Mild Climate Systems

Integral Collector Storage

Passive ICS systems (batch water heaters) are the simplest solar water heater. Cold water flows under normal water pressure to the bottom of the tank, and hot water is taken off the top. Whenever there's a call for hot water, hot water moves from the top of the solar batch heater as cold water is pushed into the bottom. Most of the ICS units produced in the United States today are progressive-tube-type heaters as opposed to single-tank units. Although the storage tank(s) of ICS systems are freeze-tolerant in normal operation, the weak point in the system is the potable water pipes running to and from the units. These systems are climate limited and are included in the mild-climate listings. (For more information on ICS systems, see HP93 & HP108.)


These systems position an insulated solar storage tank higher than the collector, relying on the principle of heat rising to move water through the system. These open-loop systems are more climate-limited than ICS systems because the small riser tubes in the collector are vulnerable to freezing. However, thermosyphon systems can be configured in a closed-loop design, using antifreeze in the collector and a heat exchanger and potable water in the tank. Because closed-loop thermosyphon systems have potable domestic water lines to and from the collector, their Achilles' heel, they are vulnerable to freezing.

The advantage of this system over the batch heater is that solar heat is stored in a well-insulated tank, so hot water can be used any time with lesser penalty of overnight losses. The SRCC lists open-loop systems as "direct thermosyphon" and closed-loops as "indirect thermosyphon." (A direct thermosyphon system is described in detail in HP97.)

Direct Pump

Used in tropical settings where freezing never occurs, this is the simplest of the active systems, using a pump and a standard tank with electrical elements teamed with a solar thermal collector. A direct-pump system is also known as a "direct-forced circulation" system by SRCC classifications. In this open-loop system, the collector-loop fluid is potable water. As with ICS and open-loop thermosyphon systems, potable water must run outside to the collector, and the associated plumbing is vulnerable to freezing. A weaker freeze link is the smaller riser tubes connected to the header tubes. They are subject to freezing before the insulated potable water lines. Direct-pump systems can easily be married to a PV module that will power a DC pump. Direct forced-circulation systems are very popular in places like Hawaii, which has mild temperatures and plenty of sunshine.

Freeze-Protection Gizmos— Caveat Emptor

In an attempt to have their systems reclassified to gain more sales, some manufacturers have incorporated freeze-protection schemes into their "mild climate" systems. The bottom line? Buyer beware if you're considering installing one of these systems in your "harsh" climate. Only two designs—drainback and antifreeze systems—offer reliable freeze protection in these areas. Here are some freeze-protection devices that have caused collectors to freeze in the past —and consequently have required expensive repairs or replacement.

Direct pump with recirculation. Some differential controls for turning pumps on and off also have a "freeze-protection feature" that can be set to recirculate water from the storage tank to the collector. The logic is that the warmed, stored water can be routed to the collector to prevent it from freezing. But this method has ruined collectors when unusually bad winter storms move in and power outages occur. Without electricity to power the control and pump, water can stagnate in the collector, and a hard freeze can burst the collector riser tubes.

Freeze valves (a.k.a. dribble valves). For freeze protection, some direct-pump, ICS, and thermosyphon systems use a freeze valve, a passive valve that is set to open at a low temperature (either 35°F or 45°F). When the valve opens, water from the municipal or well system enters the collector, and the near-freezing water in the collector dribbles from the valve onto the roof or the ground. Although this strategy is perhaps more reliable than recirculation systems, it is far from fail-safe. Hard (mineral-laden) water can eventually clog the valve, and poof!—the supposed freeze protection is gone.

Draindown valves, which were incorporated into direct-pump systems all over the United States, have been one of the worst hiccups in solar-thermal history. At a preset, low temperature, a controller activated the valves to divert water in the collectors to drain outside. However, like freeze valves, draindown valves were prone to failure due to corrosion, hard-water deposits, and clogging. Typically, the first winter freeze ruined the collector—when the valve failed, the collectors remained full of water and froze.

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