Replaced a dead Jandy with a Hayward Universal H-Series Low NOx heater. The Jandy died after 6 years, so wasn't to impressed. The Hayward is low profile and doesn't require a stack, which is nice. An in-law who is certificed HAVC gas and does pool work was impressed with the build quality, and said it is essentially the same as a small high efficiency furnace.

If you will be heating much, you will also want to use a solar blanket, and figure out a way so that it is easy to use. Per the US gov't of Energy figures, using one can cut your gas bill by about 85% when heating. There is a chart for locations, including Chicago with cover vs. no cover comparisions. The actual dollars will vary depending on you set-up but the % difference is striking.

http://energy.gov/energysaver/articl...g-pool-heaters

We actually use the heater to ensure good conditions through the season, which is so short here, not so much to extend the season.