Here in the desert, near Phoenix, AZ, I have a 9 year old Jandy heat/chiller heat pump. At the time of pool install, I decided on an HP vs propane. No natural gas option was available in my subdivision. I did some research and a propane heater or NG was extremely expensive.
I have been pleased with the heat pump for heating the pool. It will heat up about 1 to 1.5 degrees F per hour. I have heated it from 65 degrees F to 92 degrees in about 40 hours. At start the ambient air temp was about 65 degrees. Cost of electricity was minimal, and my electric bill only went up about 1 dollar per day. The concerns with a heat pump are the meantime to failure. I have had 3 service calls in the 9 years since the install. Just recently there is a gushing leak at the bottom of the metal canister that sits over the heat exchanger. According the Jandy tech, although the heat pump manufacturers tout titanium heat exchangers for reducing corrosion, the stainless canister is not titanium. I am about to open up my heat pump and see what is leaking pool water when the filter pump is on.
Here's what I know so far and didn't know when the pool was installed in 2007!!...
-- If you are thinking about installing a heat pump, do some research on the popular brands (AquaCal, etc), especially for manufacturers that will have replacement parts available into the future and not change the design so often that spare parts become scarce. Talk to the certified repair dealers. Find out how many certified repair dealers are in your area for any given brand.
-- Absolutely install a bypass system. If the heat pump fails and it leaks, you will want to be able to bypass it so you can continue to run the filter pump. This bypass becomes extremely important for another reason. The most common cause of catastrophic failure of the HP is the pool chemistry. The chemicals, chlorine, salt, and radical pH changes will corrode the components. You can bypass the heat pump and NOT be pumping pool water through it when it's not being used to heat the pool.
If the heat exchanger fails, it appears that the average cost to repair could be over half the cost of a brand new heat pump.
-- Buy a heat pump cover. Acid rain will corrode and can cause premature failure. Cover the heat pump when not in use.
-- A cover on the pool to reduce heat loss is a big plus. Here in the desert we can have ambient air temp swings of 30 or 40 degrees.
Overall, I have been happy with the Jandy HP, and main plus is I can extend the calendar use of the pool. For example, I can heat the pool up in the beginning of May, and have in fact, heated up over a Thanksgiving week end. Here in AZ, the summers gets hot, so the pool will get up to 93 to 95 degrees. I did turn on the chiller function one July 4th week end to cool it down from 94 to 88 because it was about 112 degrees F ambient. I have calculated my cost to run the heat pump and when my electric bill only goes up by $30 to $40 for the that month, I feel it's definitely worth it.
We have a Trane heat pump for our AC and heating the house. Although heat pumps become less efficient at around 40 degree F ambient temps, it works very well, even when we get down to below 32 degrees F ambient.
As mentioned, I would definitely do some cost calculations, potential repair options, parts availability, etc. Also, the newer heat pumps are quieter, but you will want to consider where you locate the heat pump, so it's location will reduce any potential fan noise issues.