Simply defined, heat pumps in climates below 35 degrees require a supplemental heating source. This heat source comes in the form of electric resistance heating located at the indoor unit. The supplemental heat can also be referred to as backup or second stage heating with the first stage being the heat pump itself.
During emergency heating, the first stage heat is not used, but instead the supplemental or second stage heat is put to work. Different thermostats and heating systems have ways of determining at what point the second stage heat should come on so as to assist the heat pump. Whatever it is, the process of determination and switch over happens automatically with no manual intervention.
It is clear from the name that emergency heat should only be used in emergency situations. This is when the first stage heating has developed a problem and therefore non-functional. For instance, of you notice the temperature in your house going below the normal level and when you step outside you realize your heat pump has been crushed by a fallen tree, you can proceed and switch on the emergency heat.
During the winter season, it is not uncommon to see your heat pump turned into an ice block and stopped working. Because the heat pump which is the first stage heat has malfunctioned, you should turn the thermostat to emergency heat as you call for service.
When you switch to emergency heat, you will immediately notice the red indicator light go on. It will stay on up and until you turn off the emergency heat. When in this mode, only the indoor unit and the backup heat will be operational. There will be no signal that will be sent to the outdoor heat pump. The emergency heat is designed to keep you going until a HVAC contractor arrives for an emergency repair.
If the type of heat pump installed in your home is an all-electric model, it will consume significant power in the emergency heat mode. This is why it is only meant for emergencies. However, if you are having an oil or gas heat pump as your backup system, the cost may not be so significant as it all depends on the cost of fuel you are using and how efficient the heating system is. All in all, the cost of running gas or oil heat pump is way less than that of running an all-electric system in an emergency heat mode.
If you find the emergency heat light on even when your heat pump is fully functional, you need to check at your thermostat because it may have been erroneously set to emergency heat. However, if you find your thermostat not in emergency heat mode and yet the light is on, your heat pump could be having a problem and getting a HVAC contractor to come and inspect it is advisable.