Question-and-Answer Resource for the Building Energy Modeling Community
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In DesignBuilder v4.7, you can edit the fluid type while editing the plant loop. It looks like you have selected a glycol option instead of water, based on your plant loop's name CIRCUITO SOLARE GLYCOL CONCENTRATION. While editing the plant loop, you can also define a glycol concentration. This sets the range of temperatures that EnergyPlus should use to calculate various properties for your glycol loop each timestep. One of those is specific heat, as indicated at the beginning of your warning (GetSpecificHeatGlycol is the part of the EnergyPlus source code performing this property calculation each timestep). From your fluid type and glycol concentration settings in DesignBuilder, the range of temperatures that EnergyPlus expects is -20 $^\circ$C to 125 $^\circ$C, and this warning indicates that there was a timestep where the glycol temperature exceeded 125 $^\circ$C and reached 126.21 $^\circ$C.
This warning message is from the first timestep where this issue occurred. Near the bottom of the error file is a recurring errors list that will tell you:
If the range of values is only a little above 125 $^\circ$C, then you may be able to ignore these warnings as that is not far past the expected glycol maximum temperature. If the range does exceed something drastic (say, 150 $^\circ$C) then you will likely need to address this issue of overheating in the glycol loop. This can be done in lots of ways (reduce collector area, adjust availability and other plant loop controls, etc.).
There are related posts with more information here and here.
