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GREEN SYSTEMS


| SOLAR SOLUTIONS | CONTINUED FROM PAGE 80 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


82


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. l


Bristol Stickney has been designing, manufacturing,


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


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.


installation can be divided into six categories, listed below, roughly in order of their importance. 1. RELIABILITY 2. EFFECTIVENESS 3. COMPATIBILITY 4. ELEGANCE 5. SERVICEABILITY 6. EFFICIENCY The success of any solar hydronic home heating installation depends on the often-conflicting balance between any of these six principles. Finding the balance between them defines the art of solar heating design.;


IN


— Bristol Stickney, chief technical director, SolarLogic LLC, Santa Fe, N.M.


this series of articles, I have been making the case that the key ingredients for solar/hydronic design and


phc december 2011 www.phcnews.com


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