Is the unit a horizontal type in an attic? The air temp coming off the cooling coil may be too low from a lack of airflow or too slow fan speed or dirty coil etc. If the air quantity is increased so does the load on the coil and the air temp rises which raises your vent temp above the dew/sweating point. Lots of those attic units use an insulated flex pipe which won't sweat. Metal pipe needs insulating.
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