Some pool guys have all been taught to parrot the relative humidity "issue" of heat pumps to sell a gas heater and that heat pumps don't work in hot places unless you have lots of humidity. Very bizarre. There is some truth that more RH is better as water in the air is a form of heat. Maybe 5% to 10% better efficiency on the heat pump the higher the RH goes but since a heat pump can be 600% more efficient than gas it really doesn't matter. Hey we have gas heaters and there are some situation where they make sense. If you ever get me on the phone I have great stories about CA pool owners fighting their contractors over heaters. I rarely seen desert state pool guys at the national pool conventions. I think that they have been spoiled by too many years of cheap natural gas and people who don't care about efficiency. We have many videos on the site of our heat pumps on our test pool in Las Vegas running all winter and they have very low humidity as you do. Warm dry states are great for heat pumps. They usually last twice as long as gas heaters, are greener as you don't have an exhaust cloud in your back yard, and you never have to worry you will hurt them with chemicals, PH or chlorine as most have titanium heat exchangers and lifetime warranties on the parts that touch the water. In some situation like in Vegas in the winter they actually work better with low RH as then the outside coil doesn't ice up as fast and the heater can heat longer without going into defrost. If you are going to heat your spa when it is below 50 degrees though you need a heat pump that will defrost with hot freon gas. Some heaters like Raypak / Rheem have that defrost feature as an option as well as Jandy heat and cool units that will work in cool weather and also cool the pool in the summer. Both are great brands. It depends on efficiency, warranty etc to pick the best one for your area. We have lots of videos of them being tested on our site in cold weather. You might want to watch the videos. I will send you a personal letter later tonight with links to them. The Raypak double coils are used all the time. They don't switch them on and off. We had good test results with them. They use a double coil and other manufacturers use a bigger coil and it all is relative as you want to look at the big picture like the COP number of the heater. The higher that number the lower the cost of electricity. With $4.00 propane and your great electric rates you will save a fortune using electric. For all you doubters out there just do the math. Figure out how much you are paying for a 100,000 BTU's. A gallon of propane at $4.00 will give you 91,000 BTU's. (Then you lose another 15% up the exhaust.) A heat pump using 6 KW for 100,000 BTU's at 11 cents a KW will cost 66 cents. You are on the right track with a heat pump for your situation. All heaters will work with your Aqualink. That is a nice system. Later tonight we will send you some info on our favorite cold weather heaters.