Richard,
In addition to what Paul has said, the amount of time the recirc mode stays on after you push the button depends on outside air temperature. In cold weather, it may only recirculate for 5 minutes; in warm weather it's longer.

It's designed (thoughtfully, I think) to prevent us from fogging up our windows in the winter by leaving it in the recirculation mode.

You can get more details in your owner's manual.

__________________
Steve
'93 400E