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[EnergyPlus_Support] GHT Slab severe errors and high sensitivity to inputs


The Ground Heat Transfer to Slab calculation gives Severe (Fatal) errors and exhibits extreme and unpredictable sensitivity to inputs. I would like to know how to programmaticly adjust my inputs to prevent these Severe errors.  I'm using Energy Plus version 6 and the Auxiliary Slab program.  The results are the same whether I use the pre-processor invoked by EnergyPlus or the slab stand-alone program.
Below my question is an example idf file for stand-alone input into the slab utility. I used this file with the San Francisco weather file (USA_CA_San.Francisco.Intl.AP.724940_TMY3.epw).  Given this idf, even slight variations in the value of the EquivalentSlab's APRatio (Area Perimeter Ratio) can lead to extreme temperature swings, even values below absolute zero (-273 C) for the calculated ground temperature. For instance:

Program   calculated
Status    temperature APR
-------   ----------- ---
Severe    -352.01 C   2.50123157894737 (value in attached idf)
Severe     230.77 C   2.501 (minute change produces wild 580 degree C swing)
Success    ~19.46 C   2.50  (temp is average of TAverage, TPerimeter, & TCore values)
Success    ~19.47 C   2.51  (it wasn't that 2.501 was too large, since 2.51 works fine)
Success    ~19.46 C   2.495 (digits of precision do not appear to be the factor)

If I hold the APR at its original value (2.50123157894737), and vary the insulation, I also get wide and non-linear variations:

Status    Temp        Insulation R-value for 1 inch thick under-slab
------    ---------   ----------------------------------------------
Severe    -284.58 C   20.0
Severe    -327.62 C   22.0
Severe    -364.49 C   25.0 (local minimum, lower temp than more or less insulation)
Severe    -331.38 C   30.0

Is there some factor in there that I need to keep within a certain value or precision range, to ensure successful runs?


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