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Types of Solar Hot Water Systems

Solar hot water systems can become very complex depending upon size and system design. More complex systems are more often used in extremely large homes or with businesses.

There are two general types of solar hot water systems used in Colorado: closed loop and drainback systems. The system should be designed according to your needs and budget. The goal is to make it as simple as possible, resulting in lower operation costs, lower maintenance costs, fewer site visits, and lower energy consumption. Both closed loop and drainback hot water systems use pumps and valves to control the circulation of the fluid throughout the system. They allow the system to run year round, without the threat of freezing. Protecting the system from freezing is critical to the proper operation of any system.

Closed Loop Glycol

The most common domestic hot water system is the closed loop glycol system. This is a sealed unit where a propylene glycol mixture transports the heat within the system. The heat transfer fluid is a mixture of antifreeze and water, which can be protected from freezing to -50°F. If overheated or stagnated, the fluid can go bad.


The second most widely used system is the drainback system. Fluid fills the collectors when the pump starts and drains the collectors when the pump stops. The benefits of a drainback system are it is protected from freezing down to -20°F, it is protected from overheating during a period of time with no power, and water is used as a heat transfer fluid.

Collectors And Tanks

Solar collectors are one of the key components of an active solar hot water system. They collect the sun’s energy and transfer that energy to the heat transfer fluid.

Flat-Plate Collectors

Flat-plate collectors are the most common collector type used in domestic water heating systems. A typical flat-plate collector is an insulated weatherproof metal box with a glass or plastic cover and a dark colored absorber plate. Mounted on the roof or ground, it consists of a thin, flat, rectangular box with a transparent cover that faces the sun. Small tubes run through the box and carry the fluid, such as water or antifreeze, to be heated. The tubes are attached to an absorber plate, which is painted black to absorb the heat. When the heat builds up in the collector, it heats the fluid passing through the tubes. Active systems rely on controllers and pumps to move the liquid between the collector and the storage tank.

Evacuated Tube Collectors

The second type of collector is an evacuated tube collector which can achieve extremely high temperatures (170°F to 350°F) which makes them more appropriate for industrial and commercial applications. These collectors have rows of transparent glass tubes, each tube containing a glass outer tube and metal absorber tube attached to a fin. The fin’s coating absorbs solar energy and reduces heat loss. An evacuated tube collector is sealed in a vacuum that minimizes heat loss and helps absorb solar energy under cloudy conditions. Because of this, there is a concern when snow and ice accumulate on the tubes, causing them to not work properly.

Storage Tanks

The storage tank is an important component of the system. In a one-tank system, solar heated water is stored in a tank with a backup heating element or a side arm boiler. In a two-tank system, solar heated water is stored in a separate tank that feeds into a conventional gas or electric water heater. Whether one or two tanks are used, the household water is heated by solar energy. System efficiency suffers from low storage volumes, so always select an appropriate size tank for extra storage. The sizing volume increases with the amount of collected energy. Pressurized tanks come in multiple standard sizes. Tanks with volumes of 40, 50, 80, and 120 gallons are common and can be installed parallel to increase storage volume.

Last updated: October 18, 2017 at 11:56 am