TOPIC: Small duct runs to plenum, why?

1997 house in Orange County Ca has a small duct inside of the plenum, under the furnace filter. I can't see where it comes from but it has a closed baffle inside. Why would one put a duct here?

I would suspect that the duct goes to the exterior to bring in fresh air. But this is making an assumption based upon insufficient information.

If you're going to talk about this you need to go find where the thing comes from and all pertinent information. Otherwise all replies to your question are unsubstantiated.

The end of that duct looks awful dirty, almost like it is the dryer duct...

David is right.  It's a California thing.  I had one on my house when I bought it.

I had not seen the pic's yet.
The closed damper is correct by the way. Even closed, they allow a lot of air by-pass. You will find most fresh air dampers closed like this.

I have seen these also and put this in my report. Most buyers have no idea what it is.

A Controlled mechanical ventilation system is installed in your home. Mechanical ventilation assists in controlling the humidity, contaminants, air borne particles, and general air quality. With single thermostat systems, periodic whole house mixing is critical to providing the best overall level of perceived IAQ (indoor air quality) and comfort.

The acceptable air quality has to be maintained by supplying adequate air of specified quality and quantity in the space. The ventilation of the home can be done by natural ventilation by means of windows and skylights or by mechanical ventilation.