Stain Around HVAC Register

I’ve posted this out here before but no one knew the answer.

Every once in a while I run across a brown stain around a ceiling mounted HVAC register. What can cause this? I’m thinking condensation caused by a closed register, or condensation from a duct loose from the boot in the ceiling. Anyone see this? Thanks in advance.


You are correct it is caused from condensation and the duct does not even have to be loose the insulation can be pulled back from the back of the boot.

It can also be the result of a low setting of the thermostat some folks just naturally like their home colder than the unit has the capability of reaching.
I have seen condensate stains like that when the outside ambient is above 100 degrees and the unit just runs non-stop during the afternoon until late evening.

I’m with Charley on this one.

I’ve seen airflow problems causing condensation on the register due either to closed damper, plugged/bullet-proof/stopped up filters, missing duct insulation either on the duct and/or removed/damaged insulation on or around the register boot.
Your example shows that most likely the ductwork is located in an Attic space, but I’ve seen the same conditions on sidewall or floor installations, where the ductwork is located in basement or crawl space areas.

It is very common here because the A/C is often on when a sliding glass door to the pool area is left open. The metal register sweats and the drywall sponges it up.

Inadequate insulation at the boot.