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[EnergyPlus_Support] Humidistat Problem With Unitary Heatpump/GSHE System

Is it true that a UNITARYSYSTEM:HEATPUMP:WATERTOAIR, being a "unitary 
system", only needs a ZONE CONTROL:HUMIDISTAT object added to provide 
humidity control?  This is what the e+2.0 InputOutput Guide seems to 
suggest (page 872, second paragraph).  

Yet when I do this I get the following warnings along with greatly 
dimished control of cooling and humidity after simply adding the 
humidistat:

	Calculated Humidity Ratio invalid (ie -0.5387) and
	Temperature out of range [-100. to 200.] (ie 406353.15)

(Just in case a UNITARYSYSTEM:HEATPUMP:WATERTOAIR system has no means 
for dehumidifying (such as controlling the airspeed over the cooling 
coil via the fan) I added a DESICCANT DEHUMIDIFIER:SOLID object to 
boot).

Without the humidistat the system controls nicely for temperature, 
albeit humidity is high for the summer design day.

The enclosure I am using is a simple 3-meter-cubed stucco/insulated 
cube with a single window.  So I know that the heatpump/GSHE system 
is large enough to handle any HVAC needs of this small space.

I've uploaded the IDF for this run (CubeSimpleWindow.idf).  Would 
anyone have any ideas as to why I'm getting these warnings and the 
consequent poor performance?  Am I making the wrong assumptions as to 
what it takes to dehumidify a water-to-air-heatpump/GSHE system?

Any thoughts would be greatly appreciated.





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