My roof faces due south to a slight southeast. The roof is in full sun from dawn to dusk. I have done calculations to make sure I don't constrict water flow, and the valves can divert full flow to the roof, or partial if I need too. I don't know how much actual heat it will produce, but any is free compared to conventional heating. So being "realistic" I could probably get better results from a manufactured design, but I'm not putting that kind of money out right now. I have put out $350 so far, but a lot of that is the check valves and the ball valves. It is a simple coil design using 100' of PE tubing. I am using 3/4 inch tubing, as I have calculated that I can divert 100% of the flow through it to equal the pressure of my 1.5" return lines to the pool if I make at least 4 coils. I am using 2" PVC as the headers to reduce back pressure on the pump. Each coil is about 4 feet in diameter, with electrical conduit bolted together to keep the coil tight (conduit made into in "x" and bolted on the four ends and center with 2.5" carriage bolt.) Each coil will be connected to the header with a pvc tee and a reducer with a 3/4 inch threaded end for the pe connector to screw into it. I'm starting with 4 coils. After this, if I need to add coils, each one will only require the tubing ($21 for 100' unless I reduce to 1/2" which is only $12), 2 PVC tees per coil, 2 reducers, and 2 pex threaded connectors (about $8). So after I get the initial four, if I need more heat, I can built additional collectors for $20 to $30 each. The only reason I used 3/4 instead of 1/2 inch is because I would need to build at least 9 to equal the same flow for 1.5". 1.5" is probably better suited, but I don't have time to build to many of these until July. I will definitely post updates to let you know how well they work, but you can do a google search to find others who have built similar designs. One in particular
http://www.thecasualtraveler.com/solar.htm is a good system with a few design flaws (from an engineering pov) the collectors are connected in series 5 at a time. The will not provide enough flow to divert all the water if desired, and will put too much pressure on the pump. He has enough coils if they were all connected in parallel (even if 2 were connected at a time in series).
The drawbacks are mostly aesthetics and time to build each one. If you can operate a cordless drill, get up on a roof, glue PVC together, and put teflon tape on a thread, you can build your own too. With the weight involved when full of water, the only roof mounts needed are those to secure the headers. A vacuum breaker at the header would help if flow was turned off the the collectors, but because of the coil design, you will probably get only half the water to drain. PE tubing expands and contracts quite a bit, so a complete drain would not be necessary. I am including a connection by filter to attach my air compressor to blow out the water at the end of the season, but it would be easy enough for me to just unthread the PE cnnections and store the collectors during the winter.
