Active Solar Heating Systems

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Active solar heating system is mainly used for domestic hot water heating in colder regions, where sun's heat energy is collected inside the solar collectors and transferred through the heat exchanger to heat the water stored inside the storage tank.

All active solar water heaters are designed to use a pump to circulate the solar thermal fluid through the system, sensors to protect and other controls which will provide higher efficiency and better performance versus the passive systems.

An active solar heating is applicable in colder regions, but to eliminate the risk of freezing you have to use the antifreeze as the thermal fluid. As the required heat or temperature is usually not met with the solar heating system, backup heating has to be provided (electrical heating elements for example).

The efficiency of an active solar water heater and its performance depends on the system design, quality of material used, how well the system is installed and maintained and the amount of time the system is exposed to the sun.

Active solar energy system for water heating can use controllers with sensors to control the pump or Photovoltaic solar collector where the pump runs only when sun heat energy is available.

Types of the active solar heating systems

Active solar heating systems are using liquid-to-water and air-to-water heat transfer to heat the water. Two active water heating systems are the most popular:

The above examples of active solar water heating systems can use Photovoltaic panels to run the pump and operate as:

Direct or open-loop system

Direct active solar energy systems utilize solar collectors where household water is heated directly, which is the reason for being more efficient than indirect systems.

On the other side the main disadvantage is that they require more maintenance due to the mineral buildup problem. In a case of the solar panels which are built from the small size tubes, scaling might clog the pipe, reduce the water flow and decrease the collector efficiency.

Indirect active system

Indirect active solar water heater systems are closed-loop systems where the solar fluid remains within a single circuit. Solar thermal fluid, glycol for example, after it is heated in the collector by the sun is moved by the pump to the heat exchanger where it transfers the heat to domestic water.

Indirect systems mainly use popular solar panels or flat plate collectors, and also evacuated tubes and concentrated collectors.

A good example of the closed loop glycol design is the Rheem Solpak active solar system.

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Active solar heating - Open-loop system

direct active solar water heating with the pump and controller

The simplest active solar water heating system is the open-loop system, often seen in the warmer regions, where freezing never occurs. The main components are solar collector, pump, controllers, sensors (i.e. temperature limit) and valves (i.e. tempering valve, isolation valves, air vent).

In the open-loop systems, solar collector is attached through the pipeline to the storage tank. Once the water is heated inside the collectors, the pump circulates the heated water and when it stops, the circulation stops as well.

The most popular and used collectors for this active system are solar panels or flat-plate collectors and vacuum tubes.

Pressurized glycol system or Antifreeze systems

Antifreeze pressurized system is an example of the closed-loop active solar heating systems, where water inside the storage tank is heated by the fluid that absorbs the sun's heat inside the solar collectors and runs through the heat exchanger.

The heating fluid is usually a 50/50 propylene-glycol and water solution that works as the antifreeze. Double walled heat exchanger must be used if the heat transfer fluid is considered toxic and single walled if the fluid is non-toxic.

Due to its ability to work in a lower temperature environment, closed-loop with antifreeze active solar heating is the only reliable solution in colder regions.

Pressurized glycol systems are the most versatile systems of all solar water heating systems. They can be installed in any direction and far from the heat exchanger. Fill and drain valves must be installed as the system is pressurized in order to add or change the fluid.

Drainback system

drainback solar water heating with one solar panel and the tank

Drainback system is another example of the closed loop systems where the circulating fluid is distilled water. The main advantage of the drainback system is its ability to empty the solar collector when the pump is turned off. Drainback active solar water heating systems can be installed in both colder and warmer areas.

The main components of the drainback system are the collector, pump, large storage tank (size depends on the demand, i.e. 80 gallons), small drainback tank (for example 10-gallon), sensors, controllers and valves.

In warmer areas when the storage tank gets really hot (but not above the maximum as it is controlled by the temperature limit sensor), the solar fluid drains out of the collectors preventing the system failure.

In colder areas drainback active systems are subjected to freeze-ups unless you put the antifreeze solution and remove all water from the collectors and piping, when the system is not collecting the sun's heat. Provide sufficient continuous slope of the solar panel and that will keep the pipes and collector from freezing.

Distilled water is used to transfer heat from solar collector to domestic water, and it is stored in the drainback reservoir. Distilled water is used due to its capability to increase heat transfer and prevent mineral build-up.

To allow complete drainage a slight tilt of the collectors is required. This is why the collector has to be installed higher than the storage tank. Flat-plate panels are the most popular choices, but they can also use evacuated tubes.

Drainback active heating systems are reliable and long lasting. If this active solar heating system is designed properly (i.e. collector and the pump size) there won't be any problem, and it will work perfect for a long time and in any climate. Drainback kits are also available for DIY projects.

Note: Adding antifreeze to the drainback system can decrease the efficiency of solar collectors. When the system is drained, thin film of the antifreeze stays on the tube walls, dries and leaves the small deposits which can build-up over time affecting the water flow and heat transfer. If only distilled water is used the fluid rarely needs to be changed.


Photovoltaic (PV) operated active solar heating is using the pump only when the sun is shining. The pump is DC (direct current) and is operated by the PV panel that converts the sunlight into direct current electricity. The PV power can be used for both direct and indirect systems. PV systems are reliable and efficient, providing hot water when other systems are shut down by the power outages.

Which one of the above active solar heating systems is right for you?

It depends on the location. Other important factors are the quality of materials used for collectors, strength, reliability... is the system properly installed and oriented.

The above active systems allow greater flexibility than the passive systems as the solar storage water tank doesn't have to be above or even close to the collectors. They can be used year round which is not the case with passive. Even with higher initial cost expect the payback in 5-10 years.

(Pictures are courtesy of and

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