Why would one A/C vent have condensation dripping while the others don't?

Asked by Ms Paula L Willoughby
Jacksonville, FL

It's a 1959 block ranch 3/2, newish 4yr old air handler and outside unit. New insulation blown in this past spring. I have to change the air filter once a month due to pets. Here in Northern Florida with a couple dogs. Why would I have condensation dripping when it hasn't before. I keep the ac at 76degress F. Average humidity and outside temp for season.


Stillman Jordan

Answered by Stillman Jordan

Stratford, CT


July 15, 2013

Ms. Willoughby,

Condensation occurs when the surface is colder than the dew point temperature of a room.

The dew point temperature of a room is dependent on the amount of moisture in the air. Moisture in the air comes from several different sources including:

  • building leakage, where air and moisture come into the house through small cracks in the walls, windows, doors, etc. Older homes have substantial amount of leakage
  • people
  • plants
  • cooking
  • shower/ baths/ etc.

So the room with one diffuser that is condensing must have one of these two things:

  1.  a colder supply air temperature coming from the vent, or
  2.  air with a higher dew point (more moisture in the air)

My guess would be that there is some extra source of moisture in the room with the condensation problem. The only way you would have significant differences in the supply air temperatures coming from the vents would be if you had really long runs of uninsulated ductwork running around your house. A good way to see if you do have uninsulated ductwork would be to inspect the ductwork and see if it is condensing on the duct work itself. If you don't see condensation on the ductwork then I would be even more inclined to believe that the problem is a higher moisture content in the room experiencing condensation.

If you would like more information on dehumidification options for your house please check the links below.


For more information:

Read "What can I do to lower the humidity level in my home?" a Q&A answered by Stillman Jordan.

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