How open-loop solar water heating systems work

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 it 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.

Both two tank and one tank systems are standard.

2 tank systems

For two tank systems, one tank is used as a preheat storage tank, which initially holds the solar heated water. The other tank is a conventional water heater, which provides auxiliary water heating when there is not enough solar energy to completely heat the water. The two tanks are connected in series; as hot water is drawn from the conventional water heater for use, it is replenished by the preheated water from the storage tank.

1 tank systems

A one tank system uses essentially the same technique as the two tank system, only compressed into the size of a single tank. One-tank systems have an electric heating element to boost water temperature when solar is not sufficient to heat.

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.

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.