 Open Loop
Solar System Operation
This is the most common type of solar hot water heating installation
in southern California.
Technically, this system is known as an "active open
loop" type. It's operation is very simple.
In an "active open loop" system, water from a large
storage tank or tanks is heated by the sun as the water passes
through solar panels. The system is called "active"
because it uses a small pump to circulate the water from the
storage tank through the collectors and back into the storage.
The pump is turned on automatically by an electronic differential
thermostat. This device reads the temperature in the collectors
and also in the storage tank. When the water temperature in the
collectors is higher than the tank water, the pump is turned
on. Water gets circulated through the panels and solar energy
is harvested.
The system is called "open loop" because the loop
of plumbing from the storage tank up to the collectors is open
to city water pressure. So the whole solar system is open to
the city water line. In a "closed loop" system, the
solar plumbing is separated, closed off, from city water. Heat
is transferred through a heat exchanger. An open loop system
is more efficient. It is also simpler, and therefore more reliable.
NOTE: Some schematic diagrams
of domestic solar water heating systems show a "Pressure
and Temperature Relief Valve" located close to the steam
vent on the solar panel return pipe on the roof. Some city codes
no longer allow for this configuration. The possibility of a
system malfunction or power failure to the pump will cause overheating
and stagnation (boiling) inside the solar panels. Having boiling
water discharging off of your roof can cause injury to persons,
pets and plants below. It is advisable to have the relief valve
located at the solar tank. If no shut off or isolation valves
are installed on the solar loop the pressure will discharge at
the tank, and the steam vent will purge the steam. When relief
valves are installed on the roof, a copper discharge pipe needs
to be plumbed to ground level, terminating 6 to 12 inches above
the ground.
Solar System
Performance
On warm sunny days, your solar water heater will produce storage
temperatures of approximately 140 degrees F.
* Note: Large amounts of heat are lost through the recirculation
lines. We have reduced this loss with two devices. First, a time
clock automatically turns off the recirculation pump between
the hours of 12:00 a.m. and 5:30 a.m. when hot water use is minimal.
Also, an Aquastat allows the recirculation pump to come on only
when the water in the line falls below 120 degrees F.
** Note: Recirculation lines are most commonly found on apartment
buildings rather than single family dwellings.
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