A glycol system is a closed-loop, active solar system. It uses a pressurized solar loop with food grade propylene glycol as the heat transfer fluid
The glycol system is very versatile, great for ground mounted systems or other situations where the pipe runs can not sloped the requisite amount for a drainback system. A glycol system needs to be sized carefully to avoid excessive overheating. The heat transfer fluid must be changed every 3-5 years.