Operative Temperature Plateaus Around 36°C Despite No Cooling or Ventilation

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Summary of your model:

• single zone (6m x 8m x 2.7m)
• no windows, no doors
• 4x walls & 1x roof : 200mm concrete + 50mm EPS ... ~R10
• 1x slab-on-grade : 200mm + finish - no insulation
• no cooling, no ventilation, no infiltration

I can pretty much reproduce your results (max 34°C ... not 36°C) - I suspect I'm not using the same Damascus weather file. Yet overall, there's very little wrong with your model.

What is likely happening is that the (uninsulated) slab+ground combo is continuously acting as a heat sink, lowering zone air (and wall+roof interior surface) temperatures. Peruse through hourly surface temperatures in the eplusout.eso file: the floor interior surface temperatures are 3 to 4°C cooler than the other surface temperatures in summer.

I altered your model by adding the same 50mm EPS insulation in the floor construction (cancelling the slab+ground heat sink effect). The floor interior surface temperatures are now only slightly below the other temperatures (e.g. -0.5°C). Yet ALL temperatures are now 7 to 8°C warmer (in summer) than the original uninsulated scenario. This can also be accomplished by setting the floor outside boundary conditions to "adiabatic". Adding day ventilation + infiltration will make things worse. Adding a west-facing window will raise temperatures well into the 40°C, I suspect.

Side note: Check the eplusout.err file warnings for both the "people" and "equipment" definitions/schedules (see here), which I changed:

People,
    ZONE ONE People,         !- Name
    GF,                      !- Zone or ZoneList or Space or SpaceList Name
    ALWAYS 1,                !- Number of People Schedule Name
    People,                  !- Number of People Calculation Method
    4,                       !- Number of People
    ,                        !- People per Floor Area {person/m2}
    ,                        !- Floor Area per Person {m2/person}
    0.3,                     !- Fraction Radiant
    0.3,                     !- Sensible Heat Fraction
    ActivitySchedule;        !- Activity Level Schedule Name

ElectricEquipment,
    ZONE ONE Equipment,      !- Name
    GF,                      !- Zone or ZoneList or Space or SpaceList Name
    ALWAYS 1,                !- Schedule Name
    EquipmentLevel,          !- Design Level Calculation Method
    400,                     !- Design Level {W}
    ,                        !- Watts per Zone Floor Area {W/m2}
    ,                        !- Watts per Person {W/person}
    0.3,                     !- Fraction Latent
    0.6,                     !- Fraction Radiant
    0.1;                     !- Fraction Lost

Schedule:Compact,
    ALWAYS 1,                !- Name
    Any Number,              !- Schedule Type Limits Name
    Through: 12/31,          !- Field 1
    For: AllDays,            !- Field 2
    Until: 24:00,            !- Field 3
    1;                       !- Field 4

Schedule:Compact,
    ActivitySchedule,        !- Name
    ActivityLevelX,          !- Schedule Type Limits Name
    Through: 12/31,          !- Field 1
    For: AllDays,            !- Field 2
    Until: 24:00,            !- Field 3
    100;                     !- Field 4

ScheduleTypeLimits,
    ActivityLevelX,          !- Name
    70,                      !- Lower Limit Value
    200,                     !- Upper Limit Value
    continuous,              !- Numeric Type
    activitylevel;           !- Unit Type

Note the activity levels (e.g. min 70 W/person) ...