Last summer I installed two 500' coils on the roof last summer and on average got an 8 degree return in pool temp for a 12x24 above ground Intex. Never got over 72 degrees, but it was a cold summer here in Southern California. I have a new plan (in addition to the coils) and can scan attach the free-hand diagram if needed.
So how about if I use the existing solar coil inlet (connected off the main pool pump outlet to the auxiliary 1HP "coil" pump, to a 40', 1.5" underground PVC pipe going to the crawlspace under the house, where, an 80 gallon auxiliary water heater would be placed (right next to the existing house water heater) I then fabricate all appropriate gas connections and then run the water heater outlet PVC line back to the existing coil return line (pool pump return)
*note coil will be isolated by ball valves in this mode.
So when Mother Nature is not working with me I can AUGEMENT the solar energy with a natural gas heat source. No different than the $3000 professional gas heaters used for underground pools but 1/6th the cost. Natural gas is cheap and if the water heater was set for say 110 degrees, 80 gallons of 110 degree water would definitely raise the water temp of a small 8000+ gallon pool that is say 72 degrees. The water heater is designed for a pressurized water system so an inlet check valve and spring loaded pressure unloading outlet valve is required. Water heaters take time to heat the water in it so I would run this for say 15-20 minutes daily to cycle the hot water out, refill with cold pool then shut it down. The gas expense should be negligible, as it is used sparingly and the pilot light is pennies on the dollar. Estimated cost of entire system (with a used water heater $250-300).
This should augment the solar coils nicely, especially when I put up the pool again this summer and fill it with frigid water off the hose tap.