I have seen a few other posts about how this is probably a fruitless endeavor. I have a 16'x32' 14,300 gallon saltwater Intex AGP and I am trying to heat it up 5-10F. I have a pool cover and that has been getting it up to the mid 80s with the high ambient air temp in the low to mid 70s, but I want to get the water temp into the 90s. I was thinking about building a secondary pump system that did not drive off of my main filter pump, run it through a few hundred feet of PEX tubing and use heat spreaders that are used for radiant floor heating, paint it all black and cover it with greenhouse plastic sheeting, and set it up along the south edge of the pool. I am thinking that this DIY setup will run me probably about $600 for plywood, pump, tubing, plates, etc. plus the electricity to run the pump when desired. I don't think I can run it to the roof and I am in the process of getting solar panels anyway which would mitigate the cost of electricity to run the pump. I saw a YouTube video of someone doing something similar with copper tubing in a single circuit and got the water temp to near boiling. I want multiple circuits of about 100' instead of one very long circuit because hot water only absorbs so much heat. I can't use copper due to saltwater. If I do 4 sheets of 4'x8' plywood that would be a total area of 128 sqft vs 512 sqft of pool surface area so only 1:4 instead of the recommended 1:2, but I am hoping for more heat transfer.