Solar Thermal Systems

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Solar Thermal Systems

Active solar thermal systems collect radiant heat energy from the sun, usually using water or a non-toxic glycol solution, though air is another option. This heat can then be stored for later use, or transferred directly to an existing heating system, be it for hot water or space heating.

Glycol Closed Loop Solar Thermal Systems

This type of system uses solar panels (also known as solar collectors) that heat a propylene-glycol mixture.  This glycol mixture is a non-toxic, food-grade anti-freeze. After being heated in the collectors, the glycol is pumped through the system, and used in a heat exchange to transfer its heat into a tank of water. This tank of water stores the heat for use throughout the day or night in all types of domestic hot water applications, including laundry, showers and infloor heating. This system also incorporates an expansion tank to accommodate the fluctuating volume of fluid due to temperature changes in the fluid. Because this system is pressurized and the glycol is physically pumped from the panels to the homes hot water heat exchanger, the panels do not have to be placed on a building’s roof, and can be placed virtually anywhere on a site.

Here is one possible design, listing components of a glycol solar thermal system.


1/6. temperature sensor

2. air vent

3. wiring to the differential controller

4. circulating pump

5/7/16. - pressure relief valve

8. temperature gauge

9. tempering valve

10. hot water output

11. cold water input

12. the tempering valve ensures water is never too hot

13. by-pass valve

14. check valve

15. cold water input to solar panel

17. flat solar panel (collector)

18. existing hot water heater

19. storage tank with heat exchanger

20. temperature differential controller

21. pressure gauge

22. expansion tank

23. flow meter

Closed loop glycol systems are often used where freezing temperatures are very common. Overheating during summer months can be an issue if the hot water produced by the system is not used, such as during extended summer vacations. A simple bypass of the check valve is sometime used, to allow the hot water to circulate into the solar collector at night and release heat (cool). The circulating pump that is used in this system, must permit the reverse flow to take place.

Drainback Solar Thermal Systems

In a drainback system panels are installed on your roof and use the sun’s energy to heat water, rather than glycol.  This water is then used in a heat exchange to transfer heat to water used in the home. To keep from freezing, a drainback system incorporates a gravity-fed mechanism that drains the fluid from the system when it’s not in use. The drainback module contains everything needed –the sensors, controller and  pumps along with the tank and heat exchanger.  In a glycol system these components may vary depending on system design.

Here is one possible design and list of components of a drainback solar thermal system.


1/6. pressure relief valve

2/15. temperature sensors

3. solar panel (collector)

4. mixing valve

5. temperature gauge

7. flow meter

8. pump - to storage tank

9. heating element (back-up)

10. differential solar controller

11. existing hot water tank

12. heat exchanger

13. pump - to solar panel

14. water reservoir - flows into panel when in use and sits in tank during drainback


Drainback solar thermal systems can be cheaper to install but it is extremely important to ensure the system completely drains, if you are in a climate that experiences freezing temperatures. In addition, a controller failure, which causes water to be pumped into a freezing solar panel, can be very expensive.


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