Building The Framework  

Assembling the frame is a snap. As you can see in Illustration A, and in Photo Six, there's nothing fancy at all. First, cut the end-frame studs to length. Then using two 12d common nails per joint, assemble two identical end frames, exactly as shown in the photo and drawing. 
One of the end frames will be at the bottom of the batch heater when it's finally installed, so it should be reinforced with metal joist hangers. Again, simply follow the photo and drawing. 
Lay flat the end frame with the joist hangers, and stand the reflector subassembly on top of it, so that the factory edge of the plywood involute pattern is flush with the top of the end frame. Nail the involute pattern to the end frame with 8d nails. Repeat the same process for the other end frame, attaching it to the opposite end of the reflector subassembly. 
There are only five boards left to add: two long side frame studs and an identical back frame stud; plus two short side frame studs. Assemble them as shown in Illustration A and Photo Five, and then reinforce the joints of the assembly with metal truss plates and plumber's straps, as shown.  

The Tank Assembly 

Assembling the batch heater's tank involves (1) adding a selective surface to the tank's exterior, (2) installing the necessary pipes and fittings, and (3) mounting the tank in the reflector frame. 
A selective surface is a special coating designed to absorb large amounts of solar energy while reradiating very little.    

PHOTOS BY JIM FREEMAN  

The type we chose is supplied as sheets of thin metal foil. One side of the foil is coated with special sun-catching black chemicals; the other side is prepasted with an adhesive. Both sides are protected with peel-off plastic backings. 
To install the foil, you first must be sure the tank is smooth and clean. Re- move any rough  spots on the tank's surface with sandpaper, and then wipe it down with denatured alcohol. When the surface is dry, cut a sheet of foil large enough to wrap around the tank once, with a one-inch overlap. Peel the protective sheet from the foil's adhesive, and gently press the foil to the tank, starting at the bottom edge and working your way up. (The first sheet of foil will cover about the lower third of the tank.) Press out any air bubbles trapped beneath the foil with a soft rag or a wall- paper roller, and use a utility knife to remove the foil from any plumbing connections on the tank that get covered over. 
The second and third sheets of foil are applied in the same way, until the entire tank is covered. (Trim off the third sheet's excess.) As Photo Seven illustrates, each successive sheet should overlap the previous one by about an inch: Simply peel back the outer protective sheet, as shown, and apply the edge of one sheet directly over the other. (Leave the rest of the outer protective sheets in place to prevent damage to the thin foil. They'll be removed in a later step.) 
The top of the tank is convex; the bottom concave. Rather than struggle to apply the foil to these complex shapes, simply paint them with black graphite paint, which acts much like a selective surface. 
While you're waiting for the graphite paint to dry, build the tank support by cutting slotted angle iron to the specified lengths, and assembling the four pieces as shown in Illustration A. It's as simple as it looks: Just bolt everything together. When it's assembled, paint the support with the rest of the black graphite paint. 
Lay the assembled support across a pair of sawhorses, and place the tank on top of the support. (Photo Eight, though for a later step, clearly shows how the tank nestles within the support.) Bolt 

the tank to the support, using heavy pipe strap wrapped around the tank's middle. Now you can plumb the tank; that is, add the pipes and fittings that will transform it into the heart of your solar water heater. Illustration E shows a cutaway view of the tank with all subassemblies labeled and in place; the five exploded views show how every part of every fitting should be assembled. Simply follow the captions and diagrams, and you can't go wrong.  
When assembling the plumbing, wrap the threads of every screw-in fitting with three to four layers of Teflon plumbing tape. (The Teflon tape provides a permanent, waterproof seal.) All other fittings should be carefully soldered. Photo Eight shows how the base of the tank will look when you're finished. 
When the plumbing's done, temporarily install the four tank support brackets in the holes you drilled earlier in the plywood involute patterns. Lift the tank (on its support) and lower it onto the brackets for a test fit. With a pencil, mark the locations where the tank's plumbing connections will have to pass through the reflector assembly.  

Photo Eight Here's what the base of the tank looks like when the plumbing is nearly complete. The tank has already been coated with the selective surface and the black graphite paint, and it's securely nestled in its framework of slotted angle irons. As soon as the reflector frame is ready, the tank can be mounted.