I have a carrier heat pump that was installed in Sept. 2010; It cooled fine in the summer but, now the outside unit is not kicking on. If you push the reset button on the outside unit it kicks on but, it is not coming on automatically. What could this problem be? We just installed a new thermostat.

What reset button are you pressing outside. If it is high head pressure there could be a problem with the charge in the unit, condenser fan not coming on, or faulty reversing valve. In Heatpump mode it reverses the flow of freon in the unit. If the valve is hanging up it could be causing a high pressure situation. If it has a low pressure reset you could also have low charge or plugged coils in either the evaporator or condensing coils. Hope this helps you.