Ok, guys I could make this a very long post, but I will try to keep things fairly short, and stick to the important facts.
First off the pool, 20x40 indoor 36,0000 gallon pool with roof mounted solar, the roof is being replaced this fall and along with it the solar is being upgraded (this has been in the works for the last year). The existing aged solar heat would extend the swim season by about 10-12 weeks from late March through mid October most years. The location is in Louisiana, climate roughly equal to Houston Texas for a nearby major city comparison (maybe a little cooler). I am hopeful that the replacement more modern solar heat system will extend the season by about 16+ weeks total (spring/fall). This creates a reasonably long swim season, and under normal conditions I would leave it at that. However this year I may NEED a heated pool in January - March, as I am recovering from a broken back. By my doctors current estimates I will be out of my brace part time and ready to start physical therapy, etc. around Christmas. According to the doctor ideally this would start with swimming and other in water exercise that would keep weight off my back, unfortunately the nearest physical therapist I have found with a heated pool is 70+ miles away. So you can see why I say I may NEED a heater this year.
Now back to those facts: Fuel options Propane or Electric (maybe Diesel Oil as I have a spare 450 gallon diesel tank, and would have to upgrade propane tank size considerably for this)? Jan - Feb highs are anywhere from upper 30's through upper 40's most of the time, with lows often above freezing, however in the last couple of winters we have had lows in the low teens for multiple days at a time, and lows in the 20's fairly common. From what I have read online heat pump units tend to work best with smaller temperature deltas, is this true, etc. This then leads to what would you suggest? (other than a 3 month tropical vacation)
thanks for the help