Kathi, are you getting the cold air flow when the furnace is completely shut down? When our furnace first kicks on, the fan, no flame yet, runs for approximately 45 seconds and then flame ignition occurs and air is heated and moved through the vents. After the set temperature is reached, the flame goes out, and the fan continues to run for another 45 seconds. During these short pre-flame and post-flame fan runs, cold air will be forced through the vents. The pre-flame air is usually cooler than post-flame. I was going to say, you would need a source of low pressure, i.e. an open window, etc., to draw cold air out of the floor vents since cold air moves down. However, I now wonder, is it possible that you heater vents are in the ceiling? Cold air could move down from them if that is the case.