I agree with Berkshire, a black rubber hose does wonders.
However, I just priced these while I was looking for a new pool heater. The prices are pretty low now so I didn't see any benefit in building my own, other than it would be fun. I bought natural gas instead since I am set up for it and I don't want to trim grass around one more thing.
The cheapest home made unit I ever seen was a piece of 4 x 8 plywood painted with black paint.
Plywood sides 6-inches high, painted black. So a black box made of plywood.
Cut up sections of 4-inch diameter PVC Pipe, 6 inches long, glued to the Plywood in a honeycomb pattern, with about 1/2-inch spacing between Pipes.
Paint them black too.
Then, a piece of clear plastic sheet, like polycarbonate or plexiglass glued to the top.
The Plexiglass and the PVC pipes also have to be silicone glued so that the inside of the pipes are dry. The sun heats up the air inside the pipes, the water flows around the pipes inside the box. The more pipes you can put in, the more surface area you have.
Also, the same concept, only a black hose coiled up inside the box, then you don't have to glue the plexiglass. A plexiglass box gets really hot in the sun.
Water goes in at one side and out the other side, or top to bottom, or bottom to top...
Charlie
