NJ:

My understanding is:

The button on the dash under the air conditioner button has two functions. When the engine is running it acts as the air recirc button. If the air conditioner is on, the air recirc will stay on continuously. If you are only running outside air, the recirc will only stay on for about 10 minutes and then kick itself off. When the engine is off, it will take remaining heat from the engine to keep the car warm.

You may want to do a search to look for some old posts on this topic.