Furnace constantly blowing cool air, but house staying at 70?

We have American Comfort and have lived in a new built home for about 8 1/2 yrs, it's as old as the home. We have noticed it only seems to blow out cool air, once in a while lukewarm air, but either way you freeze if you are under a vent(in the ceiling). We are a one story home. Somehow we manage to stay at 70 all the time, but I don't understand why the air is so chilly that is coming out. I know it isn't only at the start or end of the furnace coming on and off because we have monitored it. The thermostat is set to Auto Fan and on heat. Any ideas what may be going on?