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.