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The mixed water heater should turn on at Setpoint - Deadband and turn off at Setpoint.

WaterHeater:Mixed,
  Mixed Tank,          !- Name
  0.151,                   !- Tank Volume {m3}
  Hot Water Setpoint Temp Schedule,  !- Setpoint Temperature Schedule Name
  ,                        !- Deadband Temperature Difference {deltaC}

If the deadband temperature difference is blank, the IDD shows the default as 0.0 C, however, the code uses 0.5 C to avoid problems with the algorithm.

WaterHeater:Mixed,

N2 , \field Deadband Temperature Difference
   \type real
   \units deltaC
   \minimum 0.0
   \default 0.0

if (rNumericArgs(2) > 0.0001) {
    WaterThermalTank(WaterThermalTankNum).DeadBandDeltaTemp = rNumericArgs(2);
} else {
    // Default to very small number (however it can't be TINY or it will break the algorithm)
    WaterThermalTank(WaterThermalTankNum).DeadBandDeltaTemp = 0.5;
}

So your simulation should not be heating the water temperature above 130 C. If you cannot find the problem then the input file will be needed to further diagnose the problem.