We turn the heat on for the first consistent cold days with highs below 70 and nights maybe below 55. Our winter thermostat temp is 68 d. but we really could go lower on that. Usually we don't need the heat until January since it can be 75 d. here at Christmas. Most years, we have a period in the late fall and again in the spring where we only sporadically need heat or a/c. The a/c comes back on maybe in late March/April when daytime temps are about 80 (thermostat set at 78).