UV lights are activated according to a unit's program; they do not need to be on at all times. UV1 is for the upper tank (the one with water ready for consumption), and it is turned on periodically to sterilize the stored water.  UV2 is for the water outlet and is activated when dispensing cold water. UV3 is turned on when the bottom tank is almost full before the booster pump is activated to send water to the filters.