What causes condinsation in you air ducts?
Condensation commonly occurs when a vapor is cooled. Water vapor
from air which naturally condenses on cold surfaces into liquid
water is called dew. Water vapor will only condense onto another
surface when that surface is cooler than the temperature of the
water vapor, or when the water vapor equilibrium in air, i. e.
saturation humidity, has been exceeded. When water vapor condenses
onto a surface, a net warming occurs on that surface.