days in your area.) Winter clouds and cold temperatures mean that there isn't all that much solar energy to be had anyway, and winter shutdowns of, say, three months reduce the heater's annual Btu output by only 15 per cent or so. We feel winter shutdowns are the most practical solution to freezeproofing:
Make sure your tank and all exterior plumbing can be drained completely.

Glazing

The best glazing material for batch heaters seems to be one of the specially designed solar plastics. (Kalwall, Filon, 7410, etc.) They're less expensive than glass, they're much easier to work with, they offer good resistance to breakage, and they have good optical and thermal properties.

Collection Efficiency

In the simplest batch heater designs, the entire inner surface of the enclosure is painted flat black to absorb solar heat. This technique works, but it's not ideal because using the box's inner surface as a collector raises the temperature of the (next page)

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Above There are only two ways in which this plumbing differs from the previous designs. First, a "vacuum breaker" insures rapid and complete drainage of the tank. Second, the inner tube, which carries hot water out of the tank, is insulated inside the tank with a loose-fitting jacket of CPVC pipe. (Full plumbing details for this design appear in the following article.)

Illustration B Horizontal Two-Tank System

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Above Here, a horizontal two-tank system forms a canopy for the home's patio doors and is housed in a double-glazed, heavily insulated enclosure of its own. Wall-mounted systems such as this and the previous one-tank version must be braced securely to carry the tank's considerable weight: Two 40-gallon tanks, for example, weigh over 750 pounds when filled with water. If you live in earthquake-prone areas, check your building codes before attempting this type of mount, or look into one of the ground mounts shown in the next two illustrations.



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Above Two-tank horizontal systems, if properly interconnected, can achieve higher output temperatures than single-tank designs because the incoming cold water is kept away from the hot outlet. Even so, these tanks still cannot take full advantage of warm water's tendency to rise. In this system, provision has been made for convenient draining of both tanks (for freezing-weather shutdowns or for maintenance) by the additions of a drain valve and a one-way valve.

 

Illustration D Vertical One-Tank System


Below This design combines the good temperature stratification inside vertical tanks with the simplicity of a single-tank design. The enclosure is mounted close to the house to minimize heat loss, and its sheathing matches the home's original siding to provide a "built-in" appearance. Our batch heater shown on the cover is based on this type of single-tank, vertical system. It uses a "selective surface" absorber coating on the tank, triple glazing on the enclosure, and a cusp reflector to provide superior thermal performance.

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