@SteveDS
it appears that you have a heat pump with auxiliary heating as well as emergency heating. Now on those older thermostats, both the auxiliary heating and emergency heating are the same heating system. The difference is emergency heat can be activated at the thermostat by the homeowner if for example the heat pump has failed. This allows the homeowner to have electric heat strips manually activated until the service company can get there to repair the heat pump. Now you also have a white wire in W2 that is your auxiliary heating. If you go down to your furnace/air handler, you will notice that the wire that’s on E on your old thermostat is also on the same one as your W1 in the air handler. Basically they’re connected together.
So now let’s talk about the auxiliary heat. Heat pumps often are installed with auxiliary heat in the air handler to allow the heat pump to continue to heat the air when the outdoor unit needs to defrost the ice buildup. When the defrost control board puts the outdoor unit into defrost mode, it changes the flow of the refrigerant to be basically an air conditioner. It uses the heat from the refrigerant coming from inside the house to help quickly melt the ice on the outside. When it does this it activates auxiliary heat so that the indoor air does not get cold. They auxiliary heat is controlled by the defrost control board on the outdoor unit. Now with a smart thermostat, if the heat pump is not keeping up with the cold air infiltration in the home, the thermostat can activate the auxiliary heat to help.
If you want to be able to have emergency heat capability, you need to upgrade to a Nest Learning thermostat.
AC Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
