Many homeowners notice their heat pump displays “Auxiliary Heat” or “AUX Heat” during cold snaps. Understanding why your heat pump uses auxiliary heat can help you stay comfortable, avoid unexpected energy bills, and know when to call for help. This article explores the meaning of auxiliary heat, the common causes for its activation, how it affects your energy usage, and troubleshooting steps you can take to ensure efficient operation during chilly months.
What Is Auxiliary Heat In A Heat Pump System?
The term “auxiliary heat” refers to a backup source of heating usually included in modern heat pumps. Heat pumps transfer heat from outside to inside your home, even in cold weather, but they become less effective the colder it gets. When outdoor temperatures drop too low, or the system can’t reach the thermostat’s setting, auxiliary heat automatically activates.
Types Of Auxiliary Heating Elements
- Electric Resistance Heaters: Most common; similar to giant toaster coils inside the air handler.
- Gas Or Oil Furnaces: Some dual-fuel systems rely on a furnace for auxiliary heat.
These backups ensure your home remains warm, but they consume more energy compared to standard heat pump operation.
How Does Auxiliary Heat Work?
In most American homes with heat pumps, auxiliary heat kicks in under these scenarios:
- Outdoor temperatures drop below the heat pump’s efficiency threshold (often 30°F–40°F).
- The system struggles to match the thermostat setting’s rapid or major increase (for example, adjusting the temperature up more than 2–3 degrees at once).
- The heat pump is in a defrost cycle to prevent coil icing, briefly relying on auxiliary to maintain warmth.
Auxiliary heat is automatic and designed to operate only as needed, but frequent or prolonged AUX operation may indicate an underlying problem.
Common Reasons Your Heat Pump Is Using Auxiliary Heat
When “AUX Heat” shows up on the thermostat, several factors could be at play. It’s important to distinguish between normal operation and system trouble. Below are the most common reasons:
Very Cold Outdoor Temperatures
Heat pumps extract available heat from the air. When it’s extremely cold outside, the system can’t collect enough heat to keep your home comfortable, triggering auxiliary heat to supplement the deficit. In many areas of the U.S., once temperatures fall below the system’s balance point (the coldest outdoor temperature where the heat pump can meet indoor heating needs unaided), AUX will turn on. This isn’t a malfunction, but a design feature for colder climates.
Large Temperature Setback Or Sudden Thermostat Increase
If you program your thermostat to lower temperatures during the day or night and then quickly raise it, the heat pump may not be able to meet the new demand fast enough. This prompts the system to temporarily activate auxiliary heat to quickly close the gap. Frequent, large adjustments can cause wasted energy by overusing AUX heat.
Defrost Cycle Activation
During cold, humid days, frostr can accumulate on the outdoor coil. The heat pump initiates a defrost cycle, briefly reversing operation to melt the ice. During this interval, the system may temporarily call for auxiliary heat to maintain comfort indoors. This is expected and typically lasts less than 15 minutes per cycle.
System Malfunction Or Maintenance Issues
Prolonged reliance on auxiliary heat can be a red flag. Potential issues include:
- Low refrigerant levels (reduces the heat pump’s efficiency)
- Dirty air filters or evaporator/coil (restricts airflow and heat exchange)
- Malfunctioning outdoor unit (ice buildup not clearing, fan failure, etc.)
- Faulty sensors or thermostats (incorrectly signaling for AUX heat)
When these issues arise, your heat pump’s main mode can’t keep up and the system reverts to auxiliary heating for compensation.
How Auxiliary Heat Impacts Energy Usage And Utility Bills
Auxiliary heat, especially electric resistance heating, uses significantly more energy than your heat pump in standard mode. A heat pump typically delivers 2–3 times more heat than the energy it consumes. By comparison, electric auxiliary or “emergency” heat is 100% efficient, converting all consumed electricity to heat, but it doesn’t benefit from the heat pump’s transfer efficiency.
Examples Of Energy Usage:
| Heating Method | Energy Input vs. Heat Output | Relative Operating Cost |
|---|---|---|
| Heat Pump (Standard Mode) | 1 kWh in -> 2–3 kWh of heat | Low |
| Auxiliary Electric Heat | 1 kWh in -> 1 kWh of heat | High |
| Dual Fuel (Gas/Oil) | Variable | Depends on fuel price |
If your system is frequently or continuously running on AUX heat, expect higher electricity bills, especially during prolonged cold weather or if a problem goes unaddressed.
Signs Auxiliary Heat Is Running Too Often
Auxiliary heat should only operate when necessary. If it seems to be on most of the time, you might notice:
- Consistently high energy bills during winter
- Thermostat frequently displaying “AUX Heat”
- Indoor temperature struggling to reach your setting even as AUX heat runs
- Your home’s air handler feels especially warm or hot when blowing air
These could point to system inefficiencies or malfunctions—not just weather-related demand.
