Schematic of Solar "Active Open Loop" Hot Water Heating System
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.
