The Difference Between Emergency Heat and Auxiliary Heat on Your Thermostat

By Manny Pozos · October 29, 2026 · 9 min

Navigating Thermostat Confusion During the First Fall Cold Snap

When the first fall cold front in October/November finally arrives, understanding the difference between emergency heat and auxiliary heat on your thermostat becomes critical for maintaining your home's comfort and energy efficiency. In cooling-dominated climates like Austin, the sudden shift from months of air conditioning to late autumn heating often catches homeowners off guard. You turn on the heat pump for the first time all season, the temperature drops rapidly outside, and suddenly an unfamiliar indicator lights up on your screen. At AC American Air Heating and Air Conditioning, our dispatchers field dozens of frantic calls every fall from local residents panicking at the sight of a new thermostat alert and prematurely switching the system to emergency mode, which wastes a massive amount of energy.

The decision point you face is whether to trust your automatic system to do its job or to intervene with a manual override. Heat pumps are incredibly sophisticated pieces of equipment designed to manage temperature fluctuations seamlessly. However, because we spend the vast majority of our year cooling our homes, the heating functions remain a mystery to many. If you find yourself staring at your thermostat wondering if your heater is broken, you are not alone. This is a remarkably common point of confusion during our team's pre-winter inspections. By learning how your HVAC and Air Conditioning Systems handle sudden temperature drops, you can avoid costly mistakes, keep your energy usage in check, and keep your home perfectly comfortable all winter long.

What It Means When Your Thermostat Display Shows 'Aux Heat'

Our technicians constantly remind customers that seeing your thermostat display showing 'Aux Heat' is entirely normal and does not mean your heating system is broken. "Aux" stands for auxiliary heat, which is a supplemental heat source built directly into your system. For the vast majority of residential heat pumps we service across Austin, this supplemental source consists of electric resistance heating strips located inside the indoor air handler. Because a heat pump works by transferring ambient heat from the outside air into your home, there are specific conditions where the primary compressor needs a little bit of help to reach your desired indoor temperature. The auxiliary mode is an automatic, secondary stage of heating designed to bridge the gap between what the heat pump can provide and what your thermostat is demanding.

It is crucial to understand that auxiliary heat engages automatically. You do not need to press any special buttons or flip any switches to activate it. The system's internal logic constantly monitors the indoor temperature, the outdoor temperature, and the setpoint on your thermostat. When it detects that the primary heat transfer process is falling behind, it seamlessly energizes the electric strips to provide a boost of warmth. Once the home reaches the target temperature, the system automatically turns the auxiliary strips off and returns to standard heat pump operation.

The Automatic Trigger: Temperature Differentials

The most common reason your thermostat display showing 'Aux Heat' activates is a temperature differential. Thermostats are programmed to trigger the supplemental strips when the indoor temperature drops roughly 2 to 3 degrees below your setpoint. If you come home to a chilly house at 65 degrees and immediately crank the thermostat up to 72 degrees, you are creating a massive differential. The system recognizes that the heat pump compressor alone will take too long to close that 7-degree gap, so it automatically pulls in the auxiliary heat to recover the temperature quickly.

To prevent the electric strips from activating unnecessarily, our team highly recommends adjusting your thermostat gradually. If you want to warm the house up, increase the setpoint by just one or two degrees at a time, wait for the system to satisfy that demand, and then bump it up again. This gradual method allows the highly efficient outdoor compressor to handle the workload on its own, keeping the energy-intensive auxiliary strips dormant.

The Role of the Defrost Cycle

The second automatic trigger for auxiliary heat is the heat pump's defrost cycle. During cold, humid weather, the outdoor unit extracts heat from the ambient air, which naturally causes condensation to form on the outdoor condenser coils. When temperatures hover near freezing, this condensation turns into frost or solid ice, blocking the airflow necessary for the system to function.