Troubleshooting Why Your Heat Pump Is Using Auxiliary Heat
If you believe your auxiliary heat is engaging too frequently, follow these steps before calling an HVAC technician:
Check The Outdoor Temperature
Is it below freezing? If so, auxiliary heat use may be expected. If not, and auxiliary heat is still active, continue troubleshooting.
Review Your Thermostat Settings And Program
Gradually increase set temperatures (1–2°F at a time) instead of raising it drastically. Programmable thermostats work best when set for gradual adjustments or holding a steady temperature to minimize AUX usage.
Inspect HVAC Air Filters
Dirty filters restrict airflow and can impair heat pump performance. Change or clean your air filter every 1–3 months during heavy use.
Observe The Defrost Cycle
If the outdoor unit is icing up frequently, watch whether the defrost cycle is operating properly. If ice isn’t clearing or the unit stays in defrost too long, you may have a malfunction or low refrigerant.
Listen For Unusual Noises
Grinding, squealing, or rattling from the indoor or outdoor unit may indicate a failing component. Prompt repairs can prevent further reliance on costly auxiliary heat.
Examine The Outdoor Unit
Look for visible ice, blockages, or debris. Clear away leaves, snow, or debris from vents and ensure there’s at least two feet of clearance.
Check The Breakers
Sometimes the outdoor unit’s breaker will trip, causing only the AUX heat to run. Reset the breaker and see if standard heat pump operation resumes.
When To Contact An HVAC Professional
After basic troubleshooting, persistent auxiliary heat usage suggests it’s time to call in professionals. Seek an HVAC technician for:
- Refrigerant leaks or low charge
- Electrical issues with outdoor unit or controls
- Sensor problems (thermostat, defrost, or temperature sensors)
- Compressor or fan motor failure
- Duct leaks or insulation issues causing heat loss
Professional maintenance can restore efficient heat pump operation and significantly lower your winter energy expenses.
Preventing Unnecessary Use Of Auxiliary Heat
Prevention is always the best solution. Here are effective tips for keeping auxiliary heat use to a minimum:
- Schedule routine maintenance—HVAC inspections and tune-ups should happen annually, ideally before winter.
- Replace or clean air filters regularly to maintain optimal airflow.
- Seal air leaks and insulate your home for less heat loss and better efficiency.
- Use a programmable thermostat—Minimize large temperature changes to avoid triggering auxiliary heat.
- Clear debris and keep the outdoor unit unobstructed.
- Upgrade when needed: If your heat pump is over 15 years old or undersized, consider a modern, high-efficiency replacement.
Myths About Auxiliary Heat
Several misconceptions circulate about auxiliary (and emergency) heat:
- Myth: “Aux heat is always bad.” Reality: It’s necessary in cold spells or for rapid warm-ups.
- Myth: “Turning the thermostat way up heats my house faster.” Reality: It only causes auxiliary heat to run longer, wasting energy.
- Myth: “More frequent defrost cycles mean better performance.” Reality: Frequent cycles may indicate a problem needing attention.
Summary Table: Reasons Heat Pump Activates Auxiliary Heat
| Cause | Description | Normal? | Action Needed |
|---|---|---|---|
| Low Outdoor Temperature | Heat pump cannot meet demand | Yes | None (expected in cold) |
| Thermostat Setback/Jump | Large or rapid increase | Yes | Use gradual/steady settings |
| Defrost Cycle | System removes coil ice | Yes | Short-term only |
| Dirty Filter/Blocked Coil | Reduces airflow/efficiency | No | Clean or replace filter, or clear blockages |
| System Malfunction | Component failure or refrigerant issue | No | Contact HVAC professional |
Choosing The Right Thermostat For Heat Pumps
A heat pump-compatible thermostat is crucial for efficient operation. Look for models labeled “heat pump with auxiliary or emergency heat” capability. These monitor temperature, outdoor conditions, and adjust automatically to minimize auxiliary heat use. Smart thermostats can optimize staging and even learn your occupancy habits, reducing unnecessary AUX operation.
Difference Between Auxiliary Heat And Emergency Heat
Many thermostats feature both “auxiliary heat” and “emergency heat” settings. Here’s the distinction:
- Auxiliary Heat: Automatic backup; system decides when it’s needed based on demand.
- Emergency Heat: Manual override—homeowner selects this mode if the outdoor unit fails completely, running backup heating only (least efficient).
Use emergency heat only if advised by a professional or if your outdoor unit is unable to operate.
How Regional Climate Impacts Auxiliary Heat
Heat pumps are most efficient in milder climates. In colder U.S. regions, auxiliary heat is a normal part of winter operation, especially during Arctic fronts or storms. In moderate or southern climates, auxiliary heat should run rarely and only during brief cold snaps.
Maintaining Comfort And Efficiency With Your Heat Pump
Understanding why your heat pump uses auxiliary heat helps you make informed decisions. By recognizing normal versus problematic operation, performing basic maintenance, and seeking professional service when necessary, you’ll stay warm, avoid surprises on your utility bill, and extend your system’s life.