Poolschoolgrad said:
Under ideal conditions, here is a comparison.
It would take 1 hour and 15 mins for a 133k btu heatpump to heat 600 gal of water from 70 to 100.
Maybe, maybe not. That 133k BTU is under ideal conditions of 80 degree ambient air temp and 80% humidity. Heat pumps are rated based on whats called a COP number. The higher the number, the higher the efficiency. What they dont have is a standard or a set of regs to go by to compare the them to each other or actually give real world BTU outputs. Unlike a gas heater, heat pump BTU outputs can vary greatly depending on outside conditions. So, that 133k BTU might only put out 80,000 or 100,000 depending on the outside temps. I promise you it wont do 133,000 when its 60 degrees out at night. Below 55 degrees or so, they are pretty useless. If it's raining and 65 degrees, useless too.
Now, a heat pump in south florida will heat your 11,000 gallon pool pretty nicely though. Matter of fact, Florida is where they originally were used as pool heaters, i believe. The other thing you have to understand about a heat pump is that they are really designed to MAINTANE water temps, not heat the water 15-20 degrees quickly and on a moments notice. I like mine in months that the highs are in the low to mid 70's and the nights get to 55 or so. Works great to keep the pool at 80-83 degrees, but it runs ~4 hours a day to keep it there. If I dropped the pool to 70, it would take a couple days to get it back to 83 again.
So lets look at real world conditions here. Say your spa was at 75 degrees and its about 70 degrees ambient in January but 55 at night. You come home from work and want to use the spa. You turn the heat pump on. You might get 100,000 BTU out of it. Ok, a 30 degree rise will take 1.5 hours, give or take. A gas heater at 400,000 BTU will take about a half an hour. With the heat pump, you would really need to keep the water hot all the time if you wanted to use the spa at all, IMO. The trade off in cost to run the heat pump continuous as opposed to the gas heat for half an hour isnt much different.
If you just wanted to heat the pool, the heat pump is the way to go in south florida. But, as I said before, its too slow for my taste for a spa. You just need to do some more research and maybe find someone in your area with a heat pump and ask them how they like it for a spa.
In answer to your propane question. No, a gas grill tank is too small! For a 400,000 BTU heater, you need at least 100 gallon tank.
So for cost. I have no idea what your electric rates are or propane is per gallon so I'd just be guessing the cost to run either in your area. Here propane is about $2.50 a gallon. To heat a 600 gallon spa 30 degrees with propane, it would cost me around $5.00 in propane or 2 gallons. For a heat pump in my area, about $3.00 in electricity.