I wanted to preheat water with solar energy before it went to my electric tankless water heater. The higher the temperature of the water going into the heater, the less electricity will be used to bring the water up to the preset temperature on the heater = money saved and it's Green!
First I re-installed my old 40 gallon gas-fired tank-type water heater. It is not even hooked to the gas line, it will just be a holding tank for the preheated water. I plumbed it so that the supply water comes into it first and then goes to the tankless heater. I'll get some more use out of it and it won't have to go see Mr. Recycler yet.
I installed a new pressure relief valve on it for safety. I popped off the plastic drain valve at the bottom of the tank and installed a steel nipple which then adapts to cpvc. That will be the cold water feed to the solar collector. Then I installed a "T" in the copper pipe between the output of the holding tank and input of the tankless heater. That is the warm water return from the collector. Don't worry about envisioning those connections right now... they will be obvious when you get to the "step 6" page.
Then, I threw together a "hot box" out of 2"x4"s and a piece of particle board. I made it to the dimensions of an aluminum framed window pane I had. I just used 3" drywall screws to hold the 2"x4"s together and 1-1/4" drywall screws to put the back on. I used a lot of silicon caulk to make it as air and water tight as possible. Painted it flat black and plumbed it with 1/2" cpvc. Then I painted the cpvc black too.
I don't know why I used this zig-zag configuration. I just started building and this was what I ended up with. I sometimes do things like that. It didn't occur to me until later that most of the solar collection boxes I see use a manifold across the bottom with several vertical tubes going up to another manifold at the top. Probably more efficient. But I'm sure this box will not be the one I keep. It was just a proof-of-concept quick-and-dirty.
Sorry about the fuzzy picture... the rest are better.