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Part II—Drainback Systems
Tom Lane & Ken Olson ©2001 Tom Lane & Ken Olson
Whether your climate freezes one day of the year or every day of the year, your solar water heating system must have 100 percent fail-safe freeze protection. If your freeze protection depends on either human intervention
or the availability of electricity, you are living in the shadow of Murphy’s law.
In from solar collector
Two types of solar hot water systems are most appropriate for freezing climates—drainback systems and closed loop antifreeze systems. Solar Hot Water: A Primer (HP 84) covered the fundamentals of solar water heating systems. Solar Hot Water for Cold Climates (HP 85) covered the principles and components of the closed loop, antifreeze-type, solar water heating system.
In this article, you will learn the inner workings and components that make up the drainback solar water heating system. Some wrenches (system installers) of the frigid north have experienced drainback system failures due to flaws in design or installation. This article presents a culmination of knowledge and experience for freeze-free operation in cold climates. The drainback system is not to be confused with the strongly unrecommended draindown system mentioned in HP84.
Draindown systems rely on an electrically operated draindown valve to fill and drain the solar collectors with domestic tap water under household pressure. In general, draindown systems have had a poor performance history, with frequent freeze-related failures due to hard water, valve malfunctions, and other drain-related failures. By comparison, a drainback system uses a pool of unpressurized water that is completely separate from the pressurized domestic water in the solar storage tank.
In a drainback system, the collector fluid, typically demineralized water, is circulated from a reservoir tank through the solar collectors whenever useful heat can be collected. When the pump shuts off, the water drains
Out to heat exchanger
The heart of a drainback system—the reservoir tank stores the unpressurized water from the flat plate collectors that “drains back” whenever the circulating pump shuts off.
62 Home Power #86 • December 2001 / January 2002
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