Solar Thermal Information

25
OPEN-LOOP VS. CLOSED-
LOOP DRAINBACK SYSTEMS:
As is true with hydronic heating,
drainback systems can be designed
as either “open-loop” or “closed-loop”
systems, as shown in figure 5-4.
Open-loop drainback systems use
a non-pressurized storage tank. The
air above the water is always at
atmospheric pressure.
Non-pressure rated tanks are usually
less expensive per gallon of storage
than pressure-rated tanks. They can
often be assembled on site. Some
use an insulated structural “shell” to
support a flexible EPDM rubber liner
that contains the water. Others are
constructed of molded polypropylene.
All piping connections to such tanks
usually penetrate the tank wall above
the water line to minimize any chance
of leakage.
Because the water in an open-loop
system is in direct contact with the
atmosphere, it will always contain
dissolved oxygen molecules. All
piping components must therefore
be suitable for contact with this
“oxygenated” water. Circulators
must be of stainless steel, bronze
or polymer construction. All piping,
fittings and valves must also be
corrosion-resistant. Copper piping
along with copper or brass fittings
and valves are common. Stainless
steel and high-temperature
composite or polymer materials are
also a possibility, provided they are
rated to withstand the potentially
high temperatures within the solar
collector circuit.
Open-loop systems will experience
some water loss due to evaporation.
The water level in the tank should
be checked monthly using the sight
glass or dip stick. Water can be
easily added to the system through
the hose bib valve at the bottom of
the collector supply piping.
minimum of 1/4" per foot slope
copper caps
roof penetrations
copper caps
minimum of 1/4" per foot slope
photo courtesy of Radiant Engineering
figure 5-2
figure 5-3