Condensation is often found dripping from HVAC ducts during the summer months under houses in the crawlspace. When crawlspace vents are left open, hot humid air comes in contact with the cold ducts creating this condition. Gallons of water can be generated daily which can lead to fungi and mold in the floor structure. If there is not a moisture intrusion issue under your house, it is best to keep the crawlspace vents closed year round.