I recommend the heat pump based upon my experience so far.
Cost of heat pump/chiller = $4,600 in 2007
Cost of electricity to *run HP = about $30.00 to $40 more per month (*only several days)
Cost of repair of Jandy Heat Pump = about $400 since installed 2007
Time to heat pool = about 1 to 1.5 degrees per hour for MY 10,000 gallon inground.
Time to heat 20,000 gallon inground pool by extrapolating data would be about 1 to 1.5 degrees every 2 hours?
Obviously there are some variables that must be considered.
My experience with a pool started May 2007, when it was installed here in Casa Grand, AZ. I did quite a bit of research by asking pool owners, pool builders, and pool service companies - before building the pool. I ended up spending the extra cost of a heat pump at about $4600. Since then I have kept records on using it.
Gas of any kind is very expensive here and learning from friends with gas heaters - $300 to $500 to heat a pool up to a comfortable temp for one day - was outside of my budget. With no natural gas option in our subdivision and propane too costly per gallon and cost of delivery, I went with a heat pump. The Jandy HP is 115,000 BTU/HR, the pool is 10,000 gallons.
After carefully tracking the ambient temp, pool temp, and relative humidity when starting the HP, the average pool temp increase is about 1.0 to 1.5 degrees per hour. Several years ago, we were in the pool on Thanksgiving Day! In the past year when Easter fell on March 23, we were in the pool.
So, I've calculated the cost per daily use of the pool, total cost pool installation, and adding the cost of the HP. It turns out that by being able to extend the "use time" - for me, the cost of the HP, maintenance on the HP, and the cost of electricity is definitely worth it. With a high use family - like active pool lovers and kids - it's a justifiable investment. If you aren't going to use the pool much, it may be considered an expensive luxury.
The cost per month when I heated the pool was only about $1 per day more than our regular utility bill. So the first time I heated the pool, my electric bill was about $30.00 higher than average for that month.
There have been a number of times when I've heated the pool from a "start heater" ambient air temp of 75 degrees, and pool temp 65 degrees. I started the HP on Thursday night (off-peak), running for 6 hours. Started HP on Friday night, running about 6 hours. On Saturday (off-peak) turned on HP at 6am, by noon on Saturday pool temp was 88 degrees. With the ambient air temp around 75d and the pool temp much higher, the pool feels REALLY good! We have a propane fire pit, so getting out of the pool, drying off quickly, and sitting around the fire pit is a nice way to spend Thanksgiving Day at home!
An added benefit -- One 4
th of July weekend (ambient air temp at noon was 105d), we had a pool party and the pool temp was a toasty 92 degrees – sunny Arizona! I turned on the HP in chiller mode. In about 4 hours the pool temp dropped to 88 degrees.
For a spa, gas might be more efficient, quicker heat up, and still affordable?
Heat pump efficiently is effected by the relative humidity, so depending on your climate and what time of the year you want to heat the pool, size of pool you are heating, cost of electricity, and how much time you have to heat up the pool, you can decide what works best for you.
Here's a link to the heat pump I have:
Jandy AE-Ti Heat pumps
You can see Jandy's data on temp vs. time chart.
That's just my worthless opinion.
Randy