How Do they Work

Outdoor furnaces use hot water heated by wood, coal, corn, or waste oil products as a medium to transfer heat. Water is heated and stored in the furnace unit and pumped underground to potentially multiple end uses as displayed in the adjacent picture.

Glycol Solar Thermal System

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.

Drainback Solar Thermal System

A drainback system is a closed-loop, active solar system not to be confused with a draindown system. The solar loop uses distilled water as its heat transfer fluid and it is a non-pressurize loop. A drainback tank is installed on the return side of the solar loop so that when the system does not run, the water collects in the tanks leaving the collectors empty and so removing dangers from freezing. A drainback system requires very careful installation. Each collector and pipe run must be sloped a least 1/4 inch per foot of run. If the installer avoids any horizontal runs or sags in the pipe where water can pool or back up, a drainback system can provide years of fool proof freeze protection and high efficiency with little to no maintenance.