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Auxiliary and Emergency Heat in Thermostats

Satyajeet Vispute
Many users of modern heating systems don't know what the terms 'auxiliary heat' and 'emergency heat', as indicated by their thermostats, mean. Learn the meaning and significance of these terms through an auxiliary heat vs. emergency heat comparison and the correct way of using a heating system.

Lockout Temperature

In many heating systems, auxiliary heat comes ON when the outdoor temperature drops below a certain threshold. This threshold is known as lockout temperature. In most cases, lockout temperature is 40ºF.
There are two most popular types of air conditioning systems: air conditioners with furnaces and heat pumps. The former doesn't use the auxiliary (AUX) or emergency (EM) settings, as the only source of heat in this case is the furnace. The only exception to this is the dual fuel systems, which are a combination of furnaces and heat pumps.
The latter, on the other hand, usually comprises one main primary source of heat (typically a heat pump), and another auxiliary source of heat (typically an electric resistance type of heater).
As there are two sources of heat, the thermostat controller needs to use the auxiliary and emergency pre-programmed modes in order to decide which one (or both) of the two to use, and when.
Most modern digital thermostats for heat pumps include both these options in their settings menu. These modes are switched ON or OFF depending upon the heating requirements of the house. Let's learn more about each of these setting modes, what they mean, and how they function.

What Does Auxiliary Heat Mean in a Thermostat?

Normally, a heating pump system utilizes a heat pump as the primary source of heat for the house.
However, when the heat pump by itself is unable to meet the heating requirements, auxiliary heat is automatically turned ON. When the auxiliary mode is activated, it is indicated on the LCD of the digital thermostat.
Auxiliary heat is obtained from an electric resistance heating element, which is typically bundled along with the heating pump in the heating system. It provides the required additional heat, thus supplementing the heat pump, allowing it to meet the heating load of the house.
Auxiliary heating activates automatically to maintain the temperature of the heater's target area (within the house) when there is a sudden temperature drop of more than two degrees. Similarly, it also activates itself when you manually increase the set temperature of the thermostat by two degrees or more than the current room temperature.
Once the target temperature is achieved, auxiliary heat is deactivated, i.e., the electric heater is switched OFF, and the heat pump continues to work without its assistance.

What Does Emergency Heat Mean in a Thermostat?

As the name itself suggests, emergency heat is meant to be used only in case the situation demands it. This mode is activated manually by the use of a dedicated button on the thermostat.
When activated, it is indicated on the digital display. When put on emergency heat mode, the heat pump is shut OFF and completely bypassed, and the entire task of heating the house is undertaken by the electric resistance heater alone.
Emergency heat is designed for special situations, such as the failure of the compressor of the heat pump, or other similar issues, which result in it not being functional anymore. In such circumstances, the emergency heat provided by the heater is used to stay warm until a technician can fix the system.

How Not to Use the Heating System

Many people are known to use their heat pump systems by always activating the emergency system on the thermostat. This is the wrong way to use it.
Though it still provides heating for the home, perpetually running the heating system on the emergency mode will mean that auxiliary heat stays ON always, while the main heat pump is cut off.
Since auxiliary heat is provided by the electric heater, it will have to work overtime, and use a lot of electricity to bear the heating load of the dwelling. This will culminate in you having to pay unnecessarily excessive electricity bills.
Also, if you find that some instrument malfunction is causing your system to frequently turn ON auxiliary heat, you must have it inspected, as this again will cost you more in terms of power bills.
Thus, auxiliary heat is provided by an electric heater which requires much more energy to function as compared to the primary heat pump.
In the emergency mode, the heat pump is cut off, and auxiliary heat is used exclusively. Hence, you must only put the heating system in this emergency mode when the heat pump malfunctions, and is unable to provide the requisite heating for the house.