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Re: [EnergyPlus_Support] About the zone air relative humidity for a zone with windoow air conditioner

Relative humidity is temperature dependent.  If your zone temperature is
higher than outside due to solar gains, there will be a depression in
interior RH even though the moisture levels of interior and exterior air
are the same.  Try looking at dew point temperatures to see if the
interior is getting the outside moisture.  At 12 ACH with no AC the
interior and exterior dew point temperatures should be essentially
equal.  The DX AC window air conditioner can also give you some poor
results for RH control also.  An over-sized unit that runs short cycles
will typically dehumidify better in the model than a load matched system
that runs full time.  In reality, the short cycle operation would
produce almost no dehumidification.  Finally, if the moisture models do
work properly, you may need to model several days to see the results you
want.  In a recent test I performed we simulated occupancy moisture
loads in two apartments, one with good ventilation and one with poor
ventilation.  The data indicated that the poorly ventilated unit took
about 3 days to "store up" moisture in the interior finishes before it
reached a cyclical equilibrium with the induced load.  Since you can not
be completely certain where the wall moisture storage starts after model
warmup, you must run the simulation several days to reach your actual
induced conditions, longer if you have high storage systems like
masonry.  In fact, a precast concrete or concrete masonry unit wall
system may take a year or more to equalize with its surroundings.  I'm
not sure that the transfer function method implemented for moisture
storage in EnergyPlus is as accurate for heavy weight construction as it
could be for light weight stud wall systems which respond faster to
ambient RH changes.  

Ned Lyon

Senior Staff Engineer
Simpson Gumpertz & Heger Inc.

41 Seyon Street
Building #1, Suite 500
Waltham, MA  02453

Phone:  781-907-9000
Fax:  781-907-9009

>>> zplin99@xxxxxxxxxxx 06/26/03 12:02AM >>>

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