At this point in time the only heat is through solar which limits the swim season. The pool did have an electric heater many years ago though, which allowed year round operation, but operating cost vs level of use, particularly in the busy holiday season did not justify replacing it when it died. With the current solar heat system (upgraded last year, old solar came down with the old roof) I was able to maintain desired swim temperature of 83-84 degrees unitl the end of October when night time temperatures started dropping below freezing, and the last swim of the year was in mid November for a neighbors teen age son's birthday, water temperature was down to 77 by then, but as you know we all had a long cold winter, so it is hard to judge yet how the new upgraded solar will do in an average year.
Ike
p.s. about half the time we don't get our first freeze here until after New Years, this year it was about Oct 20th. I can't really answer for Chicago, a lot depends on the weather, I can tell you that here even with our very cold winter last winter, going 2-3 days at times below freezing and lows in the teens a number of nights that the air in the pool house never dropped below freezing and that the water only made it down to the upper 40's.