Pipe organs are very large and complex instruments made of various materials, such as metals, woods, leather and cloth.  It should come as no surprise that the environment in which a pipe organ is installed will affect both its tonal and mechanical performance.  The main three areas of environmental concerns for a pipe organ are temperature, moisture, and dust/debris.

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TEMPERATURE AND ORGAN TUNING

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One of the biggest misconceptions with organ tuning is that the technician requests the room be set to a specific temperature for their own comfort, when in fact, it is temperature that causes the greatest changes in an organ's overall tuning.  While other factors may come into play, a pipe organ will generally be in its best tune at the temperature the pipes were tuned at.  Even if a client chooses to turn the heat in a performance room down during the week, and then turn it back up for weekend services, as long as the pipes have enough time to acclimate, the organ will sound in-tune.

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Organ tuning is most affected by seasonal temperature changes.  While the room temperature can be set at a constant year-round, it does not guarantee that the temperature at the actual organ pipes will remain constant as well.  For instance, pipe organs that are installed in chambers (rooms within a building specifically built to enclose organ pipes) are often not climate controlled in the same manner as the rest of a sanctuary or performance room.  In fact, it is not rare for pipes in an organ chamber to be cold in the winter, but warm in the summer despite a performance room's temperature being set the same for both seasons. 

 

The pitch of the organ pipes varies directly with temperature.  As the temperature goes up, so does the pitch of the pipes. This change is approximately 2 cents per degree of temperature change.  The weight of a column of air within an organ pipe diminishes or increases with a fall or rise in temperature, with the effect equivalent to that of changing the length of the pipe.  The pitch of reed pipes is affected less than that of flue pipes by changes in temperature.  The smallest pipes are most affected by the change in temperature.
 
We generally recommend that an organ be tuned a minimum of twice a year for most instruments:  once in the fall/winter, after the heat is turned on in the building, and in the spring/summer, after the heat is turned off.  Air conditioning should be on if the building would be cooled during a performance.  We do not tune for the Easter holiday on two calls per year.  If you feel that your instrument needs to be tuned for Easter, we recommend that you consider 3 calls per year, rather than sacrificing either as normal spring/summer or fall/winter tuning.
 
Since the pitch changes with temperature, the organ will be flat or sharp to A440 pitch, depending on the temperature at the pipes and the difference from when it was initially set.  When the organ is tuned, we do not change the pitch.  We bring the instrument in tune with itself.  If the organ requires repitching, a special repitching call is generally required.  Repitching usually entails regulation work as well, and sometimes pipes may have to be cut to bring them in tune at the desired pitch level.  We also recommend that the organ be pitched during the heating/cooling season in which overall pitch is most vital.

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RELATIVE HUMIDITY

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Unlike pianos, where tuning is most affected by humidity, pipe organ tuning is much less affected when a room's relative humidity changes.  That being said, many, if not most, organs contain wood pipes, which may be affected by excessive humidity changes in a manner that will affect the overall tuning of those ranks.  Since the length of a pipe affects its overall pitch, excessive humidity (which can cause a wood pipe to expand) or excessive dryness (which can cause a wood pipe to shrink) will also change the dimensions of wood pipes, especially the small ones, in a way that can affect their pitch.

 

Also, some wood pipes are tuned with wood stoppers that are held into place by friction.  If the stopper becomes dry and shrinks, it can slip out of tune as well.

 

Beyond tuning, however, drastic changes in relative humidity can cause countless other mechanical and structural issues with a pipe organ.  Organ wind chests are made of wood and incorporate materials like leather to create their complex mechanical systems.  When wood is exposed to a lot of humidity, and then dried out, it can cause the wood to warp and split, which can cause very major and very costly issues with a pipe organ. 

It is recommended that the room that a pipe organ is installed maintains a relative humidity between 40% and 60%.  The closer the relative humidity is to 50%, the better, and the less the humidity shifts over time, the more you will be preserving your instrument.

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DUST AND DEBRIS

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Another environmental issue that can greatly affect a pipe organ is dust and debris.  The collection of dust within a pipe organ is inevitable over time, however, owners of pipe organs should be very cautious when it comes to creating excessive dust around their instruments.  Dust can build-up on pipes and cause issues both with tuning and speech.  Excessive dust can also cause mechanical problems such as ciphers (notes that stick on when they are not played), especially if dust gets into an organ's wind chest

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It is best practice to minimize activities that can create dust and debris near a pipe organ.  Always avoid sweeping or stirring up dust around an organ blower that is turned-on.  Always contact your pipe organ service company prior to performing any sort of construction in or around a pipe organ that could create saw or plaster/drywall dust.  NEVER try to remove the dust from organ pipes on your own, as pipes are incredibly delicate and even the slightest contact to a small pipe's tuning slide will knock it out of tune.  It is always best to contact your organ technician if you feel that your instrument is in need of protection from dust.