Setting any air conditioner below normal room temperature (about 20-21º) generally isn't a great idea. When the room temperature is too cold, the refrigerant in the coil can't absorb the amount of heat it's supposed to, which drives the whole system's temperature downwards which can lead to problems (freezing of coils in more extreme circumstances) and reduce the lifespan.
It's the same reason you don't want to close a bunch of registers; if not enough air is passing over the cold coils, they don't absorb as much heat as they're designed to and it can lead to problems. ACs are generally designed to work best at room temperatures of 21º+.
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