AC comes on in winter, presumably to help dry out the air and reduce/prevent fogging. In summer the recirc comes on when the fan is off and.. I don't know, sufficiently hot or something. Car starts with recirc on, and as it decides on its own when to do it and when not to, I could be driving 5 minutes with no venting assisting with car cooling before I noticed and turned it off. Again, I have no problem with it doing that when the AC is on. I can't imagine how it would help with the AC off.

It can be set to be made manual. Did this and much happier now.

And some here claim there's no humidity sensor so I don't know about that part.