I'm no HVAC guy, but I do know why the problem is occurring.
When you run a heat pump in winter, ice naturally builds up on the coils. The unit has some type of electronics to monitor this, which puts the heat pump into defrost mode. In defrost mode, it melts the ice and then goes back to normal operation. Often when this happens, you will see steam coming off the outside unit.
This happened on an older GE heat pump unit I had about seven or eight years ago. The unit was about twelve years old, and iced up. In my case, the defrost cycle was controlled by a timer, which had gone bad.
Refrigerators also have a defrost timer to prevent the coils from icing up. For some reason, your defrost cycle is not working.