For solar thermal applications, the heat transfer fluid can be subjected to high temperatures (0ver 150C). At these temperatures, the molecules of glycol can degrade, usually leading to a drop in the corrosion protection through acidificatin of the liquid. Regular monitoring of the heat transfer fluid, if only by the simple measurement of pH, is highly recommended. We stress also that pH-indicator strips are not suitable for these measurements. It is recommended to use a pH meter. Intervals of 1 year for checking the pH are appropriate.
To reduce the risk of degradation, you can also think about using other heat transfer fluids which no longer contain glycol but a raw material with similar molecules, with improved thermal stability. The ultimate risk, if the liquid no longer circulates and stagnates in the sensors, is the "tarring" of the product. If this happens, there are specific products to clean and safe-guard the sensors.