How do I assign the boundary condition of a wall adjacent to a pool?

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My suggestion would be to use the temperature schedule method of specifying the other side coefficient. You just need to define a temperature schedule for the pool and add that as the Constant Temperature Schedule Name.

In that case, most of the other required fields have the appropriate defaults - mostly 0, except for Constant Temperature Coefficient which defaults to 1.

That only leaves only the Combined Convective/Radiative Film Coefficient field as a required field with no default.

From the Engineering Reference:

Field: Combined Convective/Radiative Film Coefficient

This is a trigger value. If the value is greater than zero, then it is taken to be the combined convective/radiative film coefficient. In this case (value > 0), the remaining fields are used first to calculate the outside air temperature for the surface and then to calculate the outside surface temperature based on the outside air temperature and the film coefficient. If this field is less than or equal to zero, then the remaining fields are used to calculate the surface temperature (not the outside air temperature).

So if the Combined Convective/Radiative Film Coefficient field is set to 0, then the wall surface is assumed to be the same temperature as the pool which seems a reasonable assumption.

That leaves us with the following object:

SurfaceProperty:OtherSideCoefficients,
      WallName,                !- Name
      0,                       !- Combined Convective/Radiative Film Coefficient {W/m2-K}
      0,                       !- Constant Temperature {C}
      1,                       !- Constant Temperature Coefficient
      0,                       !- External Dry-Bulb Temperature Coefficient
      0,                       !- Ground Temperature Coefficient
      0,                       !- Wind Speed Coefficient
      0,                       !- Zone Air Temperature Coefficient
      PoolTempSchedule;        !- Constant Temperature Schedule Name