Anyone know what is wrong?
When you replaced the t-stat did you use a HP t-stat. Did you use the O terminal?
What is happening is the outdoor unit, the HP is supposed to energize or de-energize a valve when calling for heat. If this valve does not work or is not wired properly at the t-stat, then it will run as if the A/C is calling only. You should have noticed cold air coming from the vents during call afor heat. If not, then the electric heat was probably coming on as well as the A/C.
Could be also the unit was stuck in Defrost. But I think it's a wiring issue or a bad reversing valve.