The term auxiliary heat (often abbreviated as aux heat or aux Ht) appears on many modern thermostats, especially those paired with heat pumps. Understanding what aux heat does helps homeowners manage comfort and energy use. This article explains the meaning, operation, and practical implications of aux heat, along with tips for efficient use and troubleshooting.
What Is Aux Heat and How Does It Work
Auxiliary heat is a supplementary heating source used to reach or maintain the desired indoor temperature when the heat pump alone cannot efficiently supply enough heat. In most systems, when the thermostat detects a drop in temperature or a need for rapid warming, the heat pump will handle the initial heat exchange. If additional warmth is required, the auxiliary heat source—commonly electric resistance coils inside the air handler or furnace—kicks in to assist.
On a heat pump system, the auxiliary heat is typically activated automatically by the thermostat based on the current outdoor temperature, indoor temperature, and the efficiency of the heat pump. The result is faster heating and improved comfort during very cold conditions, when heat pumps lose efficiency.
When Your Thermostat Uses Aux Heat
Aux heat generally engages in these scenarios:
- Outdoor temperatures are very cold, reducing heat pump efficiency.
- The thermostat calls for rapid setback recovery and needs faster warmth.
- The system is in defrost mode, temporarily limiting heat pump output.
- There is a call for heat that exceeds the heat pump’s immediate capacity.
Many thermostats display a status indicator such as “AUX,” “Aux Ht,” or “Emergency Heat” when auxiliary heat is active. The switch to aux heat may be seamless, but it often results in higher electricity consumption and a warmer canister or blower area as resistance coils generate heat directly.
Aux Heat vs. Emergency Heat: What’s the Difference?
Some thermostats distinguish between auxiliary heat and emergency heat. Aux heat supports the heat pump and runs to boost comfort. Emergency heat, in contrast, uses only the auxiliary heat source and does not rely on the heat pump at all. Emergency heat is designed for very harsh environments or failed heat pump operation and typically consumes more energy.
Understanding this distinction helps homeowners avoid unnecessary energy use. If the thermostat remains in “Emergency Heat” mode longer than expected, it may indicate a system issue or a need for service.
How Thermostats Control Aux Heat
Thermostats manage aux heat by monitoring indoor temperature, outdoor temperature (via system inputs), and the heat pump’s operating status. If the setpoint is not met by the heat pump within a short time, the thermostat will engage the auxiliary heating elements. Advanced thermostats optimize this by using intelligent scheduling, outdoor temperature data, and humidity readings to minimize energy waste.
Some thermostats allow you to customize aux heat behavior, such as setting a minimum outdoor temperature threshold for aux activation or choosing a higher energy-efficiency mode that relies more on the heat pump. Familiarize yourself with your model’s settings to balance comfort and efficiency.
Tips For Efficient Use Of Aux Heat
- Keep thermostat setpoints consistent to reduce frequent cycling between heat sources.
- Use a programmable schedule to avoid large temperature swings during absences.
- Ensure proper insulation and sealing to minimize heat loss, reducing the need for aux heat.
- Consider a heat pump with a high-efficiency auxiliary system and proper refrigerant charge for optimum performance.
- Regularly replace air filters to maintain airflow and system efficiency.
Energy impact: Aux heat typically consumes more electricity than the heat pump because electric resistance elements convert electricity directly into heat with near 100% efficiency, but at a higher operating cost compared to a heat pump’s efficient heat transfer, especially in milder conditions.
Common Issues And Troubleshooting
- If aux heat stays on longer than expected, check for unusually cold outdoor temperatures, large indoor temperature swings, or thermostat programming conflicts.
- Ensure the outdoor unit is free of debris and the refrigerant level is correct; low refrigerant can force the system to rely on aux heat.
- Verify that the thermostat’s settings are correct and that the “Aux Ht” or “Emergency Heat” option isn’t selected unintentionally.
- If you hear loud or unusual noises when aux heat engages, or if comfort doesn’t improve, contact a licensed HVAC technician for inspection.
Frequently Asked Questions
Is aux heat bad for my system? Not inherently. Aux heat is a designed operating mode that improves comfort when the heat pump alone is insufficient. However, prolonged reliance on aux heat can raise energy costs, so optimizing the system for efficiency is beneficial.
Can I prevent aux heat from engaging? Some thermostats let you set a minimum outdoor temperature for aux activation or adjust heat pump performance parameters. Improving building insulation and using a smart schedule can also reduce the frequency of aux heat use.
What should I tell a technician? Note when aux heat engages, any temperature discrepancies, thermostat model, and recent changes to settings or performance. This helps diagnose whether the issue is a thermostat setting, airflow, or refrigerant concern.