To protect itself and maintain efficiency, the heat pump will temporarily reverse its operation, switching back into cooling mode to send hot refrigerant out to the outdoor coils and melt the ice. Because the system is technically in cooling mode during this brief period, it would normally blow cold air into your home. To prevent you from feeling a freezing draft, the system automatically turns on the auxiliary heat strips to warm the air before it reaches your vents. Once the ice is melted, the defrost cycle ends, the system returns to normal heating mode, and the auxiliary strips turn off.

The Role of Emergency Heat: A Strictly Manual Override

While auxiliary heat is an automatic function of a healthy system, emergency heat is a strictly manual override designed for catastrophic equipment failure. Emergency heat is not a mode your system will ever enter on its own. It is a setting that must be physically selected by the user on the thermostat interface. Understanding this distinction is the key to managing your home's heating efficiency.

When you switch your thermostat to "Em Heat," you are completely disabling the outdoor heat pump compressor. You are telling the system to ignore its primary, most efficient method of heating your home. Instead, the system bypasses the outdoor unit entirely and relies 100% on the indoor electric resistance strips to generate warmth. Because these strips are designed only to be a temporary, supplemental boost, forcing them to carry the entire heating load of your home puts a tremendous strain on your electrical usage.

The definitive rule for using this setting is right in the name: it is for emergencies only. You should only activate emergency heat if your outdoor unit has suffered a complete mechanical failure, has been crushed by a falling tree branch, or is completely encased in a solid block of ice that the automatic defrost cycle cannot melt. If the outdoor unit is physically incapable of running, switching to emergency heat provides a temporary bridge to keep your family warm while you wait for a professional to arrive. Routine use is never advised, which is why we stress the importance of scheduling fall maintenance. Investing in routine HVAC maintenance and tune-ups before winter hits is the best way to ensure your outdoor unit remains reliable enough that you never have to touch the emergency override.

Mechanical Deep Dive: Aux Heat vs. Emergency Heat

To truly understand the impact of your thermostat settings, we need to look at the mechanical differences in how these two modes consume energy. A standard heat pump is incredibly efficient because it does not generate heat; it simply moves existing heat from one place to another. This process of heat transfer allows a heat pump to deliver up to three units of heat energy for every one unit of electrical energy it consumes. Electric resistance strips, on the other hand, generate heat directly by passing an electrical current through a metal element, much like a giant toaster. This direct generation process operates at a one-to-one ratio, making it far less efficient than the heat pump compressor.

When your thermostat display showing 'Aux Heat' lights up, the system is running the highly efficient outdoor compressor AND the indoor electric strips simultaneously. The compressor is still doing the heavy lifting, while the strips are just providing a temporary boost to recover the temperature quickly. Once the temperature is reached, the strips shut down, and the compressor continues its efficient operation.

When you manually select Emergency Heat, the system runs ONLY the electric strips. The efficient compressor is locked out. This dramatically increases your home's energy consumption. One of the most common behavioral mistakes our crew sees during the first fall cold front in October/November is homeowners manually switching to emergency heat simply because the house feels a little chilly and they want it to warm up faster. This panic-driven decision leads to massive energy waste. As your honest local guide to home comfort, AC American Air wants to save you from this costly mistake. Unless a tree has literally fallen on your outdoor unit, leave the thermostat set to its normal heating mode and let the automatic auxiliary function do its job.

Why Mild Texas Winters Cause Heat Pump Panic

To understand why thermostat alerts cause so much anxiety, we have to look at the specific climate context of Austin and South Texas. In our extensive experience servicing HVAC systems across the region, we know that our area experiences a predominantly long, intense cooling season, followed by relatively mild winters. Because the baseline winter temperatures in Austin rarely stay below freezing for extended periods, heat pumps are the absolute perfect heating solution for this climate. They operate with incredible efficiency in the 40-to-60-degree range, easily extracting enough ambient heat from the mild Texas air to keep homes comfortably warm without ever needing supplemental help.

