Sustainable Building Technical Manual - Etn-presco.net

from the storage tank, or by flushing the collectors (drain-down). Since the recirculationsystem increases energy use while flushing reduces the hours of operation,direct-circulation systems are used only in areas where freezingtemperatures are infrequent.– Drain-Down Systems. These systems are generally indirect water-heating systems.Treated or untreated water is circulated through a closed loop, and heat is transferredto potable water through a heat exchanger. When no solar heat is available,the collector fluid is drained by gravity to avoid freezing and convectionloops in which cool collector water reduces the temperature of the stored water.– Indirect Water-Heating Systems. In these systems, freeze-protected fluid is circulatedthrough a closed loop and its heat is transferred to potable water through aheat exchanger with 80 to 90 percent efficiency. The most commonly used fluidsfor freeze protection are water-ethylene glycol solutions and water-propyleneglycol solutions.– Air Systems. In this indirect system the collectors heat the air, which is moved by afan through an air-to-water heat exchanger. The water is then used for domestic orservice needs. The efficiency of the heat exchanger is in the 50 percent range.Direct-circulation, thermosyphon, or pump-activated systems, require highermaintenance in freezing climates. For most of the United States, indirect air andwater systems are the most appropriate. Air solar systems, while not as efficient aswater sytems, should be considered if maintenance is a primary concern since theydo not leak or burst.5❑ Consider a pre-heat or full-temperature system.– A low-temperature solar water-heating system can be sized to provide only hot-waterpreheating. When hot water is needed, the warm water from storage is boosted tofull temperature with a conventional gas or electric-based hot-water system. Thesesystems can be relatively simple, with reduced collector size, lower insulation levels,and small boosting system, making them attractive options.– Higher-temperature solar hot-water systems can be designed to provide full-temperaturehot water. A conventional gas or electric backup system is used onlywhen there is no sun for extended periods. A high-temperature system can savemore in fuel costs, but with the tradeoff of more expensive equipment.❑ For systems using water as a collection medium, consider the following issues:.– Prevent stagnation. If a system is allowed to stagnate in direct sun, very high temperaturescan quickly result, causing collector materials to deteriorate rapidly. andcausing closed piping or storage tanks to burst from excessive pressure. Stagnationcan be avoided by venting or slow circulation of some water to keep the collectorscool; a drainback system can also be used.– Provide freeze protection. Freeze protection is important, even in nonfreezing climates,because an extreme weather event can cause substantial damage to a system.In desert climates, systems can freeze even on relatively warm nights because theirheat radiates outward to the cool night sky, dropping the system temperature tofreezing. The strategies used to protect a water system from freezing are determinedby the main system type (e.g., direct-circulation, draindown, or closed-loop).– Avoid calcification and corrosion. Calcification is the buildup of minerals inside acollector and its pipes caused by "hard" water circulating through the system. Opensystems that circulate city water are especially vulnerable. Mineral scales eventuallyclog the system, reducing flows and pump efficiency. Water can also be slowly corrosiveof both metals and organic compounds. Gaskets and sealants can be quitevulnerable. Closed-loop systems can compensate with buffering chemicals to maintaina neutral pH.– Plan for leaks. Any failure of a water-based system is likely to result in a leak.Provisions should be made to contain all possible leaks and prevent water from