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What is this pattern of discoloration around my ceiling AC supply vent, and how can I stop it from happening again if I repaint?

photo of ceiling AC supply vent with radial discoloration of surrounding ceiling

Background: In September 2016, I had an air conditioning system installed in my house (built in 1893, located in Massachusetts, USA). This included all new ductwork (we have hot water heating so there were no pre-existing ducts). The air handler is located in the attic and there are 5 supply vents upstairs and 3 downstairs, with a central return in the upstairs hall.

Around the end of 2017 I first noticed some brownish discoloration starting to form around this vent. It has continued to expand in 2018 and 2019.

This type of discoloration is only visible on this particular vent, and none of the others. This is the vent for my living room and it is the largest of the 3 downstairs vents. This is also the only downstairs vent mounted in a ceiling paneled in fiberboard (masonite, I think). There is also a beam and wall near this vent, which may be impeding airflow, especially in the directions where the discoloration points. I doubt there have been any leaks/drips above this area.

I suspect the staining is some sort of condensation; could it be happening in the fiberboard, caused by over-cooling because of the nearby beam and wall?

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It's a water stain, either from a leak as Ed stated or condensation. Check the ductwork for leaks. Once you have fixed the problem, brush on two coats of stain blocker, prime and then paint. Whether you have to paint the entire ceiling will depend on how well you can match the paint.

  • I don't see how there could be a leak there, but condensation is a possibility. The ductwork to this location consists of an insulated flex duct (in the unconditioned attic) which connects to a rigid, uninsulated duct that runs straight down through a bump-out in the bathroom wall. I guess when the A/C is running that rigid duct gets cold enough to collect condensation on the outside, which then drips down into the ceiling panel? What's the right way to fix it -- tear open the bump-out and insulate? – Shimon Rura Nov 4 '19 at 14:24
  • A leak can happen on the opposite side of the house and come out here where the vent exits. – Gunner Nov 4 '19 at 15:51
  • @Gunner I agree with that in theory, but it's extremely unlikely in this particular location. Also, the spot seems to grow in the summer but not other times of year, which suggests the condensation explanation may be right. I could perhaps check the condensation theory by removing a switchplate and inserting an inspection camera into the bump-out that contains the rigid duct (once AC season starts up again). – Shimon Rura Nov 4 '19 at 18:21
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To me that looks like a water leak that is being pushed by the air flow. I would be checking for a leak above that location.

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