However, this high baseline efficiency means that when a severe, sharp cold snap does roll through, locals are simply not used to seeing auxiliary heat indicators on their thermostats. When the first fall cold front in October/November drops temperatures from 75 degrees down to 35 degrees overnight, the heat pump is suddenly forced to work much harder than it has all year. The system automatically pulls in the auxiliary strips to handle the sudden load, the "Aux Heat" light pops on, and homeowners immediately assume something is broken because they haven't seen that light in twelve months.

We want to reassure our Austin neighbors that brief periods of supplemental heating are completely normal and are already factored into the overall seasonal efficiency of your system. Your heat pump is designed to handle the mild days with peak efficiency and lean on the auxiliary strips during the rare freezing nights. As long as the "Aux" indicator turns off once the house reaches the target temperature, your system is adapting to the Texas weather exactly as it was engineered to do.

Frequently Asked Questions About Heat Pump Thermostat Settings

Why does my thermostat say Aux Heat?

It indicates the system is automatically using supplemental electric strips to help reach your desired indoor temperature. This happens naturally due to a 2-3 degree temperature drop in the house, or because the outdoor unit is running an active defrost cycle to melt ice off the coils. It is a normal function of a healthy heat pump and does not signify a breakdown.

When should I turn on Emergency Heat?

You should only manually engage this setting when the outdoor heat pump unit has suffered a complete mechanical failure. It is designed to act as a temporary bridge to keep the house warm using only the indoor electric strips while you are waiting for a professional repair. If the outdoor unit is running, you should never use this setting.

Does Emergency Heat use more electricity?

Yes, significantly more. Electric resistance strips are far less efficient than the heat pump's standard heat transfer process, operating at a one-to-one energy consumption ratio. Relying on the emergency setting for days at a time will result in noticeably higher energy usage, which is why it should only be used during an actual equipment failure.

How long should emergency heat stay on?

It should only stay on as long as it takes for a licensed HVAC technician to arrive and repair the outdoor unit. It is not designed for prolonged, multi-day use under normal circumstances. If you find yourself needing to use it for an extended period, you should schedule a professional diagnostic immediately.

Is auxiliary heat the same as emergency heat?

They use the exact same electric heating strips located inside your air handler, but the operational method is completely different. Auxiliary mode runs alongside the outdoor compressor automatically to provide a temporary boost; Emergency mode shuts off the compressor entirely via a manual override, forcing the strips to do all the work.

Is it normal for Aux Heat to run constantly during a freeze?

During extreme, sustained below-freezing temperatures, the heat pump may run continuously with frequent auxiliary support because the outdoor air holds very little heat to extract. However, if temperatures outside are mild (above 40 degrees) and the auxiliary heat runs constantly, it may indicate a refrigerant leak, a failing compressor, or a sensor issue requiring professional attention. If you are experiencing strange operational cycles, knowing what to do if your thermostat clicks but your heater won't ignite or run properly can help you determine when it is time to call for service.

Secure Reliable Heating Performance Without the Guesswork

Understanding the difference between emergency heat and auxiliary heat on your thermostat is the easiest way to prevent unnecessary panic and energy waste when the temperatures drop. Remember that seeing your thermostat display showing 'Aux Heat' is simply your system's automatic way of providing a little extra warmth during a cold snap or a defrost cycle. Emergency heat, on the other hand, is a manual override reserved strictly for actual mechanical emergencies when the outdoor unit has failed completely.

If you suspect your heat pump is leaning on the auxiliary strips too often during mild weather, or if your outdoor unit is making strange noises and struggling to keep up, it is always better to be safe than sorry. Do not rely on the emergency setting to mask an underlying mechanical problem. Instead, schedule a professional fall HVAC system diagnostic with our experienced team to secure reliable heating performance. A clear, reassuring explanation of your system's health from a licensed AC American Air technician will ensure your home stays perfectly comfortable, no matter what the Texas winter brings.