My longtime friend moved recently and his new place has an in-ground pool. He's a bit* of an Engineering nerd he has decided to build a solar heater from scratch. Him being a neophyte at the art of pool science though, I felt it my duty - despite being someone who never got his own custom exchanger working - to share my wisdom. *a lot Some ideas I'm pretty sure of, some less so. But it's got my curiosity up. Looking for feedback. In descending order of confidence: 4. His heat exchanger core is 50' of black garden hose. Obviously, thinner, more conductive tubing would be better. I would go with 1/4" copper tubing myself. Greater surface area per unit volume, better thermal conductivity. We all agree, but it's out of budget for now. 3. He intends to secure his heat exhange array to a large sheet of plywood. Truth be told, we both agree that it would be good to have a heat sink (something more heat absorbent than plywood), and embed the array in a 1/2" or so of conductive substrate of some sort. (Heat exchange by direct conduction will be far more efficient than heat exchange via radiation from a reflective surface). However, such a solution is also not in the budget at this time, so plywood back it is. He intends to paint the plywood black. My suggestion is to paint it silver, or better yet, line it with tinfoil. Failing an actual heat sink, you don't want your backing to absorb rays, you want it to reflect rays back to the hose. 2. They are using black hose, to absorb heat. I suggested clear hose would actually let more solar radiation in. Upon reflection though, I recanted. Clear hose will also let much of that same solar radiation right back out the other side. Even with a reflector, much radiation will never make it into the system. 1. They are making what I consider and excessively complex setup that takes cold water in to the array and recirculates it - in a secondary loop off the main plumbing, and powered its own secondary pump - within the heat exchanger until it has reached a certain temperature, then releases it back into the pool. Being a nerd with an electronics bent, he has constructed a control box from an arduino, a temperature probe and an actuator valve, that automagically opens and shuts when the water temp reaches the desired values. I am impressed to say I have seen it working, and - for this reason alone, I'd like to see his solution functional - however, I don't think he is gaining anything with his secondary loop. Water running continuously through the system will draw heat from the solar array just as efficiently, no matter how fast it flows. (In fact, the faster the better, to a point) i.e. even if a single pass raises the temp by, say, only one degree, it will be made up for by the sheer throughput. Heat transferred is heat transferred, whether it be in 5 gallons of throughput or 50. Matter of fact, 50 is better - the higher he raises the temp., the more heat loss from the system. 0. Finally, something I've been toying with on my own: exchanging heat directly on the pump. It puts out a lot of waste heat. I'm thinking a coil of pipe off the main line, running through some sort of liquid-filled bag or pillow**, draped *directly* over the pump should transfer some heat, and do so with little fuss and muss. ** self-contained fluid-(prob'ly water)-filled bag acts as the substrate/heat sink. Form-fits to pump, with flexible copper pipe running in one corner, winding around lots of times, and out another. i.e. Fluid contents of bag are static, and not part of pool water.