One thing to consider, is the pump run time with a heater, and/or a VSP. A Heat Pump will require longer pump run times. If it takes on average 6 hours daily to keep my pool temperature constant, it would take 3 hours with a gas heater (Assuming no solar cover), but I have to tell you that they are a PIA to put on and take off daily (especially with no help). I have also found that running the heat pump from 11 a.m. until 5 p.m. seems to produces faster heat times, given more efficiency as temperature increases.
While the heat pump runs longer, so does the pump. My calculations with a single speed at 3,450 RPM's is $.33 per hour. At 6 hours, my pump will cost $1.98 per day. The heat pump is around $1 per hour, or $6 per day. In total, the cost is at $8 per day, that is assuming one can keep the pool at the constant temperature of 84-86 degrees. This summer, I tried to maintain the temperature at 84. The overnight drop was only a few degrees. I found that the hotter the water is, the faster it heats up, even with the heat pump.
Assuming a gas heater can heat the pool up in 3 hours, plus the pump run time of 3 hours ($1 in cost per day), can a gas heater consume less than $7 of gas per day? Based on what I have been told by people who own gas heaters locally, the average gas bill runs between $450-$600 per month. In the end, I believe a heat pump will cost a lot less.
Although, if you like being in the water when temperatures are in the lower 70's, and lower 80's, then a gas heater would be a better choice, but a more expensive one. For my needs, no one goes swimming when temperatures get that low, especially with school starting the first week of September. The pool rarely gets used during the school season, which places limitations on the pool and this is one of the reasons why I decided on the heat pump along with clearance issues.
