You have a heat pump and during winter when they are in heating mode, the outdoor coil can ice up. When the defrost control board detects the ice, it places the unit into defrost mode. In defrost mode, the outdoor fan is turned off, the reversing valve is switch to cooling mode, and that makes it an air conditioner. So, we need to turn on Auxiliary heat; typical Aux heat is electric heat strips. The jumper between E and W2 is a way to force the electric heat strips to be turned on when the heat pump is not keeping up while in heating mode. When the temperature falls 3-degrees below the set point, the the thermostat can turn on stage 2 heating (W2). So whoever installed that thermostat programmed it to have two stages of heating in addition to the heat pump. So when it gets behind, it will supplement the heat pump with heat strips in the air handler.
With a nest learning thermostat, you will not need a jumper between E and W2. You will simply have Auxiliary Emergency Heat. When the wire is defined as Emergency Heat, the thermostat can be placed into emergency heat mode when say the outdoor unit fails or otherwise breaks down. In this way your not without heat and therefore emergency heat has to manually be turned on.
Now with Auxiliary Heat it is simply booster heat when the outdoor unit is in defrost mode or needs a heating boost when the indoor temperature continues to fall when the heat pump is not keeping up. Auxiliary heat is automatically controlled.
I hope this helps.
The AC Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
