Thermosyphon Heat Exchanger Willson Bloch copyright 1991 Willson Bloch Prior to the advent of thermosyphoning heat exchangers, designers had two choices when installing a closed-loop solar hot water system. One: they could use a storage tank with an internal or jacketed heat exchanger, or two: they could use an external tube-in-shell heat exchanger with a pump. Internal Heat Exchanger Solution one seemed like the best way, but it posed two problems. Heat exchanger equipped tanks were very expensive; and when tank replacement became necessary, the heat exchanger went too. Secondly, because of its internal location, the heat exchanger was impossible to descale of its mineral buildup. This rendered it less and less effective as the mineral coating grew thicker and thicker while often accelerating the deterioration of the tank itself. External Tube-in-shell Solution two was an external tube-in-shell heat exchanger with applied pump to extract the heat from the solar fluid to return it to the storage tank. This solved both problems of the internal heat exchanger but only by adding a pump with its parasitic electrical consumption. Also, the pumped heat exchanger didn't work very efficiently because the pump always pumped at the same speed regardless of the available solar radiation and corresponding solar fluid temperature. Thermosyphon In 1984, Noranda Corporation released the first thermosyphoning, external heat exchanger. The design was good, but the materials used in its construction were below standard despite the International Association of Plumbing and Mechanical Officials, (IAPMO), stamp of approval. Note: Drain, waste, and vent pipe, (DWV), and test caps were approved for use in household, (150 psi), situations. Also, the set of installation instructions that were included with the heat exchanger showed that Noranda hadn't really researched their product well. I purchased several of the Noranda exchangers; and through experimentation, discovered the method of installation that produced optimum results. I especially liked the way the exchanger heated the storage tank from the top down rather than gradually bringing the whole tank up to maximum solar temperature by the day's end. The thermosyphoning method meant that hot water would be available to the user much earlier in the day. Days of marginal solar radiation would produce some useable hot water in the upper portion of the tank rather than the whole tank at only lukewarm temperatures. Also, the thermosyphoning action works proportionally with the amount of available solar radiation. With its vertical mounting position alongside the storage tank, cleaning the exchanger is a breeze when installed with two shut off valves, a boiler drain, and fill plug. A simple 30 minute task can clean off mineral build up and return the exchanger to like-new condition. This means that a properly built thermosyphon heat exchanger will last a lifetime with only miniscule care. Homebrew Noranda never did upgrade their thermosyphoning hat exchanger; and when the solar tax credits died, they got completely out of the heat exchanger business. Another company in Florida came out with another type of thermosyphoning external heat exchanger that served as an elevating pad for the tank, but its accessibility to cleaning didn't please me. I decided at that time to build an exchanger similar to the Noranda design but with improvements in size, efficiency, and especially in pressure-durability. It is now commercially available, and some typical installation diagrams as well as cleaning procedures follow: Richard diagrams go here Cleaning the Heat Exchanger (HX) 1. Close, (clockwise), the upper and lower shutoff valves. 2. Open drain valve at bottom and remove threaded brass plug at top to drain the water from the HX. 3. Close drain valve and fill the HX with one gallon of white vinegar. Top off with water so that HX is completely filled. 4. Reinstall the brass plug. 5. Turn on the solar system so that solar fluid heats up the HX till it is hot to the touch, then turn the solar system off; and let the HX sit for 20 to 25 minutes, (longer if HX is really scaled-up). 6. Open the drain valve and remove the brass plug to drain out the vinegar and water solution, and then replace the brass plug. 7. Leave the drain valve open and open the upper shut-off valve to flush out any remaining vinegar and water solution, then close the drain valve. 8. Open the lower shut-off valve and you are finished. You might double- check that both shut-off valves are open to be sure that the HX can begin thermosyphoning otherwise your pressure relief valve will blow off. Access Willson Bloch, Sunburst Horizons Co., 22580 Hwy 184, Dolores, CO 81323-9111 ¥ tele: 303-882- 4558.