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 When freezing conditions are encountered, a control valve in the collector loop
shuts off the water pressure to the collectors and exposed piping. The valve allows
the water in the collectors to escape to a drain. In larger systems, three or more
solenoid valves replace the single control valve used for residential applications.
Currently available collector loop control valves are typically not reliable. If water is
left in the collectors during freezing conditions, they will probably be ruined. For this
reason, draindown systems are not known for their high reliability in climates with
frequent freezes.
2.5 BASIC CONFIGURATION OF THERMOSIPHON SYSTEMS
If solar collectors are filled with water and exposed to the sun, the water will heat up,
become less dense, and try to rise. If the outlet of the collectors goes up to a
storage tank, the heated water can flow up into the top of the tank.
If the cooler bottom of the tank is piped back to the inlet of the collectors, a flow of
water will start. This "thermal siphon" can move enough water through the
collectors to heat the contents of a properly sized storage tank. (Figure 2-14)
This system is called a "thermosiphon" system. The collectors, piping and tank are
all outside and filled with water. Heat losses can be significant, and the potential for
freezing exists. For this reason, this system is used only in areas which rarely have
FIGURE 2-14
A Thermosiphon
System
OPERATION
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2.5 THERMOSIPHON SYSTEMS
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