Definition
Glycol, usually non-toxic propylene glycol mixed with water at 30–45 %, is the antifreeze heat-transfer fluid used in closed solar collector loops and outdoor heat pump pipework in climates with frost. Solar-grade mixtures include inhibitors and must withstand stagnation temperatures around 200 °C without degrading. The fluid should be checked every few years for pH and freezing point and replaced when it has aged.