i agree with spensar. without a heater, you'll slow down the loss of temp (my pool was still 77 today..enough heat during the day to keep it up to that i guess) but it was a hot day..over 80..but, no heater to maintain a bit higher warmth at night. as you've probably found out..2 or 3 degrees in the pool can make the difference between swimmable or not..swim until you can't take it anymore, i guess..but don't expect that the pool is going to gain 3 or 4 degrees in a moderate climate where it's cooling off to 50s -low 60s at night.