In many places, even on a warm summer day the pool may be uncomfortably cold. Lots of thermal inertia, and if it's shaded at all it may not get enough solar en

In many places, even on a warm summer day the pool may be uncomfortably cold. Lots of thermal inertia, and if it's shaded at all it may not get enough solar energy to warm up. Without a heater the swimming season can be very short indeed. Also, it's pretty darn awesome to fire up the pool once a year in the winter, like for Christmas, and host a swimming-in-the-snow party. Wouldn't want to keep it heated, but for a couple days it's not too bad.