[EnergyPlus_Support] Swimming pool simulation

["solarmech" <solarmech@xxxxxxxxx>]

Dear group!

We are trying to simulate a natatorium!
we have two zones: one for the pool and one for the hall. 
We want to simulate the water in the pool with the use of the object
Surface:HeatTransfer:InternalMass. 
when defining Water as a material and then as a construction the 
simulation stops with the following message:

** Severe  ** CTF calculation convergence problem for POOL WATER
   **   ~~~   ** The potential causes of this problem are related to 
the input for the construction
   **   ~~~   ** listed in the severe error above.  The CTF calculate 
routine is unable to come up
   **   ~~~   ** with a series of CTF terms that have a reasonable 
time step and this indicates an
   **   ~~~   ** error.  Check the definition of this construction 
and the materials that make up
   **   ~~~   ** the construction.  Very thin, highly conductive 
materials may cause problems.
   **   ~~~   ** This may be avoided by ignoring the presence of 
those materials since they probably
   **   ~~~   ** do not effect the heat transfer characteristics of 
the construction.  Highly
   **   ~~~   ** conductive or highly resistive layers that are 
alternated with high mass layers
   **   ~~~   ** may also result in problems.  After confirming that 
the input is correct and
   **   ~~~   ** realistic, the user should contact the EnergyPlus 
support team.
   **  Fatal  ** Program terminated for reasons listed 
(InitConductionTransferFunctions)

why is this?
what can we do to change it?
Is the aproach correct?
We also have the latent gains in the pool scheduled with a "steam 
equipment" object in the "internal gain" group.
Please comment.

PS
the material, construction and internal mass used:

MATERIAL:REGULAR,
    Water,                   !- Name
    VerySmooth,              !- Roughness
    2,                       !- Thickness {m}
    0.602,                   !- Conductivity {W/m-K}
    998.2,                   !- Density {kg/m3}
    4180,                    !- Specific Heat {J/kg-K}
    0.9,                     !- Absorptance:Thermal
    0.7,                     !- Absorptance:Solar
    0.7;                     !- Absorptance:Visible


CONSTRUCTION,
    Pool water,              !- Name
    Water;                   !- Outside Layer


Surface:HeatTransfer:InternalMass,
    water,                   !- User Supplied Surface Name
    Pool water,              !- Construction Name of the Surface
    ypog_Pool,               !- Zone Name
    1350;                    !- Surface area {m2}



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