My a/c unit is blowing cold but takes forever to reverse to heat; it was working a few days ago but when I turned the heat on, it would keep running as if the coil would not heat up... am I correct? The unit is 8 years old and was working fine. I might add that it is a 2 ton unit, Amana, supplying air/heat to 800 sq. Ft. What's the problem? Maybe the thermostat?