Aux Heat vs. Emergency Heat: What's the Difference on a Heat Pump?

If you have a heat pump, you have probably seen AUX HEAT or EM HEAT flash on your thermostat at some point during a cold stretch. For most homeowners, the first reaction is to wonder whether something is wrong. In most cases, it is not. But understanding what each of those indicators actually means, and when one of them should concern you, is worth knowing before the next cold snap arrives.

Auxiliary heat and emergency heat use the same backup heating equipment inside your home, but they operate in completely different ways, serve different purposes, and have different implications for your energy bill and your system's health.

How a Heat Pump Heats Your Home

Unlike a furnace, which generates heat by burning fuel, a heat pump moves heat. In heating mode, it extracts heat from outdoor air and transfers it indoors. This process is highly efficient in moderate temperatures, which is one of the main reasons heat pumps are popular in climates like North Georgia, where winters are relatively mild.

The limitation is that as outdoor temperatures drop, there is less heat energy available in the outside air for the pump to extract. Below a certain temperature threshold, the heat pump alone cannot meet the home's heating demand. That is where backup heat comes in.

Most heat pump systems include a backup heating source built into the air handler, typically electric resistance heat strips, though some dual-fuel systems use a gas furnace as the backup. This backup system exists for two reasons: to supplement the heat pump when it cannot keep up on its own, and to serve as a fallback if the heat pump fails entirely.

What Is Auxiliary Heat?

Auxiliary heat, shown as AUX HEAT on most thermostats, is your backup heating system working alongside your heat pump. The keyword is alongside. When aux heat kicks in, the heat pump is still running. The backup heat strips simply add capacity to what the heat pump is already producing.

Aux heat activates automatically. You do not turn it on, and you cannot prevent it from running when conditions call for it. Your thermostat manages the switch based on a few different triggers:

• Outdoor temperature drops below the heat pump's efficient operating range: This threshold varies by system but is typically around 35 to 40 degrees Fahrenheit. When it is cold enough that the heat pump cannot efficiently extract enough heat from the outside air on its own, auxiliary heat supplements it.
• The thermostat is set to raise by more than 2 to 3 degrees at once: A heat pump heats gradually and efficiently at steady temperatures. If you suddenly raise the set temperature by a large amount, the system needs aux heat to close the gap more quickly than the heat pump alone can.
• The outdoor unit enters defrost mode: In cold, humid conditions, frost accumulates on the outdoor coil. The heat pump periodically reverses its cycle to defrost the coil, temporarily halting the production of warm air. Aux heat runs during this defrost cycle to keep your home comfortable while the unit clears itself.

Seeing AUX HEAT on your thermostat during a cold night or after adjusting the temperature is normal. It means your system is working as designed.

What Is Emergency Heat?

Emergency heat, shown as EM HEAT or EMERGENCY HEAT on your thermostat, is a manual mode that completely shuts off the heat pump and runs only on the backup heating system. The heat pump plays no role while emergency heat is active.

This is the critical difference: auxiliary heat works with the heat pump; emergency heat replaces it.

Aux Heat vs. Emergency Heat: The Key Differences

1. How it activates: Aux heat turns on automatically based on temperature and thermostat demand. Emergency heat is activated manually by turning a thermostat setting.
2. What happens to the heat pump during aux heat? The heat pump continues running, and the backup system supplements it. During an emergency heat event, the heat pump is shut off entirely.
3. When to use it: Aux heat runs whenever needed in cold weather, which is normal. Emergency heat should be used only when the heat pump has failed, and you need heat while waiting for repairs.
4. Energy cost: Aux heat is less efficient than normal heat pump operation, but still benefits from the heat pump running simultaneously. Emergency heat is the most expensive heating mode because the heat pump, which provides the efficiency advantage, is completely bypassed.
5. What the thermostat indicator means: The AUX on your display indicates the system is automatically supplementing. EM HEAT means you or someone else has manually engaged emergency mode, or the heat pump itself has a problem that triggered the indicator.

When AUX Heat Running Is a Warning Sign

Aux heat running occasionally during a cold snap is expected and healthy. Running the heat constantly, even on days that are not particularly cold, is a different situation and usually points to a problem with the heat pump itself.

If your thermostat shows AUX HEAT for extended periods while outdoor temperatures are above 40 degrees, a few things may be happening:

• The heat pump is not operating efficiently: Low refrigerant, a dirty coil, a failing compressor, or a reversing valve problem can all reduce the heat pump's output, causing the system to lean on aux heat more than it should. The heat pump may appear to be running while actually producing very little heating capacity.
• The outdoor unit is frozen and not defrosting properly: A heat pump that has accumulated ice and cannot complete a defrost cycle will lose its ability to extract heat. The system stays in aux heat continuously, trying to compensate. If you see significant ice on the outdoor unit on a day above freezing, the defrost system needs attention.
• The thermostat is set too aggressively: Raising the thermostat by large increments frequently will trigger aux heat more often than necessary. Gradually setting back the temperature rather than all at once helps the heat pump operate without constant backup.
• The system is undersized for the home: A heat pump that is too small for the square footage it serves will rely on auxiliary heat regularly, even in moderate cold. This is a sizing problem that only a new installation can permanently correct. Learn more about how to properly size your HVAC System.

A helpful rule of thumb: if you are seeing AUX HEAT consistently on days above 40 degrees in Georgia, it is worth a professional diagnostic visit before the colder months arrive.

A Note on Dual Fuel Heat Pump Systems

Some heat pump systems use a gas furnace as the backup rather than electric heat strips. These are called dual-fuel or hybrid systems. The aux and emergency heat logic works the same way, but the backup source burns gas rather than running electric resistance coils.

In dual fuel systems, the backup heat is typically more efficient than electric heat strips, which changes the cost calculation somewhat. Whether frequent heat running is a concern depends on how the system is configured and on fuel costs in your area. If you are not sure which backup type your system uses, a technician can tell you during a maintenance visit.

Questions About Your Heat Pump? MR. HVAC Can Help

If your heat pump is running on aux heat more than it should, your EM HEAT light came on unexpectedly, or you are not sure whether your system is operating normally, MR. HVAC serves Canton, Woodstock, Roswell, Alpharetta, and Cherokee County with honest heat pump diagnostics and repair.

Call us at (770) 213-4111 or schedule a heat pump service visit, and we will tell you exactly what is going on. If your system is due for seasonal maintenance, our heating maintenance service includes a full inspection of both the heat pump and its backup heating components.