From personal experience, anything under 70*F is dangerous for ball pythons.
At 66*F you're practically begging for a respiratory infection...

Here's what I did this year, I have a space heater in my closet with my snakes in there. I keep the door shut to my room as well to hold in the heat. It doesn't take much to get the closet nice and warm for the snakes and temps are in the upper 70s-82*F.