a mention in Kottick’s “The Harpsichord Owner’s Guide”.  I live in Maine, so humidity is VERY low in the winter and high in the summer. I experimented with various humidifiers and dehumidifiers which allow you to set the relative humidity target. Unfortunately, I found them all to be extremely inaccurate to such an extent as to be useless.
 
I wanted a "set it and leave it" solution. I purchased a humidity controller from a company in Canada called ThermoMart. Here’s a link to the actual item….

Moisture Humidity Controller Hygrometer Electronic Humidistat Control Switch for Relative Humidity level of Greenhouse Incubator Greenery Curing Chamber Greenhouse Incubator Dehumidifire Humidifier Home Basement storage house Damp Garage Attic <http://www.thermomart.com/humidity-control-System-Thermostat-measure-hygrometer>

I have found this to be spot on. It has quite a few control settings, including one to calibrate the actual humidity reading. I have mine calibrated and set to turn on at 49% and off at 52%. Being on the far side of the room, away from the instrument, the readings at the instrument are usually between 50% and 52%. The controller turns power on and off as necessary, so all you do is wire it to its own outlet which it sends power to as necessary. The unit has a humidify or dehumidify setting.. so when I set it to dehumidify it turns on at 52% and off at 49%. Most importantly I have a “dead time” set to 5 minutes so that it won’t turn on again for at least 5 minutes, thus avoiding frequent on-offs which destroy dehumidifier compressors. 

Once you’ve gone to the work of wiring the unit, which is a straightforward task, (I’m not an electrician by any means) it runs itself. My harpsichord is happy to report a temperature variance of 2 degrees and a humidity variance of 3% in the last 7 months. 

-Bob Lorenz

> On May 5, 2016, at 7:08 PM, Rob Brooke <[log in to unmask]> wrote:
> 
> Tom, your harpsichord seems to have been happy at 50% relative humidity but the difference between 50% and 20% is fairly large. My instruments are in Florida where everybody air conditions in the summer and there are a few days in winter when heat is needed. The indoor relative humidity range for us here is between 40% and 50%, though it could go higher in the summer, depending on the level of AC. The only times I have had to use a dehumidifier is limited to the shop when I am drying a soundboard prior to installation.
> 
> The tolerance for any harpsichord to changes in humidity has to do with where it was built (England, Florida vs drier climes) and how the builder designed to increase that tolerance. The adverse effect of low humidity is usually a split soundboard. High humidity can result in bulging or sunken soundboards and the sticking of action parts (keys, jacks, jack tongues) as the wood swells.
> 
> Every harpsichord is different but in your case, I'd get a humidifier for use during the dry months and try to keep the humidity at or above 40%. In Florida, I would worry if my ambient humidity got much lower than that. Your nasal tissues will also thank you for the humidity increase...
> 
> Best of luck.
> 
> Rob
> 
> -- 
> ROBERT BROOKE HARPSICHORDS
> 4214 18th Ave W
> Bradenton, FL 34205
> 
> www.robertbrookeharpsichords.com
> 
> 941-746-6725
> 
> 
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Note:  opinions  expressed on HPSCHD-L are those of the  individual con-
tributors and not necessarily  those of the list owners  nor of the Uni-
versity of Iowa.  For a brief  summary of list  commands, send mail to
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