Heat would blow for about 10 seconds, but then returned to cool air?

We bought our car in August, 2007. We are not aware of what repairs were made before we got it. As the cold weather has arrived, we tried turning on the heater once the engine was warm. Some small amount of heat would blow for about 10 seconds, but then returned to cool air. I am guessing it may be some relay or sensor. Thanks for your help. Incidentallly, I tried entering a small donation, and it would not accept it.