Before you select a system, you’ll need to classify your climate area as either “mild” or “harsh.” For the purposes of the table (see In This Article), we split the list according to each system’s freeze tolerance—or lack of it—and set the dividing line at 10°F.
However, since water freezes under different conditions of temperature, pipe size, liquid flow, insulation, and time, there are no hard-and-fast divisions—it’s very difficult to give a rule that will work everywhere under all conditions.
In “mild” climates, where freezing conditions are uncommon, potable water can be used directly in the collector loop. In SRCC lingo, this is a “Type I fluid system.” These mild-climate systems have no heat exchangers, and are usually simpler and less expensive, with few components. However, because of their limited freeze tolerance, these systems, which include integrated collector storage (ICS) systems, direct forced-circulation systems, and open-loop thermosyphon units, are generally limited to installation in Hawaii and the southernmost part of the United States—states that border Mexico or the Gulf of Mexico.
The rest of the United States is classified as falling in the “harsh” climate, since the probability of freezing is far greater. These areas are best served with true freeze-tolerant systems, which have heat exchangers, and are either drainback systems that use potable water (Type I fluid system) or nontoxic antifreeze systems (Type II fluid system). The SRCC catalog refers to these systems as “indirect forced-circulation systems.”
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.)