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Re: [EnergyPlus_Support] Humidistat Problem With Unitary Heatpump/GSHE System



The documentation for ZONE CONTROL:HUMIDISTAT does not tell the whole story.  
For unitary systems which have a humidity control option, the humidity control 
option must be activated and a ZONE CONTROL:HUMIDISTAT object added.

But not all unitary systems have a humidity control option.  Only 
FURNACE:HEATCOOL, UNITARYSYSTEM:HEATCOOL, and 
UNITARYSYSTEM:VAV:CHANGEOVERBYPASS support humidity control through the 
"Dehumidification control type" field.  UNITARYSYSTEM:HEATPUMP:WATERTOAIR does 
not support humidity control

In your model, the desiccant dehumidifier has been placed after the heat pump.  
When the desiccant dehumidifier turns on to control humidity, it also warms the 
air, so this is why the zone temperature control is no longer working.  If you 
choose to use a dessicant dehumidifier, it should be placed upstream of the 
heat pump, either in the outside air system or in the main branch.  A sensible 
heat exchanger is usually place immediately after the dehumidifier to reject 
some of the sensible heat gain from the desiccant wheel prior to applying 
mechanical cooling.  See examplefiles DesiccantCVRh.idf and 
DesiccantCVRhZoneRHCtrl.idf.

Mike


On 24 Apr 2007 at 1:41, energyplus99 wrote:

> 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.
> 
> 
> 
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
> 
> EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the
> Documentation link and press the "search" button.
> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 


========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx





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