A Thermosphoning Collector System

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First, the convective loop will work properly only if the bottom of the storage tank is at least a foot and a half above the top of the collector. This head is necessary to build pressure in the system and to help prevent back-flow at night. [EDITOR'S NOTE: Steve Baer's companyIf the modern solar water heater seems too costly and complex, give some thouaht to . . . Zomeworks, Dept. TMEN, P.O. Box 712, Albuquerque, New Mexico 87103-recently introduced a convection check valve that allows the collector top to be posi tioned as high as the top of the storage tank and the bottom of the collector as high as the bottom of the storage tank. The device is profiled in MOM's Market place, on page 146.]

Furthermore, the connections to the storage tank need to be properly located in a convective-loop solar hot water setup. The cold line to the collectors should exit at the bottom of the tank, and the hot return must enter near the top. The cold line, however, shouldn't be placed in such a way that its effective head will be less than that of the hot water return.

Because thermosiphon systems rely on (relatively) weak convective forces to provide their circulation, they need to be built with generously sized tubing to reduce pipe friction. Depending on the distance between the collector and the storage tank, then, the pipes should be at least 3/4" in diameter, and 1 " would be preferable. (All pipes must be thoroughly and equally insulated.) In addition, the feed and return lines should be pitched evenly from the collector to the tank (without any significant dips that could catch air bubbles).

Finally, if you're going to construct collectors similar to the ones shown here, either the units should have their headers built in at a slight angle, or the entire collector must be tilted (about 5° of rise toward the hot water outlet side should work out well).

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