gkruske said:
As for extending the swim season, don't expect that. I found that even though an 88 degree pool felt nice in September, a breeze with a 65-70 ambient temperature is enough to make you want to get out.
In my opinion, a heater doesn't extend the swim season, just makes it a lot more comfy.
This is definitely a personal preference. My wife uses the pool as a therapy pool so has it be around 88F (not lower than 86F when in use). We use solar most of the time, but with gas assist she is able to use the pool into an extended season from April through November. Outside that range, the colder outdoor temps make using the pool very unpleasant (and the gas heating costs get extremely expensive) so she switches to an indoor community center pool at that point. Average monthly data where I live are the following for my 16,000 gallon pool with a mostly opaque safety cover and FAFCO solar panels and (unfortunately) 80% efficient gas heater:
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
.56..62..64...68..74..78...81..81..80...75..64..56 ... Avg. High Temp
.40..44..44...45..48..52...54..54..54...50..46..41 ... Avg. Low Temp
.49..54..55...58..62..66...69..69..68...64..56..50 ... Average Temp (slightly more weight to daytime temp)
7.9 5.8 4.9 2.0 . 0.3 0.2 0.1 0.1 . 0.4 2.1 5.6 6.3 ... Average monthly rainfall in inches
3.1 3.9 5.0 6.2 . 6.8 7.0 7.3 6.9 . 6.2 5.0 3.5 2.9 ... kWh/m
2/day from the sun (30-year average for month; panels facing south tilted at 38-15=23 degrees)
. - .. - .. - ... 6 ... 2 .. 1 .. 0 .. 0 ... 3 .. 8 . 11 .. - ... Gas Avg. Therms/day used for pool heating (1.67 Therms input per ºF increase)
. - .. - .. - . 300 100 . 50 . 0 . 0 . 150 400 550 . - ... $ Cost @ $1.68/Therm
Without any pool heating, including sun exposure, the water temp would roughly be the average day/night temp. Even with sun, however, an uncovered pool heats up some from sun during the day, but generally loses most of that heat at night from evaporation (depending on humidity) so the water is not much warmer than average air temp (< 5F higher). Use of a pool cover that still lets light through typically gives a 10-15F temp increase. Using a black plastic flat mat type of solar system in conjunction with a cover typically gives 20-25F temp increase (depending on solar panel area and time of year) or even higher depending on the insulation of the solar cover.
Our use of gas is mostly for days that are cloudy and when the air temps get lower, so usually April, May, October and November require the most gas assist with very little during the June to August period (June and September are iffy depending on the amount of cloudiness). I would guess that trying to heat the pool with gas in December and January would cost between $750 and $1000 per month, February and March probably around $500 per month. If we were to do that, we would probably get a more thermally insulating secondary cover, but the air temps are too cold, it rains too much, so is a moot point unless we were to get an enclosure (and that's also expensive -- we've looked into it).
[EDIT] I think I have 11 4' x 10' SunSaver (39.5 sq. ft. effective area) and 1 4' x 8' Revolution (31.6 sq. ft. effective area) panels and my pool is 15.5' x 32' (496 sq. ft. area) for solar area that is 94% of pool area. [END-EDIT]
Everyone's situation is different, but this can give you some idea of how it works for us, which admittedly is unusual due to the higher desired swimming pool temperature and the requirement for nearly daily swim exercise.
Richard