Open Loop Solar Hot Water Heating Systems
 

Open-loop systems work by circulating water through the solar collectors to a water storage tank using a small circulating pump. The pump is activated by a differential thermostat controller that senses when heat is available in the solar collectors or if a solar pump is used, it will be activated by sun rays hitting the PV solar panel. A solar storage tank connects to the existing system or existing hot water heater and feeds the preheated solar water into the system.

Where pipes can freeze, the solar collectors and feed lines are usually protected from freezing by automatic drain down controls. These Drain down systems allow the water in the pipes and panels to fall safely back out of the solar collectors and feed pipes to prevent freezing. These types of systems are "open loop" since the energy collection loop is not separate from the rest of the hot water system and sharing the same water.

Open-loop systems have less cost because they are generally less expensive to install and they do not require an anti-freeze solution or heat exchangers. They are also more efficient than closed-loop systems since they heat the water directly from the solar collectors. Sort of cutting out the middle man as it were.

However if you live in a colder climate where there is concern about prolong freezing cold nights and days, you should consider a closed-loop system. Because if there is a failure of a valve, you could really cause major damage to you solar collectors. Thereby out weighing all its advantages.  We do not recommend open-loop system in regions where there is deep freezes.

system configuration

Open-loop and draindown systems are configured according to energy demands, solar energy availability and other factors. Open-loop systems generally consist of at least these parts:

 1. Sensor
 2. Air Vent
 3. sensor wire
 4. circulating pump (aC or DC)
 5. Pressure Relief Valve
 6. SB Sensor
 7. Pressure Relief Valve
 8. Thermometer
 9. Tempering Valve
10. hot water output line
11. cold water feed line
12. Temperature control valve
13. Bypass valve
14. In-line check valve
15. solar collector inlet
16. Pressure Relief Valve
17. solar collector
18. existing hot water tank (optional)
19. Rheem Storage Tank
20. Controller

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How an open-loop solar hot water
heating system works
 

Open-loop and drain down systems use solar energy to preheat water. This description applies to no particular installation or vendor's system, but is representative of open-loop and drain-down systems installed in homes in general.

The systems consist primarily of a water tank and a solar absorber array (to collect heat from the Sun).

How they works:

• The solar collector panels are heated by the Sun's rays;

• The heat transfers to the water flowing through the panels;

• The heated water is pumped to the storage tank until it is used --such as when hot water pours from a faucet etc.

• The water is returned to the solar collectors for reheating.

Water completely fills the pipes in the loop. In fact, excessive air bubbles in the pipes can cause airlocks --stopping pumping action.

The differential controller monitors the difference between the water temperature in the solar tank and the surface temperature of the solar panels. The water only circulates through the system when the panels are collecting a usable amount of heat from the Sun's rays.

Open-loop systems usually have one or two tanks.

Draindown systems

Draindown systems are drained during periods of near freezing temperatures to protect the collectors and piping from freeze damage. Draindown systems may have a manual bypass valve configuration, or an automatic draindown feature.

Automatic drain-down systems

When temperatures drop to near freezing levels, the collectors and piping system are automatically drained of water to prevent freezing. The system relies on gravity-flow drain down for freeze protection.

When the pump is not running, the water remains in the pipes and collectors at utility pressure. During periods of freezing weather, the pipes and collectors are subject to ruptures from freezing water (turning to ice). To prevent freeze damage, the system is designed to automatically drain down or "dump" its water when the temperature reaches a predetermined low. The piping is installed with a slight decline grade to ensure drainage; The flow passages in the collectors are sloped for the same reason. Automatic drain-down systems are usually designed to drain down even if the power has failed.

In drain down mode, auxiliary heating can maintain hot water in the tank to prevent freeze damage to the tank area. When the air temperature rises to a safe level, the open/close positions are again reversed, the system refills, and the solar heating mode is resumed.

solenoid valve automatic draindown configurations

In some models, two solenoid valves are used to control the drain-down process. One of the valves is normally open, allowing water to circulate to the collectors, while the other valve is normally closed, keeping the water in the piping system. During near-freezing temperatures, the open/close positions of the valves are reversed; the valve which allows circulation through the system closes, and the valve which prevents the water from leaving the system opens, draining the pipes and collectors.

single automatic draindown valve configurations

In some models, a single "draindown" valve replaces the two solenoid system. The Sunspool draindown valve was popularly installed in the 1980's, but now is no longer available. Some replacement parts are still available.

Manual draindown systems

Manual draindown systems usually consist of a two manual valve configuration that resembles the solenoid valves, except that each valve is hand cranked into open and closed position.