As the temperature of an inactive system drops below the solution's freeze point, ice crystals will begin to form. Since water in the solution will freeze first, the remaining glycol solution becomes more concentration yet remains fluid. Combined with ice crystals, it forms a flowable slush and its volume will expand. With adequate expansion space in the system and proper glycol concentration, system damage will not occur. Inhibited glycol heat transfer fluids provide adequate burst protection typically at concentrations of 30% or more.
Diluting heat transfer fluids to less than 30% of glycol may reduce inhibitor concentrations to levels too low to provide adequate corrosion protection and dilutions of less than 25% may be at risk of bacterial contamination. Glycol acts as a natural bactericide. The maximum recommended concentration of glycol is 60% for efficient heat transfer since relative heat transfer efficiency may be sacrificed.
To read more about concentration and burst or freeze points check out our post "Difference between burst protection and freeze protection".
The most notable differences between propylene glycol and ethylene glycol are viscosity and toxicity. Ethylene glycol-based fluids are less viscous than propylene glycol-based fluids and they generally provide superior heat transfer efficiency. Ethylene glycol is typically preferred for heat transfer applications unless toxicity is a concern. Propylene glycol-based fluids are used in applications where contact with drinking water or food and beverages could occur or when mandated by law because of its low acute oral toxicity.
Not sure which type of glycol is in your system? Contact Glycol Blender to see if you qualify for free testing.
The basics on glycol and glycol blends.