I agree with Dave about first seeing why the central HVAC is not delivering the proper RH levels to your display area.  You state fluctuations due to the building's main air conditioning units.  This leads me to suspect that you do not have RH control, but only T control in the building.  Generally in this situation, when the air is actually being cooled, the RH will drop.  As soon as the AC shuts off, the RH begins to rise.  Generally, the larger the capacity of the AC unit compared to the cooling load, the greater the RH fluctuations.  

So, the first step would be to verify that RH control exists, and if it does not, add it to the main system.  This must be central dehumidification or reheat on larger systems, not just AC.  Some people will try to tell you that AC IS RH control, but it is not, as the "control" is a by-product of T control, and varies uncontrollably.  But they will still insist it is RH control.  In similar situations with AC only, I have measured 20% RH fluctuation in a 5 minute period, and on a different day with different outdoor conditions (or night instead of day), different RH levels and fluctuations will occur.

Small dehumidification units that must be emptied are a very risky proposition.  I'm a big fan of portable units, but only if they can be installed so that they are connected to a drain.  This does not have to be sophisticated.  For example, in some spaces, they can be located high on a wall so that they can gravity drain to a sink with a piece of garden hose.  If you use a larger commercial portable dehumidifier, they can be located out of sight and ducted to the conditioned space, or can be added directly your central air handling system.

If dehumidifiers must be emptied, be sure it is done on a regular basis, generally every day, before they fill and shut off.  If this happens, it will create a large, rapid RH rise.  When they are eventually emptied, a rapid RH fall will result.  This could cause more damage than not using them if allowed to occur regularly.  Establish a definite schedule, and have a clear understanding of who is responsible, and have back-up responsibility for times the primary person is away or unexpectedly absent.

In your specific situation, portable dehumidifiers may not be effective, as the central air handler will immediately incorporate the dehumidified air in the display area and mix it into the overall building air.  Thus, your portable unit(s) will have to try to dehumidify the entire building.  Whether they will work will depend upon your specifics, and can not be further evaluated on this list.

As for specific recommendations of dehumidifiers, generally the larger they are, the louder they are.  This is basic physics - more air must be moved.  Small units may be quiet, but probably will not work for your needs.  Another consideration is the low temperature limit that you will need.  Most residential units will frost up permanently between 60-65 degrees.  Certain units are made for cold locations, such as basements, and will remain operable into the high 30s.  Commercial units generally are operable to colder temperatures.  I am currently on a job site and away from my office, but would be glad to provide a couple of suggestions when I return if you are interested.

Humidification has much the same concerns as dehumidification, including the effect of the central system, possibly introducing fluctuations, where to place them, etc.  It is with portable humidifiers that you find problems such as bacteria growth and deposits on objects.  These can occur, but can be controlled by purchasing the right type of unit (evaporative instead of ultrasonic), and by adding biocides.  HOWEVER, humidifying the air at cold temperatures can cause condensation within the walls of the building.  This can lead to rotting of wood, spalling of masonry, peeling paint, or other problems.  For these reasons, I generally do not recommend it for smaller institutions.  The decision to humidify must evaluate all of the relevant factors.  This can not be done on this list, and therefore, I suggest that you do not humidify unless you have a site visit by a competent conservator who gives you site-specific approval.

Good luck, and first focus your efforts on improving the central system!

Marc Williams
President, American Conservation Consortium, Ltd.
www.conservator.com


---- "Jenner wrote: 
> I checked the listserv archive, and it doesn't look like this has been
> discussed for a few years, so....I'm curious about what the current
> thinking is on small portable humidifier and dehumidifier units.
> 
> We have a display area that does not have its own climate control, and
> the humidity tends to rise and fall dramatically, depending on what is
> happening with the building's main air conditioning units.  
> 
> Although my preference would be for a console unit hooked into the
> building's drainage and water supply, so that we don't have to
> constantly refill and empty pans, I don't think it's going to be
> possible.
> 
> I don't want to just buy a cheap home unit, however, because I keep
> coming across cautionary tales about bacteria growing in the units,
> about humidifiers depositing lime on collections, etc.
> 
> Are portable units acceptable when there are no other alternatives?  If
> so, any recommendations for humifiers and dehumifiers (separate units or
> combinations) that are good in a museum setting, do the job, do no harm,
> and are relatively quiet?
> 
> Thanks for your insight,
> 
> Nancy Jenner
> 
> =========================================================
> Important Subscriber Information:
> 
> The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).
> 
> If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).

=========================================================
Important Subscriber Information:

The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).

If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).