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GREEN SYSTEMS SOLAR SOLUTIONS


Freeze danger from thermosyphon reverse flow


BY BRISTOL STICKNEY CONTRIBUTING WRITER D 80


uring extreme weather conditions, strange and unexpected things can happen to a solar heating system. A good example occurred last winter


during a record cold snap, when the temperature outdoors dropped suddenly into the negative double digits (F). This was followed by an epidemic of frozen and broken water pipes throughout our region, which can be expected during record-breaking cold weather. There were also a few instances of frozen and broken solar heat exchangers, which could not be so easily explained. They were mounted indoors, in the mechanical rooms, next to the other heating equipment and were properly filled with the correct mixture of antifreeze. One of these is shown in Figure 41-1, cut in half to reveal the freeze damage inside.


The phenomenon of thermosyphon reverse flow is well known in the solar heating world. It is common knowledge that liquid in a closed loop will circulate when driven by a temperature difference.


Thermosyphon reverse flow — two pipes The phenomenon of thermosyphon reverse


flow is well known in the solar heating world. It is common knowledge that liquid in a closed loop will circulate when driven by a temperature difference. The warmer fluid, with lighter density, tends to rise, and the colder fluid tends to drain


While this only happened to a few systems in different


locations, they all had some piping details in common. All of the offending heat exchangers (HTX) were located in residential solar combisystems with heat exchangers mounted at the bottom of a closed glycol loop. A typical solar heat exchanger installed like this is shown in Figure 41-2. They all used hydraulic separators (rather than a primary loop) as the “flow center” that connects all the secondary loops together. They all had roof-mounted, flat- plate solar heat collectors connected to the boiler room with a relatively short and straight pair of glycol supply pipes (1” copper, well-insulated). None of the other plumbing indoors or outdoors was damaged. Figure 41-3 shows the typical piping configuration used


in all of the troubled installations. All of these systems worked normally for years before the unexpected freeze damage occurred. It turns out that the sub-zero liquid antifreeze itself caused the water side of the heat exchanger to freeze overnight. How does this happen and how can we prevent it? Let’s take a closer look at the details.


“downhill” if the piping allows. At night, if unchecked, cold fluid from outdoors “sinks” down one solar supply pipe, while warmer fluid, heated by the warmth of the boiler room, rises up the other supply pipe, causing continuous circulation through the two pipes. The cool fluid usually flows in the reverse direction from the daytime pumped flow. To prevent this, a spring check valve is virtually always included in the solar loop, labeled as spring check 2 in Figure 41-3.


e Turn to STICKNEY on p 82


phc december 2011 www.phcnews.com


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