Closed Loop versus Drain Back Systems
There is some debate over the merits of a drain back style
solar water heating system versus a closed loop glycol solar water heating
system. Both have their merits and both
will do the job of heating hot water.
However in Canada and the Northern USA we do not supply or recommend drain
back systems with evacuated tube collectors.
Here are the reasons we feel a closed loop solar heating system is
better than a drain back system in freezing climates.
What is a drainback
system? A drain back system uses
water as its heating method not glycol.
The water in the system is in an open loop and as such drains itself via
gravity back to a storage vessel when it has reached it shut off temperature or
is too cold. This is how the system
handles both overheating and freezing.
In the evening when there is no solar gain to be had, the system
activates a drain solenoid that drains the water in the collectors down to a
smaller storage tank called the drain back tank located in the basement or
utility room. In this type of system
water is not left in the collect as it has been drained. When the solar collector has energy to
deliver, a pump is activated and pumps the water through the heating
system. In the event of overheating, the
system will drain itself so no water is in the collectors to be over heated.
What is a closed loop
system? A closed loop or sometimes
known as an “active” solar system uses a mixture of glycol and water and is
pressurized with in the system. This
requires a pump station to fill the system initially (all our solar water
heating kits come with the fill pump).
The system is pressurized to between 20-30 psi. In a closed loop system an expansion tank
with a rubber membrane is present and absorbs the expanding and contracting of
the heating system as it heats up and cools.
Hydronic in floor heat is a good example of a closed loop system as is
any boiler system. Because glycol is
used in the system, the heating fluid can be left outside without worrying
about freezing. A closed loop system
handles overheating by shutting off the circuiting pump and forcing the system
into what is known as “Stagnation”. In
this state the heating fluid in the collector turns to steam and this added
volume of steam is absorbed in the system by the rubber membrane in the
The drainback has an advantage in that is uses water as the
heating fluid. Water has a slightly
better heat co-efficient of performance than glycol and as such can produce
slightly more heat energy than a system with glycol. However on the other side a closed loop
system once installed has very little maintenance. Because the glycol/water is in a separate loop,
it is not effected by oxygen and does not evaporate. A closed loop system also has no vertical
head pressure to overcome as it is not affected by the forces of gravity that
an open loop or drain back system must overcome. Thus the pump requires less energy increasing
A solar vacuum tube gets very hot and can reach 300 C. This becomes an issue in a drain back system
where water is filled and drained in a system based on the heat in the
collector. If the collector was filled
when it is too hot, it would instantly flash the water to steam as it enters the
collector. For this reason drain back
are typically found with flat panel collectors that do not reach the high
temperatures that vacuum tubes can achieve.
A drain back system has critical
weakness in cold climates. It
relies on a mechanical valve to open and close the system to drain itself. Any mechanical item is prone to failure and a
failure of a drain back system in freezing climates would result in nearly total
loos as the collector and piping would be completely destroyed by the expanding
ice. For this reason we feel the risk
of damage is too great to overcome the small advantages of a drain back system
and as such we do not offer any type of drain back solar water heating systems.