parts are clearly shown in illustration A, and that you should turn to that illustration to see exactly what the component is, where it goes, and what it does.
Next, you should obtain any necessary permits and gather together the materials you'll need. We've listed all the components and their dimensions in one place (at the end of the article) for use both as a "shopping list" and for the dimensions you'll need as you build your batch heater.
That done, you're ready to build and install your passive solar water heater, using the following step-by-step procedure as a guide.

Drawing The Cusp
The cusp reflector is constructed from two specially curved plywood involute patterns, two sheets of thin hardboard (such as Masonite), and a reflective surface of aluminized Mylar. Construction begins with the layout and cutting of the two involute patterns, which create the special curve of the cusp reflector when the hardboard sheets are bent and attached to them. Photos One through Six provide an overview of this process and will help you visualize the next few steps.
Start by cutting the two involute pattern blanks to size. (Again, see the parts list for complete material specifications and dimensions.) Next, lay one of these blanks lengthwise on a level, flat surface with its smooth side up and with the smoothest edge (the edge cut at the factory) facing toward you. Locate and mark the midpoint of this factory edge, and use a carpenter's square to draw a perpendicular line all the way across the width of the plywood. Your line should divide the plywood into equal right and left 'halves, each 25% inches long by 20 inches wide. The water tank will rest squarely on this center line.
To help center the water tank, (which measures 14 inches in diameter), draw two parallel lines each seven inches to either side of the center line. Lift your (empty) water tank and place it on the plywood's center line, between the two lines you just drew. Now, keeping the tank squarely on the center line, position it so that it's set back one inch from the factory edge. (See Illustration B.) When the tank is properly positioned, run a pencil around its circumference to trace its outline on the plywood.
Now you're ready to draw the actual cusp. You'll need three feet of thin, flexible wire (don't use string, which could stretch and deform the shape of your cusp), and a heavy, black pencil. Tightly tie one end of the wire to the pencil near its point, and then position the pencil's point as shown in the illustration. Wind the rest of the wire around the tank in a clockwise direction, and

firmly tape the wire's loose end to the tank.
Next, as shown in Photo One, swing the pencil away from the center line, keeping an even tension on the wire at all times, and you'll automatically draw the curve that's just right for your tank. Run the pencil's point right off the factory edge.
Now, reposition the wire and pencil to draw the mirror image of the curve you've just created: When you're done, you'll have drawn an "involute curve" shaped like a rounded letter W. The central point of the involute is the "cusp" from which the reflector assembly gets its name and its characteristic shape. You can now remove the tank from the plywood and set it aside for later use.
The involute you've drawn is an ideal form, and must be slightly modified to meet real-life conditions. First, you should "open up" the curves to admit the maximum amount of sunlight. It's easy: Make a pencil mark on the factory edge exactly 25'/4 inches to the right of the center line. Next, take a straightedge and draw a line between this point and the original involute curve, so that the line just grazes (is tangent to) the curve. (See Illustration C.) Repeat the same procedure at the other end of the plywood. In effect, you're opening the original involute almost to the outer corners of the plywood sheet.
Illustration D and Photo Five are both for later steps, but they show why the next modification is needed. A 2 X 2 will span the distance between the "points" of the cusp to provide the necessary support for the two thin hard-board sheets. Because 2 X 2s are cut square, the curve of the cusp must be changed from its original narrowly pointed shape to a squarer shape that matches the 2 X 2's right angles. This only takes a minute: Use a carpenter's square to redraw the point of the cusp to a precise 90-degree angle.
By examining Illustration D closely, you'll see how the original, sharply pointed curve of the cusp will be recreated in a later step with strips of aluminum flashing. You now should draw a reference mark to help mount the flashing accurately in that later step. Photo Two shows how: Cut a short piece of four-inch aluminum flashing, and position it at the intersection of the plywood's center line and the tank's circle. Bend the flashing slightly to follow the actual involute pattern, and make a pencil mark where the flashing ends on the involute pattern. This mark is the "flashing layout line" that will be used in a later step.
The last step before actually cutting out the involute pattern is to mark the location where the tank support

Illustration B Drawing The Involute And Cusp

Above The involute is the spiraling curve, and the cusp is the point where the two halves of the involute meet. To draw these complex curves, all you need is a length of wire and a pencil: Take a bird's eye view of your tank as it rests on the plywood sheet and pretend that its circumference is divided like the face of a clock. The center line of the plywood should bisect the tank from 12 o'clock to six o'clock; and the six o'clock position of the tank should be recessed one inch away from the plywood's factory edge. Tie one end of a three-foot length of wire to a pencil, and place the pencil's point on the plywood's center line at the 12 o'clock position. Wind the wire clockwise around the tank and tape the wire's loose end to about the 9 o'clock position. Now swing the pencil away from the center line, as shown, and you'll automatically draw the proper curve. Then simply reverse this process to draw an identical curve on the other side of the tank, and your basic involute and cusp is complete.

ILLUSTRATION BY SALLY ONOPA

Illustration C Altering The Involute
For Top Performance

Above The involute and cusp must be altered slightly for construction ease and for optimum solar performance. Following the dimensions given in the text, use a carpenter's square to "open up" the involute nearly to the edges of the plywood sheet, as shown. Next, use the square to reduce the original sharp point of the cusp to a 90-degree angle. This will make fabrication of other parts of the reflector assembly much simpler in later steps.

ILLUSTRATION BY MARY WEST