Sweating AC Vent

While performing a 6 month Maintenance Procedure, I noticed that one of my customers had a problem with a couple of his AC grilles sweating and staining his ceiling.  Usually this is caused by the register box or AC duct not being insulated properly, or by excessive moisture inside the air conditioned space.  I examined the register boxes and ducts and made sure they were insulated properly.  I told my customer how important it was to have and use bath fans and kitchen exhaust fans.

Two ways to get moisture in a house are showering and cooking. 

The vent fans need to be ducted to the outside of the house.  It is important that you use your fans while showering or cooking to remove the moisture.  I told the customer to use fans and watch the area closely and I would call in a couple of days to see if this resolved the problem. I called back in a few days and talked to my customer.  He told me they were still sweating.  Sometimes you really have to dig deeply to find solutions to problems with houses.  Sometimes you have to repair several things to resolve the problem and sometimes, when you have it resolved, you are not sure which item you repaired actually resolved the issue.  Sometimes it is a combination of several things repaired that work together to provide a solution.

I returned to the house to take another look.  I had to think outside of the box.  The Heater closet was in a hallway not far from the sweating registers.  I noticed that there were two rooms that opened to the hallway that had their doors closed.  I asked my customer if these doors usually stayed closed and he said yes.  One was his son’s room and the other was a catch all room.  I told him to leave the doors open and see if that helped.  I called him in a couple of days and he told me that they were not sweating since he left the doors open.  Closing the doors affected the return air circulation and caused the grilles to sweat.  I’m not sure exactly why this happens but I have seen this situation several times in my career.

The grilles affected are always close to the return air grill on the heater closet.  I returned to his home and repaired his ceiling by Kiltz painting the affected areas.  It was a blown acoustic ceiling and luckily it had not started peeling off.  The Kiltz paint killed the stains and was a very close match to the ceiling color.  If the blown acoustic had started peeling then that would have been another problem that can be difficult to deal with.  But, that is a story for another day.

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