Living in Minnesota and not having a pool heater, I have a very short swim season. I've been thinking of ways to make a solar pool heater. I'm thinking of trying to use black plastic irrigation pipe to make a collector like some have but instead of leaving it open, I was thinking of putting it in concrete painted black. I would love to hear others thoughts on the concrete idea. I was thinking that since concrete will absorb the heat from the sun, the many pipes running inside and through the concrete would absorb the heat from it which would then transfer to the water running through. Any thoughts? I got this idea from a photo I saw of a pool being built that was having pipes being run all under the concrete decking so that heat could be absorbed from it to heat the pool water.