I have some Andersen double hungs and they are controlled by a very thin
sash cord, hard to see even if you know it is there. On mine, there is
a double channel guiding the window, and the cord is well hidden in the
channel closest to the outside; it gets there through a break in the
upper frame, that almost looks like they just cut the frame a little too
big for the channel divider. If the cord was broken, I think it would
retract and you would not see anything. Do you have another window
without the problem? If so, look at it and see if there is a sash cord.

wrote:
Thanks. It's a double-hung window - installed 5 years ago. I cannot
seem to find the maker - but it looks like the anderson double-hung 200
series. I do not see any "pulley/weight" system. The top window falls
on its own as soon as it is unlocked - it is supposed to slide down
only upon pulling it down. I do not actually see any mechanical system
after it slides down (or after pulling down on the top window). Not
sure how much more details to provide...