Drainback solar heaters for cold climates

There are two main types of solar heating systems for cold climates: drainback solar heating systems and Antifreezing solar closed-loop systems. This page covers the drainback solar systems, a relatively cheap and simple to install and maintain solution.

Components of solar Drainback Systems

A drainback module contains a solar collector, a controller, pumps, a large storage tank (say 80 gallon), a small drainback reservoir tank (10 gallon/38 liters), an heat exchanger (usually internal to one of the tanks) and some sensors and accessories.

The solar collectors

Modern drainback systems tend to use evacuated tube collectors. The collectors are typically installed on the roof and use the sun’s energy to heat distilled water (though is colder climates, it may be used a glycol antifreezing mixture).

How a solar drainback system works

Drainback systems use a pump to circulate water in the collector loop, which drops into a reservoir tank when the pump stops. They also use a differential controller to start the pump (or the pumps) and to route the distilled water into the system. This process begins whenever the sensors attached to the collector detect useful sunlight available (which happens when the temperature in the collector is higher than the temperature of the water at the bottom of the storage tank).

Circuit and design constraints

The system routes the distilled water from the reservoir tank to the solar panel on the roof and back, in a continuous loop. As the distilled water passes through the solar collector, it heats up, and once in the drainback reservoir tank (or in the storage tank, depending on the designs) it transmits its heat to the potable water – usually through a heat-exchanger present into the tank. Pumps will continue circulating the water through the system as long the controller detects useful solar heat. The on-off cycle of the pumps continues throughout the day, but the controller will stop the pumps whenever the system senses no useful solar heat (namely at night).

When the system is not pumping, the solar collector is empty and the distilled water is stored in the drainback tank, usually located just above the solar storage tank.

Design and storage tank location

The design of the drainback solar heating systems rests largely on the gravity-fed principle (that's why the system is called unpressurized). The draining of the fluid from the collector into the drainback reservoir is based on that principle, which demands drainback systems to be carefully installed; the collector should be placed higher than the storage tank, and the system should have sufficient slope in terms of piping in order to avoid freezing problems.

Unless the system involves a second pump to route the distilled water between the two tanks, it also demands the drainback reservoir to be above the storage tank, eventually in the attic (a solution that may demand the use of glycol, if the attic is subject to extreme temperatures).

Advantages: reliability and energy efficient

Drainback systems are very reliable and require very little maintenance. The heating fluid (distilled water) rarely has to be changed, due to the gravity-fed mechanism that drains the fluid from the collector into the drainback reservoir.

These systems are very energy efficient, both on moderate, hot and cold temperatures. They allow larger collector areas per gallon than pressurized glycol systems.

The SRCC (Solar Rating and Certification Corporation: a certification entity that sets solar equipment standards for the solar industry), recommends these systems for cold climates due to their simplicity and long life.

That long life and reliability are largely explained by the use of gravity and water instead of pressure mechanisms and glycol. The system has less moving components (valves, air vents…) than pressurized antifreezing systems, and is immune to pump fails damages.

Disadvantages

A possible downside of this system resides on the large or relatively large pumps it demands, especially if the design involves two or more stories and pumping the water between them and to the height of the collectors on the roof.