More types of Solar Water Heaters
Open Loop systems, continued:
Draindown systems - not
adviseable.
These systems were primarily made in
the 1970's, and featured a unique
device central to their operation,
called a draindown valve. Water
is supplied to the collectors directly
from the cold supply line.
When the thermostat detects that the
temperature at the collector has fallen
to 40 degrees F, under instruction by
the system's controller, valves are
closed which isolate the collectors
from the water tank.
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It then opens another valve to initiate
a drainage of the collectors, in the
event they may freeze and burst if the
air temp drops further. These
systems are not recommended for
interfacing with new solar water
heaters, as they require the most
maintenance.
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TIP: ** Water,
unlike most other liquids, expands as it
freezes **
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Closed Loop Systems
In a closed loop system, the water used in the home is
physically not the same water that is circulated up to the
solar collectors on the roof. That loop of the system
(that is 'closed' to the outside world) is usually actually
a mxture of water and ethyene glycol (food grade
antifreeze).
- Closed loop antifreeze heat exchanger - This is
the system described on the previous page
that transfers the heat from a collector's het transfer
fluid, through a heat exchanger, to the house's water
supply, and stores it in a tank. There is one
potential problem with this system. Despite being
closed, it actually can require changing of the solution in
the collectors from time to time. Why? Because
when the system sits and has done its job for king and
country and no more water is demanded but the temperature
continues to rise, it can actually break down te glycol
solution in the collectors, causing all sorts of scaling
and trouble, even with a PTRV installed. The system
also requires something known as a charging pump to
pressurize the piping while removing air bubbles.
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DrainBACK system (very different from
drainDOWN) - In a drainback system, there is
a closed loop present. The heat transfer
fluid is sent to the collector and back down to the
water eater where it transfers its heat through a
heat exchanger in the tank. When the threat
of freezing is present or there is no more sun for
the day, the system drains the collectors down into
the tank. This keeps the empty collectors
from storing any fluid when not needed. There
is a slight pocket of air in the system to account
for the drainage, but there is not a recurring
supply of fresh oxygen continuously being
introduced into the circulation loop to cause
rusting problems.
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Thermosiphon - In this system, the collector
is placed below the tank. The natural
effect of heated water is to rise. When this
happens, the heated water in the collectors rises
up to take the place of the cold water in the
tank. There are no moving parts here, except
the pump that carries water into the
home...although if sufficiently high enough on the
roof, the pressure from this tank may be
enough. Note that this system is not all that
quick, and if the tank is outside, even with the
best insulation it may not hold its water at the
same temperature as efficiently as other methods.
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