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Drain-back vs. Pressurized Glycol Systems

This comparison discusses the differences between two of the most common Solar Thermal (Hot Water) System configurations; Drainback Systems and Pressurized Glycol Systems.  This comparison is provided provided by Alan J. Paul, CEM and Founder of APEX Thermal Services, specializing in Solar Water Heating Systems and located in Highland, NY (Ulster County). 

Which one is better?  Well. . .  It Depends!  In a nutshell, with a solar thermal drainback system, the solar loop (which includes the collectors, heat exchangers and connecting piping) is only partially filled with heat transfer fluid.  This design allows the fluid to reside in a dedicated tank inside, when the solar pump is not running.  Solar Thermal Drainback System - Typical Shelf-Mounted Drainback Tank.  APEX Thermal Services' solar hot water installation, Town of Newburgh, Orange County, NYSolar Thermal Drainback System - Typical Shelf-Mounted Drainback Tank. APEX Thermal Services' solar hot water installation, Town of Newburgh, Orange County, NYThis design ultimately protects the heat transfer fluid and the solar tank from over-temperature conditions.  More on this later!  The solar loop within pressurized glycol systems (which includes the collectors heat exchanger and connecting piping) is pressurized and completely filled with heat transfer fluid.  This arrangement provides less protection from overheating the heat transfer fluid, which can accelerate the breakdown of the food-grade antifreeze commonly utilized in solar and space heating applications.  Pressurized glycol systems can benefit from a larger than average solar tank, which can help avoid over-temperature conditions.  In addition, a heat dump of some type, can be designed into the system, allowing excess or unneeded heat to be rejected to the outdoors, into a radiant slab, into a pool, etc.  Solar hot water system overheating most typically occurs when occupants go away on vacation.  If no hot water is used in the house, solar tank temperatures will continue to rise and rise, day after day.  Some solar controllers have a vacation setting, which operates the pumps at night, to cool the solar tank(s), to reduce/eliminate the potential for overheating.  Another issue stacked against pressurized glycol systems is if there are frequent black outs, especially during summer.  Without fluid circulation, the heat transfer fluid in your collectors will basically turn to steam.  Occasional occurrences are the norm, but it this happens often, you may wish to specify a drainback system.

Basically, the design of your home or business and the location of the solar water heating collectors and tanks, as well as the piping path (drain-back systems need all piping pitched downhill, toward the drain back tank) may determine which type system you install.  Also, if you desire an Off-Grid (PV-pumped) Solar Hot Water System, you will most likely need to go with a pressurized glycol system (PV pumps are not capable of lifting the weight of the heat transfer fluid, as required within drainback systems).

Solar Water Heating System - Note the neat installation of piping, ball valves, mixing valve, gauges, pipe insulation, Town of Newburgh, Orange County, NYSolar Water Heating System - Note the neat installation of piping, ball valves, mixing valve, gauges, pipe insulation, Town of Newburgh, Orange County, NYDrainback systems are typically more piping intensive than pressurized glycol systems.  But, if you are planning on installing a heat dump on a pressurized system, the amount of piping can be closer to equal or could exceed that of drainback systems.  Myself and other professionals give drain-back systems the nod for performance.  Barely noticeable disadvantages include that the panels and piping must be tilted slightly (to assist in fluid draining).  This could potentially be noticeable from the ground, if you stare long enough.  The main advantages for installing drainback systems include a slight operating efficiency advantage, antifreeze longevity and (by design) elimination of system overheating conditions.  Drainback systems also make a lot of sense if a larger solar heating system is being installed for a space heating or radiant heating application.  The drainback tank allows the collectors to rest idol in the summer, rather than needlessly pumping hot fluid to keep a large array and associated heat transfer fluid from overheating.  Advantages to installing pressurized glycol systems include system simplicity, smaller pumps, about one third the pumping power, PV-driven (off-grid) pumping becomes an option, and a potential to use smaller piping to collectors.  Contact APEX Thermal Services at 845 430 5825 to help you decide which solar hot water system is best for your situation.