No, nothing comes from the center vent, which is at it should be. I think when it was working properly, nobody noticed that nothing came from the center. It was only when it was misbehaving and people were uncomfortable that anyone noticed cold air from the center. I wrote this post just to give people an idea of how to replicate an intermittent duovalve symptom to help them in their diagnosis. My daughter was able to verify that the disconnected duovalve caused the same airflow from the center vents as she had experienced while the car was away.

But oddly enough, this problem was first reported to me (remotely, of course) during the previous winter. When I got my hands on the car in the spring, I found that the fresh air damper's vacuum actuator was leaking, causing various noises. I replaced it successfully and was confident that all of the problems were solved. I did test the heat during the summer to be sure, but the duovalve problem never occurred while I drove it, so I mistakenly thought that everything was normal.

It occurs to me as I write this that if I didn't have a duovalve, I might have simply unplugged the A/C compressor for the winter as a temporary, seasonal solution.

I do have the old duovalve - I was planning on taking it apart, but haven't gotten around to it. I'll give some thought as to whether or not I'm willing to sell it instead.