Freeze danger from thermosyphon reverse flow

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

During 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.

 

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 (page 28) 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.

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 “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.

If spring check 2 is missing or fails to shut properly, cold glycol can pass backwards through the pump and reach the heat exchanger. A pair of red and blue arrows shows the direction of the thermosyphon flow at night around the solar glycol loop. When this happens, freezing cold glycol can enter the heat exchanger backwards through the pump, causing the water side of the heat exchanger to freeze. In our perplexing example, the check valve was doing its job, and that side of the heat exchanger was not damaged.

Thermosyphon reverse flow — single pipe

Thermosyphon flow is also possible inside a single pipe. Under moderate temperature conditions and in smaller pipes, the effect is hardly noticeable. But under extreme temperature conditions, with larger diameter pipes, especially in straight vertical pipes, the resulting flow can be substantial. The enlarged pipe diagram in Figure 42-3 shows the flow pattern inside a single pipe with two pairs of red and blue arrows. This is what happened during our record cold snap. The evidence can be seen in Figure 41-1, where all the freeze damage has occurred on the left side, at the solar glycol “hot pipe” connected to the top of the solar collector. In all of these examples, this was the pipe without a spring check valve. Single pipe reverse flow can be blocked by installing another spring check valve on the solar hot pipe, shown as Spring Check 3 (in green) in Figure 42-3. This will prevent cold fluid from “falling” directly into the HTX down the vertical pipe.

Preventive measures — summary and discussion

This experience pointed out a certain vulnerability that exists in this piping detail that does not exist when using a primary loop HTX system, such as the Combi 101 mentioned in previous columns. When the water side of the HTX is placed in the flow path of the primary loop, it is freeze-protected by the constant flow around the loop, due to the persistent operation of the heating system. It would take a major power outage or shut-off of the backup fuel for freeze danger to occur. In our example, when pump P1 shuts off at night, there is no flow in the water side of the HTX.

The installation of Spring Check 2 is mandatory, and it is probably a good idea to install Spring Check 3 any time the HTX is located at the lowest point in the glycol loop or is connected with direct vertical supply pipes. Single pipe thermosyphon flow can also be blocked by terminating the bottom of a vertical pipe with a horizontal run followed by an uphill run, creating a “U-trap” for the migrating cold fluid. (Remember to allow for air vents when adding bends that can trap air in the solar supply pipes.)

A freeze protection controller can always be added to any boiler room for a little extra peace of mind. This can be as simple as mounting a snap disk or capillary tube low limit switch on the heat exchanger pipe to run the circulator pumps to provide freeze protection by forced circulation. An electronic temperature control could also do this job.

It is interesting to note that, when the solar collectors are wall-mounted or ground-mounted at the same level or below the level of the boiler room, the problem of reverse thermosyphoning tends to just go away.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute recommendation or endorsement.

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.