If your thermostat shows AUX or AUX HEAT, the system is normally calling for backup heat automatically because additional heating is needed. EM HEAT or Emergency Heat is different: it is normally selected manually and typically stops normal heat-pump heating so the backup heat source carries the heating load.
Normally comes on automatically when the thermostat or HVAC controls determine that additional heating is needed.
Normally selected manually and typically prevents normal heat-pump heating while the configured backup heat source is used.
What Is Auxiliary Heat?
Auxiliary heat is backup heating that the system can turn on automatically when additional heating is needed. The thermostat may display AUX, AUX HEAT, or Auxiliary Heat.
Homeowners normally do not switch AUX Heat on themselves. The thermostat or equipment controls decide when it should operate based on indoor temperature, thermostat settings, runtime, outdoor conditions, and system configuration.
In many all-electric heat-pump systems, auxiliary heat is provided by electric resistance heat strips inside the air handler. In a dual-fuel system, the backup source may instead be a gas furnace. The exact sequence depends on the installed equipment and control settings.
What Is Emergency Heat?
Emergency Heat is a thermostat mode available on heat-pump systems configured with backup heat. It may appear as EM HEAT, Emergency Heat, or simply EM.
On a typical heat-pump thermostat, Emergency Heat is selected manually. When activated, it normally prevents the heat pump from providing its usual compressor-based heating and uses the configured backup heating source instead.
AUX Heat and Emergency Heat may use the same backup heating equipment. The key difference is how the backup heat is activated: AUX is normally automatic, while Emergency Heat is a manually selected operating mode.
The Main Difference Between AUX and EM Heat
AUX: usually automatic.
EM Heat: usually selected manually.
AUX: heat pump may continue running.
EM Heat: normal heat-pump heating is typically locked out.
AUX: provides additional or alternative heating.
EM Heat: relies on backup heat instead of normal heat-pump heating.
No. Neither display alone proves that the heat pump is broken.
When Auxiliary Heat Can Turn On Automatically
Seeing AUX Heat for a limited period can be normal. The exact trigger depends on the thermostat, heat-pump controls, equipment capacity, outdoor conditions, and the heating load of the home.
The heat pump may need additional capacity to maintain the set temperature.
Some thermostats bring on backup heat when the requested temperature rises several degrees.
Backup heat may help restore the scheduled indoor temperature more quickly.
The controls may stage backup heat when the system cannot keep up with the setpoint.
Some heat pumps also use auxiliary heat briefly during a defrost cycle, but defrost is a separate operating sequence. For that topic, see Heat Pump Defrost Cycle: What Is Normal and What Indicates a Problem?
When Should You Use Emergency Heat?
Emergency Heat is not a “heat faster” setting. It is generally intended for situations in which normal heat-pump heating should not be used or is unavailable and the backup heating source needs to carry the load.
For example, a technician may recommend Emergency Heat if the outdoor heat-pump section should remain off until a repair is completed. The correct use depends on the specific equipment, thermostat, and failure condition.
If the thermostat unexpectedly shows EM HEAT, confirm that Emergency Heat was not selected accidentally. Changing from normal Heat mode to Emergency Heat changes which heating equipment the thermostat is allowed to use.
On systems with electric resistance backup, Emergency Heat can use considerably more electricity than normal heat-pump operation. It should not be left on routinely simply because outdoor temperatures feel cold.
Does AUX Heat Mean the Heat Pump Is Broken?
No. An AUX Heat message alone does not mean the heat pump has failed. Backup heat is part of the normal design of many heat-pump systems.
- Short AUX operation
- Colder outdoor conditions
- Recovery after a thermostat setback
- A large increase in the thermostat setpoint
- AUX runs for long periods in mild weather
- The home cannot maintain the set temperature
- Backup heat use suddenly increases
- The outdoor heat pump does not run when the installed control sequence indicates that it should
Why Auxiliary Heat May Run Too Often
In the relatively mild winter conditions common in South Orange County, long periods of AUX operation are worth investigating when they cannot be explained by thermostat recovery, unusual weather, or the intended control sequence of the installed system.
Thermostat and Control Problems
A heat pump can be mechanically capable of heating correctly while incorrect thermostat settings or control wiring cause backup heat to operate more than intended.
- incorrect heat-pump equipment type selected during thermostat setup;
- incorrect compressor or auxiliary-heat staging;
- incorrect balance-point or outdoor-temperature settings;
- miswired thermostat or equipment-control terminals;
- sensor or low-voltage wiring faults;
- thermostat compatibility or control-board problems.
If AUX behavior changed after thermostat replacement or HVAC control work, see heat-pump thermostat installation and setup in South Orange County.
Heat Pump Problems That Can Increase Backup-Heat Use
If the primary heat pump produces less heat than expected, the thermostat may request backup heat more often. Possible causes include airflow restrictions, compressor or inverter problems, outdoor-fan faults, sensor or control issues, reversing-valve problems, or refrigerant-system faults.
An AUX message by itself cannot identify which problem is present.
If the main symptom is cool air coming from the vents rather than an AUX or EM Heat message, see Why Is My Heat Pump Blowing Cool Air in Heating Mode?
How Technicians Diagnose Excessive Auxiliary Heat
Diagnosis starts by determining whether the AUX call is normal for the current thermostat settings, outdoor conditions, and equipment configuration. A technician may then check thermostat staging, control signals, outdoor-unit operation, indoor airflow, backup-heater operation, sensors, and heat-pump performance.
On dual-fuel systems, balance-point settings and intentional heat-pump lockout must also be considered. An outdoor unit that is off is not automatically evidence of a fault if the control sequence is intentionally using the furnace instead.
For detailed troubleshooting, see heat pump repair and diagnostics in South Orange County.
When Heat Pump Service Is Needed
Schedule service when AUX or Emergency Heat behavior is accompanied by signs that the normal heating system is not operating as expected.
- AUX Heat runs for long periods during mild weather without an obvious reason;
- the home cannot maintain the thermostat setpoint;
- the outdoor heat pump does not run when the installed control sequence and current conditions indicate that it should;
- Emergency Heat is the only mode that can maintain indoor temperature;
- the thermostat shows a fault, a breaker trips repeatedly, or the equipment develops abnormal electrical or mechanical symptoms.
If backup heat keeps running in mild weather or your heat pump cannot maintain the set temperature, MaksBuilder can diagnose the thermostat, controls, airflow, and heat-pump operation in South Orange County.